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 Traditional Coachpainting


In the old days, the art of coachpainting was a much slower affair than it is now. Back then the painting of carriages or motor vehicles took a month or so to complete, from start to finish, using synthetic enamels.
Nowadays, however, the process is expected to take a week or less, the only way this can be accomplished is by cutting down on preparation time and reducing the amount of coats applied.
The number of coats can vary from anything between three and fifteen and the condition of a wooden or metal surface plays an important part in regards to the number of coats required: roughness, porosity etc.
It is true that quality work can still be produced, even when spending less time on preparation and materials.
Nevertheless, the overall finish will still lack the superior shine and colour depth associated with traditional coachpainting standards.
The information provided here attempts to draw together both the theoretical and practical aspects of both coachpainting and vehicle spray painting methods.
The key factors to consider in obtaining a perfectly smooth finish ready for paint, on any surface are:

  • Cleanliness.
  • Preparation.
  • Priming.
  • Filling.
  • Undercoating.
  • Glossing.
  • Polishing.

Obviously the smoother the surface the less build up of paint required:
Porous surfaces undoubtedly require more care with preparation so that the layers of paint material built up are sufficient to fill the rough wood or metal surfaces. Wood is a natural fibrous material, on which it is relatively easy to obtain a really good grip for subsequent paint layers to adhere to. This grip can reasonably be described as tenacious if the preparation has been carried out correctly.
Metal however, has always proved to be far more difficult for paint to adhere to, due to the ultra-smooth, often oleresinous surface, This is why modern acid etch primers are used to force the initial grip of primer to stick as tenaciously as with wood.

Synthetic paint consists of the following to make the paint medium:
Pigment solid (colour)
Fluid vehicle (Linseed oil, china oil, alkyd, polyurethane)
Thinners (Turpentine, white spirit)
Driers (metallic salts of, cobalt, manganese, lead monoxide)
Mixed together provides a liquid medium.

The early natural types of pigment were earth pigments, umbers, yellow ochre or sienna.
Ochre which can be burnt to create what is known as burnt umbers which gives different pigment shades.
Mineral pigments such as china clay, chalk or barytes are all white: these particular pigments provide excellent colour fastness and are relatively cheap.
Colour or light fastness means a colour has less tendency to fade in sunlight although this is more likely to effect paint containing cheap pigments not expensive ones.

Lead was once used as the main ingredient in paint, nowadays it is more likely to be found in glazes or ceramics, although lead paint can still be purchased for use on listed buildings.

The pigment solid is suspended in a fluid vehicle that does not dissolve. Oil based paint and varnish dry slowly by oxidation when exposed to oxygen which is when the oil and oxygen combine leading to a hard dry skin.

Modern resin based synthetics: Alkyd for example dry by a process called polymerisation which is a chemical reaction where molecules join together forming one component as the resin and the simultaneous evaporation of solvents (usually turpentine) take place.

It is worth noting that to obtain optimum performance from any paint material (if possible) the paint should be applied at room temperature 68-72 Fahrenheit 21-23 Celsius, This temperature not only ensures correct drying and application time it also allows moisture to evaporate completely.
Paint material applied in temperatures too hot will cause the material to become sticky and dry too quickly whereas applying in temperatures too cold or damp stiffens the material causing runs or blooming.

Two finely applied thin coats are much better than one heavily applied thick coat that may give a rough brushy appearance.

Primers:

The term primer has become a much misused name to describe a primer coating, Primer is a primary or foundation coat offering the first layer of protection prior to building up subsequent paint layers.
There are many different kinds of primers which include primer fillers, corrosion inhibiting, adhesion promoting, sealing or isolating primers.
The first coat of primer is the most important layer of any paint structure as it has more than one function to perform. It must bond to the surface with enough strength to allow additional coats to be applied over the top and allow sufficient movement for expansion and contraction of the surface whether it is wood or metal.
The entire painting system or process depends on the priming coat it is therefore in the interest of the coachpainter or spray painter to use a good quality primer designed specifically for the job in hand.

There are several synthetic primers suitable for steel as follows:
Red Oxides, general purpose primers,
Zinc Phosphates, usually contain anti corrosive pigments.
Galvafroid, Zinc rich paint comparable to hot-dip galvanizing.
Corrosion inhibiting primers give exceptional rust preventing results even on lightly rusted areas.

Primers are generally porous because they lack oil which is what provides the protection from the environment, Synthetic primer should be painted over as soon as possible preferably within a fortnight otherwise it will become too hard and will not allow itself to become part of the paint system.

Aluminium which is described as an oily metal in terms of painting requires etch priming to provide a suitable key to promote paint adhesion.
Acid etch is a corrosion preventive phosphoric acid that can be applied by roller or spray, it is not a substance that can be successfully applied by brush mainly because it is a rapid drying alcohol base formulated to spread thinly and therefore very evenly over the surface.

Aluminium (or Birmabright used on Land Rover) is classed as a reactive metal and like iron it reacts with oxygen causing surface corrosion. Aluminium left in its bare metal state corrodes producing aluminium oxide which is a form of corrosion that will completely cover the surface and yet to some degree protects it from further corrosion. This white aluminium oxide powder formed on bare aluminium can be scraped off, which will lead to further corrosion because the oxide is naturally weak having little or no strength. This process is called anodising.

However anodising aluminium, by an electro chemical process for example, physically alters the metal surface and produces a really tough dense layer of oxide offering maximum protection to bare aluminium.

Sealers or barrier coats are designed to isolate paint substrate reactions from volatile solvents. An example would be if you wanted to spray cellulose over a synthetic base you simply apply a sealer coat first so that when cellulose is applied over the sealer coat because it has isolated the synthetic substrate, so the strong volatile nitro-cellulose thinners will not be able to attack or soften the synthetic substrate.
These sealers are alcohol based so they cannot be undermined by alternative solvent based paints being applied over the top.
You can apply synthetic or oil-based paints over nitro-cellulose or acrylic substrates but you cannot apply cellulose or acrylic paints over synthetic substrate because the solvents will act like paint remover and remove the softer synthetic layers of paint.
If this is the case then an isolation coat is required to form a physical barrier between the old substrate and the new to stop solvent reaction from occurring.
Oil-based paint and varnish are synthetic and not soluble in their own solvent which is turpentine or a similar synthetic thinners, cellulose and acrylic solvent are resoluble.

Iron ore in its natural state does not rust. The rusting process only starts after being wrought when the ore is converted into iron or steel, which is then susceptible to rusting caused by moisture. When the metal is exposed to moisture the rust or ferrous coating, which forms on bare metal as a reddish coating, must be treated immediately to prolong the conversion. Halting or at least slowing down the conversion process is achieved by applying a primer coat initially then subsequent coats of various paints in a bid to improve long term protection from the elements.

However if the surface has become deeply pitted in rust the best way to deal with it is to have the area sandblasted and then use one of the many phosphoric rust converters on the market, which will neutralise rust and keep the metal free from ferrous oxide for a considerable amount of time.

Severely rusted areas that compromise the overall strength of a panel should be cut out and a new section welded in situ. This procedure may be beyond the scope of the average DIY person who is perhaps intent only on providing a vehicle with a repaint without wishing to perform more than just cosmetic paint repairs.

To provide a good base on wood (after rubbing down the surface ready for paint and before any paint is applied) the surface should be wiped over with a solution of boiled linseed oil, goldsize and turpentine or white spirit. The solution should be rubbed well into the grain, impregnating the wood fibres, or until it becomes dispersed and when dry it will prevent the wood grain from raising when further coats are applied on top.
Bringing a wood grain surface up to a first class standard requires more paint material to be applied to fill in the grain, thus building up the surface. It is recommended that an oily primer is used on wood to soak in and ensure adhesion for subsequent coats of paint.

The traditional way of filling the grain of wood was with a knifing filler or brushing filler. Modern methods use a premixed or powdered decorators type filler for wood obtainable at hardware or paint outlets.
Knifing filler or stopper is spread into the grain using a filling knife (usually three inches in width) which is similar to a wallpaper scraper but with a much thinner and flexible blade which allows filler to be spread evenly across the surface.

Polyester filler should be used on metal but it is a harder filler.
After a suitable filling has been applied it is recommended that a stain or guide coat of different colour be added so that when rubbed the high and low areas can be seen. When the surface shows no sign of high or low spots the finish has reached its final rubbing level-this may take several applications.
Rubber rubbing blocks can be used to good effect on flat areas and when rubbing curves a one quarter sheet of wet-or-dry paper can be wrapped around a rag or piece of mutton cloth which will follow curves naturally.
Rubbing in one area too hard or too long or at different pressures should be avoided to eliminate the possibility of rubbing through which will only introduce moisture and cause wood to swell or metal to rust.

When rubbing with a rag or single piece of wet-or-dry your hand must rub in a sideways direction. Rubbing in a forward or backwards direction produces unwanted finger-marks that will show as furrows.

When the rubbing down is finished it should be washed off with clean water and thoroughly dried with a chamois leather and allowed ample time for the moisture to evaporate completely. It is recommended to leave areas that have been wet flatted overnight to thoroughly dry otherwise moisture may cause blistering or peeling to subsequent coats. Initially it is quite practical to rinse bare metal with hot water because this will heat the metal and increase moisture evaporation.

Waterproof rubbing down paper called wet-or-dry is ideal for rubbing as opposed to the old traditional method of pumice stone or cuttlefish bone and it comes in many different grades for almost every surface.
Rubbing down with wet-or-dry paper should be done wet in conjunction with a bucket of clean water with either soda or a small amount of washing up liquid added to lubricate the paper and prevent clogging. Otherwise paint deposits will build up and cause scratching.

During the rubbing process, copious amounts of water must be used to keep the area clean from rubbing slush which, if left to build up, will result in scratching to the surface. The rubbing slush should not be allowed to dry on the surface-it must always be washed off and leathered dry as it contains salts or alkaline. The dried flatting water or sludge can be difficult to remove and may scratch the surface whilst attempting to wash or rub it off. A propriety panel-wipe or pre-wipe is a very rapid-drying degreasing solvent that should be used to clean and inspect flatted areas just prior to paint, removing any contamination (including fingerprints) that can cause adhesion problems.
Panel-wipe can also be used to inspect the surface as it is wiped over wetting a panel which creates a temporary gloss until the spirit dries, usually within a few seconds.
Turpentine can be added to panel-wipe, this slows down the rapid drying so that a surface will remain wet and look glossy long enough to inspect the area.

To obtain the finest quality work it may be necessary to apply filler to certain areas: usually low spots or deep scratches.
Applying a layer of primer will satisfy the porosity of bare wood and discourage suction, generally improving the surface. If the porosity or suction is not satisfied the surface tends to leave a heavy brush mark appearance when painted over which will require even further rubbing to eliminate.
Adding more oil will solve the brush mark appearance slightly but the paint will take longer to dry and remain softer for a longer period.

Rubbing down:

Do not confuse the term rubbing down with polishing.
Rubbing down means the surface is literally rubbed down to a matt finish using coarse abrasives with a view to paint.
Polishing means polishing the surface with ultra-fine abrasives, which enhances an already present shine.

The task of rubbing down requires a moderate amount of skill or the surface may become damaged by rubbing too hard or choosing the wrong grade of wet-or-dry paper. Older paint surfaces are generally harder to rub down and will need extra care when choosing the correct grade of rubbing paper in order to rub down properly.
Linseed oil varnishes and polyurethane should always be rubbed down wet to stop rubbing paper melting the substrate,clogging paper, scratching and generally ruining surfaces during the rubbing down process.
Wet rubbing eliminates any dust which night otherwise float round as well as lubricating the paper. It also cools the surface which stops melting or clogging.

Cuttlefish bone, pumice stone and pumice powder were the traditional rubbing mediums used on synthetic based paint or varnish before wet-or-dry paper was invented. Cuttlefish bone in particular was less likely to damage a paint surface because of its natural soft composition but it requires preparation before use.
Pumice powder is also graded by grit. Dental pumice powder is the finest grade, coarser grades of pumice can be used to flat paintwork.
Attempting to polish with wet-or-dry papers on soft synthetics is too harsh and will almost certainly rip up the surface even when rubbing down wet.

Rottenstone is derived from decaying limestone (hence the name) and was used initially to polish wood finishes. However it can also be used to polish synthetic finishes providing the paint film has thoroughly dried or cured. Oil lubricants (linseed oil or tung oil, also known as China oil, for example) can be used with rottenstone on a piece of cotton wool or felt pad.

Various grades of wet-or-dry grit are available ranging from very coarse to very fine. Low numbers represent coarse grit, high numbers represent fine grit.
Examples of common wet-or-dry grades are as follows:
120, 180, 220, 240, 320, 400, 600, 800, 1200, 1600.

Production paper however must be used dry and its purpose is mainly for use on filler or very rough paint areas that require machine rubbing.
Production paper is very coarse and the popular grades are as follows:
40, 80, 100, 120, 180, they can be used either by hand or they can be used with flat or orbital sanders.

The number corresponds to the grade of grit per inch so a typical 180 grade would produce far more visible scratches per inch than an 800 grit.

Rubbing down with any grit produces scratches than can be measured by the number or type of grade used. The finest grade of paper produces tiny fine scratches, unseen by the naked eye, whereas coarse papers will produce deep wide scratches easily seen with the naked eye.
The coarse scratches can be removed in most cases simply by rubbing with finer grades of wet-or-dry until the surface has become sufficiently rubbed to eliminate the scratches ready for painting.
Is is important to flat with the finest paper when applying varnish because varnish in particular consists mostly of oil which will sink into the flatting scratches, thereby weakening the film. While the effect might not be immediate, it will eventually show as deep scratches when the substrate finally settles.
In certain cases it is better to rub down with linseed oil and not water to avoid scratching the surface too much. Rubbing with oil leaves a nice silky sheen particularly on wooden sections where water contamination would be a major concern.

When a particularly old painted or varnish surface is peeling, blistering, cracked or simply just perished it has to be removed completely whether on wood or a metal surface.
A blow lamp is an ideal method of burning paint off wood surfaces but not very successful on metal because the heat can distort metal panels. Removing paint from metal surfaces should be done with chemical removers.
However certain wood finishes that require the wood or natural grain to be shown through varnish should not be burnt off with a blow lamp but removed with paint removers instead.
There are two types of paint remover to choose from: one is water soluble, which can be washed off with water (preferably hot) and the other is solvent based, which should be washed off with white spirit or turpentine.

Paint consisting of linseed oil visibly ages usually after five years when the substrate becomes chalky. Modern paints now tend to use alkyds and acrylic resins as opposed to traditional linseed oil.

Application of undercoats and colours:

The application is probably the most important quality because the material has to spread evenly and hold up without forming runs or sags.
Adding about 10% of linseed oil to gloss or about 20% gloss to an undercoat helps it flow out but it will soften the finish so extra drying time should be allowed.
Tekaloid synthetic formulation, particularly undercoat, dries so hard that it effectively closes the pores of primer etc.
You can over thin undercoats without having a detrimental effect on the quality but over thinning gloss colours or varnishes should be avoided because it will dull the shine and soften the final finish, which should be hard.
Gloss is a heavier material than varnish and, although it will spread easily enough, it requires less working as it sets up quicker than varnish. The less it is worked or played with the better it will flow out.
It is possible to actually create brushmarks instead of eliminating them simply by over brushing either a gloss or varnish. If the material appears too thick or sets too quickly a small amount of genuine turpentine or white spirit should be added.

Do not stir varnish or paint too vigorously or bubbles will form that can be transferred to the object being painted and ruin the finish by creating air bubbles that may not flow out properly.
Some pigments tend to leave brush-marks behind and others actually help them flow out smoothly.

Brush-marks are the most likely cause in spoiling a finish and should be kept to a minimum. This can be achieved by using only finely ground pigments thinned with turpentine to a suitable consistency before being applied. Camel hair is the softest type of bristle and can be best used in the application of heavy pigmented undercoat to reduce brush-marks.

If an undercoat is found to set too fast to allow laying off then it should be layed off as you go without any further laying off in a criss-cross way. Once a finish starts to pull or drag on the brush it is unlikely to flow out properly and should be left to set.

Certain colours actually increase brush-marks as they dry and this may be due to thinning with white spirit. This ploughed field effect is more noticeable at the point where the bristle has touched the surface, leaving the paint film somewhat thinner at the low furrows than at the higher ridges. The remedy is to use a slower drying turpentine instead.
It is not always possible to paint primers or undercoats without leaving brush-marks, because of the high pigment and low oil content. These particular paints do not flow out and should therefore be applied in different directions to build up evenly with each brush stroke layed off horizontally and then vertically.
Perhaps a better way to explain this would be to apply one coat layed off vertically, allow to dry, then apply the next coat layed off horizontally, repeating the process horizontally and vertically.

Quick setting paints should be applied in vertical strokes or strips, with each full stroke performed in a wavy fashion to blend into the previous stroke, without horizontally crossing and keeping the edge wet in a steady downward direction.
The undercoat should seal and obliterate a colour or primer underneath, building up the surface for subsequent layers. One or several layers of undercoat can be applied depending on the standard of work required. Usually one or two coats of undercoat are sufficient for inexpensive jobs but for high class work several may be needed, applied thinly to achieve a very smooth starting base. When the level of undercoat build up and obliteration has been achieved the surface is ready for gloss.

Some colours tend to float, greens and blues in particular. This is when a pigment raises to the surface during application, changing the colour slightly, which usually shows as darker streaks and can sometimes be seen as each brush-stroke is drawn across the surface. A substrate suffering from pigment floatation will look similar to a zebra crossing only the stripes will be coloured.

Floating can usually be cured by extending the open time, for example by adding a little turpentine or raw linseed oil, allowing the material to spread thinner and more evenly and thereby reducing flotation.

The finished gloss surface should be completely free from seediness. Seediness is when a coloured film has dried after solvent evaporation leaving behind a sandy finish which is actually caused by a coarse pigment. Coarse pigments are associated with cheaper paints where the pigment has not been ground finely enough.

Most colours can be wet flatted, this may be required on certain colours when a varnishing coat is necessary to provide optimum durability (although this usually applies to reds and greens).

It may be necessary to apply more than one coat of colour (especially if a good wet flatting is required prior to varnish) to ensure a solid all over colour. It is quite common in the trade to wet flat a colour and then apply varnish over the top.
Sponge flatting is a method used to wet flat a colour surface just prior to varnishing, particularly on delicate colours, when rubbing with anything other than a wet sponge would damage the surface.
Some lighter colours may need varnishing but a clear varnish would be recommended or else the varnish will darken the colour.

The theory behind the painting process is to build up a series of paint layers which become hard yet still remain flexible. The finishing coat, either gloss or varnish, should be the most flexible to sustain the influence of what the environment can thrust upon it.
The finishing coat in particular should be workable on application otherwise it is not up to the job and should therefore be discarded no matter what other qualities the paint may possess.
Poor or bad drying is caused by bad ventilation. Factors such as low temperature, damp atmosphere, oil or grease contamination or poor lighting can all affect the drying process.

Runs or Sags:

These are caused by too much paint material being applied. Temperature can also affect drying times which can lead to runs or sags because the paint is allowed to stay wet too long and will start to slip off the surface.
The paint consistency can also affect the drying process. If a run cannot be pulled up whilst still wet it will have to be left until completely dry then flatted and repainted or varnished.
It may be possible to stipple a run with your paint brush providing the paint has not started to set.

Sinkages:

Sinkages usually occur after a few months have elapsed and may be due to the whole paint system failing as a result of continued shrinking,
To avoid shrinkage the paint system layers should be left for an extra week allowing each applied coat to thoroughly harden off properly before subsequent coats are applied over the top. The only cure to sinkage is to flat and repaint or varnish the area.

Blistering or peeling:

This is usually caused by trapped moisture which when heated by sunlight will expand and burst through the substrate in the form of blisters.
Peeling is generally caused by moisture where sufficient time has not been allowed for the surface to dry thoroughly prior to painting.

Cleanliness:

Do not brush the floor prior to painting final gloss or varnish as dust will be introduced into the atmosphere which will settle on paintwork and spoil the finish.
A good idea is to soak the floor just prior to painting or varnishing because this will help keep dust to a minimum. There should be no sudden movement try to avoid having other people walking around the paint shop or painting area disturbing dust or introducing dust which is shaken off clothing.
Clean overalls are recommended especially on final coats where there is less room for errors.
A synthetic paint or varnish finish should only be washed in clean cold water and leathered off dry.
Water should not be allowed to dry on the paintwork as it may contain impurities which can cause colour staining or bleaching.

Varnishing:

Varnish should be applied with a Namel-Var brush designed specifically for applying varnish and the task of varnishing should be carried out on a warm, dry, still day with no wind.

paintbrush

paintbrush

The Hamilton traditional oval shape Namel-Var brush is ideal for use on concave and convex surfaces. Extra long seamless steel ferrule consisting of an extra thick head of 100% selected pure black China bristles which gives maximum pick-up and is perfectly suited for woodstains and varnish.
To achieve the cleanest finish possible two paint kettles should be used when varnishing, the first kettle contains the varnish you actually apply and the second kettle is used to scrape the brush to keep varnish in the first kettle clean. This also removes particles from the varnish brush if it picks any up during use.

Polyurethane varnish in normal conditions dries in about eight hours or less and is not always suitable for quality brush work. In my opinion polyurethane is more suited to varnishing floorboards or less quality work.

A good quality coach varnish should have a low viscosity, full body, flow out freely and be hard enough to flat down in about sixteen hours.
Varnish should possess a much longer open window allowing for complete flow out and must not run or sag after application, However extra attention should be given to where it may gather on screw heads, rivets, mouldings etc.

Paint or varnish can be successfully strained through a piece of nylon stocking into a purpose-made paint kettle. The stocking can be cut into lengths of approximately six inches with a knot tied at one end then turned inside out so the knot is in the inside as you pull the cut piece of nylon right over the paint kettle.
Paint or varnish should never be used directly from the original container: an estimated amount should always be poured into a paint kettle or similar receptacle suitable for the purpose. The lid should be replaced immediately after the amount has been decanted so the paint or varnish does not thicken by solvent evaporation.

Long oils have a longer open window than short oils and this applies to both paint and varnish. The longer the open time the greater chance of a good flow out and the easier it will be to remove bristles or flies, and if picking out straight away with the edge of your brush the painted area will still flow out.

Oil provides elasticity and life to a paint but must be strengthened by gum resin or rosin. There are various grades of gum resins ranging from soft to hard.
The gum is melted down and dispersed in a solvent or oil-generally the harder the gum the better the oil. Too little resin and the finish remains soft, too much resin and the finish becomes brittle.
Modern paints are now manufactured ready for use straight out of the tin as whereas the coachpainter traditionally had to make paint from scratch by grinding and mixing pigments.

During the paint manufacturing process it is generally noted that resins which melt at low temperatures are used in cheaper, lower quality varnishes. Resins melting at higher temperatures, that take longer, are the type used in top quality expensive varnish ideally suited as a finishing vehicle or carriage varnish.
Varnish consists of gum, oil, turpentine and drier but obviously lacks colour or pigment. The rosin or resin gives it toughness, oil gives elasticity and the two produce the brilliance; the turpentine thins and the driers dry. Some cheap varnishes use over cooked resins and when these dry they leave behind a dulled sheen and a very thin coating instead of a full body varnish.

Poor quality varnish will appear sandy or have a dusty finish when dry This is caused by metallic stearate sediment on the bottom of the varnish were the material has not been processed thoroughly. It is advisable not to shake the contents of varnish because this not only mixes unwanted sediment (if present) back into the varnish but also introduces air bubbles.
Sandiness is more likely to be found in quick, hard-drying varnish that contains extra driers but the consequence is undesirable stearates which are (undissolved driers destroying the varnish quality) leaving an imperfect finish and premature dullness.
These stearates or foots can be strained out using a paper filter placed over a clean tin and allowing a day or two for the varnish to pass through. The filtering should be done at lower than room temperature (about 60F) to stop the stearates from melting and therefore allow them to pass through the filter only to reappear again when the varnish heats up naturally.

It may be an advantage to apply paint or varnish to a test panel first to see how it performs before attempting to varnish a large area. The simple test panel however must be in the same state of preparation as the intended panel work to ensure a comparable working surface with regard to porosity, suction, etc. It should also be painted at the same temperature.
Varnish is applied in much the same way as a colour coat and the edge must be kept wet, The laying off time is automatically extended, however varnish always allows you extra time for application because it contains more oil than pigment. This allows you to lay off in multiple directions instead of the single wavy method used when applying faster drying materials.

Polishing with rottenstone or pumice:

Use moderate pressure when rubbing with the felt pad in straight strokes, checking frequently as you go to avoid rub through. Continue rubbing until you obtain the level of gloss required: this can range between matt, semi-gloss and full-gloss depending on the amount of rubbing or the grade of powder used.
The rubbing sludge should be removed with a clean damp cloth and the oil removed with Naphtha or turpentine. When polishing with pumice or rottenstone you can either sprinkle on as a powder or make up a polishing slurry using water or oil and adding the powder. Simply mix to achieve a creamy consistency. Rottenstone is finer than pumice stone.

When painting or varnishing the amount of dust floating about in the working atmosphere is not always noticeable but note should be made that sunlight can pick out minute particles and these particles will inevitably fall onto paintwork, spoiling the finish.

The condition of the surface to be painted will play an important part in how the finish will look: the porosity, type of undercoat used, roughness of surface, even the colour can affect the overall finish.

Driers:

Too much drier can spoil the paint or varnish. It is strongly recommended to only apply the correct percentage of drier when adding to paint material. An overdose of driers can undermine adhesion and damage the finish.
Oil-based paint dries by oxidation as it absorbs oxygen from the air but the oxygen absorption process is very slow indeed.
Speeding up the absorption process is done by adding drying agents like terebine to encourage oxidation in exacting quantities.
The surface of the paint film in direct contact with oxygen dries first. This can be increased by providing alternative pure airflow or heat. Painting a vehicle outdoors is not always a satisfactory method if you wish the paintwork to stay clean and free from contamination.
Many vehicles are in fact painted outdoors by the DIY fraternity and the drying process is dramatically increased by outdoor pure airflow often without the awareness of the person applying the paint.
If the paint film is too thick or applied too heavily the outer film will dry and form a skin before the under surface has a chance to dry. As the under surface dries it shrinks, pulling the already dry outer surface with it and leading to crinkling. This can happen to oils if too much driers are added forcing the outer film to dry too quickly.
This process does not necessarily apply to undercoats having a higher pigment content and less oil-these tend to dry more powdery without a skin.

Terebene is a concentrated liquid form of drier where the principal components manganese or cobalt are dissolved in linseed oil and thinned with turpentine, which can be added to oil paint or varnish.
Terebene, sometimes called terebine, can be added to white and other similar light colours without causing discolouration.
White pigments used in modern paints are zinc oxide and titanium dioxide. Titanium dioxide is a pure white with excellent opacity.
White lead based pigmented paints were banned during the 1960s and were replaced with a none-poisonous pigment called titanium white.

Turpentine:

Turpentine is a commonly known dilutent for synthetic oil based paint but should also be noted for its other lesser-known properties.
Turpentine is obtained from the pine tree and the sap extracted by steam distillation. After the steam distillation process the turpentine attracts more oxygen. Turpentine is also a drying agent, not just a thinner. Gum turpentine is obtained by distilling the tapped living pine extract.

Adding copious amounts of turpentine in an attempt to ease out thick paint only increases the drying time, it does NOT reduce it.

However turpentine substitutes and white spirits will shorten the drying process, not necessarily increase it, and although white spirit can be used to thin primers or undercoats it should not be used to thin gloss enamels or varnish as it will disintegrate the resins.
White spirit is derived from crude oils of a paraffin base and should not be relied upon to hold up in paint finishes but should be used instead as a cleaning agent for brushes etc.
Turpentine should be water white absolutely clear and after evaporation should dry without leaving a residue. White spirits and turpentine substitute leave behind residues.

Paintbrushes:

It is false economy to buy a cheap brush-especially when you have spent a considerable amount of time on preparation only to have it spoilt by the poor performance of a brush. It is always best to purchase the most expensive brush, leaving you safe in the knowledge that this is one factor you do not have to worry about with regards to performance.

A Namel-Var brush contains soft pliable bristles perfect for spreading the lighter varnishes across panel work.

Enamel or gloss brushes contain hog or china hair which has a stronger bristle for spreading the harder to spread gloss across the surface.
Finishing gloss enamels and varnish should be dry overnight.
The faster drying paint or varnish finishes should never be used on vehicle work. Vehicle finishes should consist of the long oil variety to be extra durable.

A paint brush is by far the most important item in producing a finish and its quality will reflect this.
The quality of a brush may not be apparent at first glance.
The best way to test the quality is through its use.
The brush should be well balanced with a good solid weight and length of bristles.
China black bristles are found in Hamiltons brushes that are strong and springy whereas cheaper horse hair brushes do not have the same spring or strength and should be avoided.
A quality brush will have a full head of bristle where the ferrule is packed tightly and remains rigid even when charged with paint and held horizontally whereas cheap brushes tend to have the centre ferrule hollow, lacking bristle, which makes them weak so that when charged with paint it will just flop over.
This type of brush would prove absolutely useless on a heavy paint or varnish.

Stand the brush upright on its bristles and push down firmly at the tip of the handle using the palm of your hand (thus bending the bristles at almost 45 degree angle) then release the pressure and the brush should immediately spring back into position with considerable force from the bristles.
This tests the firmness of the bristles: if they do not push back up then the brush is not going to spread paint out evenly and should be discarded.
All too often cheap brushes labelled pure bristles on purchase are misleading because although the brush may contain a small percentage of pure bristle the remaining bristles are limp and simply incapable of producing a suitable paint job. They will simply slide over the paint surface, instead of pushing the paint evenly across it causing an uneven surface which will inevitably lead to runs or sags.

Brush cleaning:

Usually paint brushes are cleaned in turpentine or white spirit if oil-based and warm water if emulsion. Either way they should also be washed out in soap and water allowed to dry thoroughly.

A brush should be stored in its original wrapper which keeps the bristles clean and helps retain the shape of the brush. The brush can also be stored in a plastic bag or container to prevent moths from laying eggs so that their grubs will not eat the bristle.
A wire brush can be used to stroke down the bristles from the ferrule to clean stubborn paint deposits without fear of damaging the bristles themselves.

Oil paint can be quite difficult to remove as it dries because it is not resoluble and sometimes it has to be scraped off the bristles. Hardened paint can often be left in the stock: this is one reason why a brush should not be wiped over the edge of a paint kettle because it forces paint into the stock making it difficult to remove if left to harden.

A brush should only be charged with paint material by dipping in the paint and dabbing the brush on the inside of a paint kettle keeping the paint material at the tip of the brush rather than the stock or ferrule.
Scraping a varnish brush out in the way previously described is acceptable because unlike gloss paint varnish takes a considerable amount of time to harden so does not pose the same problem. It is much easier to clean.

Oil paint and varnish, although not resoluble in their own solvent (which is turpentine) can be dissolved in solvent brush cleaners, but I prefer to use my own solution of cellulose thinners and turpentine mixed at 75% turpentine, 25% cellulose. This mixed solution is still powerful enough to dissolve hard oil paint but will not damage delicate bristle. The turpentine or white spirit will neutralise some of the powerful strength of cellulose thinners that can make the bristles go floppy or even rot if left standing in neat cellulose for too long.
Certain synthetic brushes consist of fibres that may melt in cellulose thinners so I would recommend checking before cleaning your brush.

Mineral naphtha is better than cellulose but it may be harder to source because cellulose is more readily available.

Primers and undercoats are usually very heavy in pigment and difficult to spread. They therefore require a firm brush that can push the paint evenly across a panel. Varnish on the other hand, being a lighter substance, only requires a soft brush to spread the substance evenly (like a Namel-Var varnish brush).
If a soft brush is used on a heavy pigmented paint it will not spread evenly, it will only slide over the top. A brush that possesses good spring and strength is ideal as a general purpose brush for priming, undercoating, glossing and varnishing. A Hamilton perfection brush consisting of 100% black China bristle would be the ideal choice to use as an all round general purpose brush.

Skin drying:

Gloss and varnish finishes form a continuous solid film when dry but are liable to skin dry, which means the outer exposed surface has dried but the surface underneath remains soft. Skin drying prevents oxygen from reaching the inner layer and is usually caused by an overdose of driers-this is more likely to occur on long oils which contain a higher percentage of oil.
The more oil added to paint the more flexible it becomes when dry; with less oil the paint film is more brittle and likely to crack.
A Drier only acts as a conveyor of oxygen it does not have the ability to oxidise the paint on its own. Oxygen must be absorbed from the atmosphere and is more effective on the area it touches.
Skin drying can be rectified by allowing the skin to dry thoroughly then flatting and repainting. However the same paint can be used again by adding raw linseed oil which will retard the drying process and stop the skin from reappearing.
There is a precise proportion of driers required to oxidise an oil paint and if the quantity is too little or too much it will only yield poor results.

Discolouration:

Colours can alter on their own even if kept in dark places and when a colour is varnished it might tint a little towards yellow.
White is more noticeable with regard to discolouration. If an object is placed on a white painted window sill and left for some time, a distinctive brownish discolouration will appear underneath the object, but the mark will disappear in a relatively short time.

White pigment increases opacity and helps resist UV radiation.

Cellulose:

Cellulose is a completely different material from the conventional air-drying synthetic oil-base materials. Cellulose or nitro-cellulose dries by evaporation of a spirit solvent, not by the oxidation process (as do synthetic oil-based paint and varnishes).
Cellulose is derived from cotton waste mixed with nitric and sulphuric acids producing pyroxyline or dinitocellulose which we now call nitro-cellulose. This is the by-product left behind after solvent evaporation during manufacturing. Coloured pigments are then added to this cellulose varnish which produces nitro-cellulose enamels.

Because cellulose dries very rapidly it should be applied by spray gun to obtain the best result. A decent brush application is not possible except on very small limited areas.
As cellulose does not dry with a particularly good shine, it has to be polished either by hand or mechanically to obtain the high gloss automatically expected from synthetic finishes.
Cellulose sets in about ten minutes but dries in about half an hour (ready for second coating) unlike a day or two for certain synthetic finishes.
The final coat should be allowed to harden for 2 hours before attempting to polish. It is easy to obtain a cleaner finish with cellulose than synthetic as it dries faster, can be reworked quicker and reduces dust contamination considerably.

The spraying process is classed as semi-skilled although preparation requires real skill.
A coachpainter is more likely to produce a skilled level of finish using a brush or spray system, whereas a sprayer is less likely to possess the necessary skills to produce a coachpainted finish this takes years to perfect whereas spraying can be perfected in a single day.

As with any proposed paint application, preparation is absolutely vital and must be thorough. There must be no trace of any foreign body on the surface, such as dust or grease. Metal in particular should not be over- handled-even your fingerprints can cause adhesion problems. This is more apparent with cellulose paint systems than oil paint or varnishes.

Paint:

Paint has been used for thousands of years and has been recognised for the following three purposes: Decoration, preservation and sanitation.

Decoration goes back as far as the stone age with evidence of cave painting.

Preservation was particularly important during the early 17th century for protecting wooden carriage work and architecture.

Sanitation was more of priority in hospitals where painted surfaces can easily be cleaned with a simple wash down but exposed wooden areas for example would harbour dirt and germs etc.

White lead powder (lead carbonate) was once the main ingredient in paint manufacture and paint originally contained more than 75% white lead.
Some pigments reflect light better than others: reds and yellows for example can appear less opaque, allowing light to penetrate right through to the undercoat or primer if an undercoat has been omitted.
Colours containing these particular pigments will need further applications to completely obliterate the underneath colour.
Is is also recommended that less opaque pigment colours should be helped along by providing an undercoat base as close as possible to the intended finishing colour.

Preparation:

Flatting down can often expose inner paint layers or bare metal, particularly if the paint area has been repaired or simply through removing rust. These areas will require extra attention prior to repainting by flatting or feathering back. This process is necessary to make good a repaired area by feathering down the surrounding area so that the hollow of a repair will be blended in to the thickness of the paint edge.

Wet flatting:


This is simply a way to flat down an area with wet-or-dry abrasive paper. Water lubricates the paper and eliminates dust.

Rust is a prime concern on panels whether it be surface or ingrained. Sand blasting is an ideal method of removing light or deep pitted rust because the small particles of sand will get deep down into the rusted area, almost completely removing the rust, leaving a clean dull area ready for treatment with a proprietary brand of phosphorus rust treatment and then primer etc.
Grinding or mechanical wire brushing are not a very satisfactory method of rust removal because neither will penetrate the metal as effectively as sand or grit blasting.
Bare metal must be primed straight away to avoid surface rust from appearing. Rust can appear within seconds, particularly on steel.
Severe rust can only be rectified by cutting out the affected area and welding in a new section or by complete panel replacement.

Guide coat:

This is a method of applying a contrasting colour to the substrate very lightly to use as a guide when flatting, very useful for showing low spots or other paint defects that may require further rectification or stopping.
A guide coat also aids the flatting process by helping you avoid rubbing too much material away It is also easier to inspect as you rub.

If you do not have access to spray equipment or aerosols there is another, perhaps preferable method of applying a guide coat called Dry Guide Coat, made by 3M. This is a cartridge that contains a powder which you simply wipe over the substrate with the supplied applicator pad.
Dry Guide Coat can be used during the flatting process eliminating masking which can be considered a major advantage. However guide coats should not consist of red or yellow to avoid bleed through.

Bleed through is a term describing a saturation in which the colour underneath can still be seen, even through the substrate has been painted over, For example, red painted over in white would reveal a pinkish colour underneath-even after several applications the colour can still bleed through. The preventative measure is to isolate the red or yellow colour first by applying a barrier or sealing coat before applying the colour white.

Bare metal can often hold contaminants even when it appears clean. Such contaminants can undermine the adhesion or durability of the paint which is why OEM panels go through a matriculate six stage cleansing process to prevent corrosion. The main ingredient is phosphoric acid which removes all traces of oil, grease, fingerprints, residues and rust. Phosphoric acid also etches into the bare metal providing maximum adhesion for subsequent paint layers.

The paint process on OEM panels is called electro phoretic and in most cases this finish should be left intact unless it has been damaged.
However I have come across a few replacement panels that required this coating to be completely removed as it was notably soft and rubbery so that any coating, even primer, applied on top would not be sufficiently bonded, (although this might have been a one-off with one cheap panel manufacturer).
If it is not always necessary to remove all the paint, it should be prepared in the usual way by rubbing down, cleaning and painting in a suitable primer.


Primary, Secondary and Tertiary colours:

Primary Colours

Theoretically there are only three colours : Red, Yellow and Blue, all other colours are made from these three.
Two primary colours mixed together will produce a secondary colour: Orange, Green and Purple.
Primary colours cannot be created by mixing other colours.

Primary

Secondary colours

Produced by mixing two primary colours. The secondary colour is halfway between the two primaries. Mixing Red with Yellow will produce Orange, Mixing Yellow with Blue will produce Green and mixing Red with Blue will produce Purple.

Secondary

Tertiary colours

Colours produced by mixing a primary colour with one of its secondary colours. Mixing Red with Orange produces a Red Orange. Mixing Blue with Green produces a Blue Green. Mixing Yellow with Orange produces a Yellow Orange.

Tertiary colours

A greater variety of colours can be achieved by varying quantities of the primary colours, for example; To make a clean bright orange the red must lean towards yellow and the yellow must lean towards red. If the red contains blue or the yellow contains blue the resulting orange will be dull because a small portion of the blue primary colour has been added which reduces the purity and brightness of the secondary colour being produced.
A similar example only this time wanting to produce a pure green: the blue must lean towards yellow and the yellow must lean towards blue. For a purple the blue must lean towards red and the red must leans towards blue.

Tertiary colours

It has been said that black and white are the negative and positive of colour.

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