Before attempting to coach paint a vehicle the paintwork or bodywork must be inspected closely to see what kind of repair method may be necessary.
Assessment is the first task to undertake before commencing any repair as the repairer or coach painter needs to know exactly what is involved.
When the vehicle body is prepared ready for painting it must be free from dirt, oil, grease etc and should be "tacked off" to remove all particles of dust or other contaminants just prior to applying paint.
Applying primer is the first step when painting either bare metal or wood and although Red Lead (the infamous metal protector) was the favourite choice years ago it's now become difficult to obtain due to lead content paints being banned in UK manufactured paint in particular.
As an alternative to Lead based paint a Red Oxide or Zinc Phosphate primer can be used on bare metal even Galvafroid which is a Zinc rich brush applied paint that gives protection very similar to hot dip Galvanizing.
The primer coat will always be heavily pigmented which often leaves a rough brush-mark finish but also provides a thick protective film ready for wet flatting the following day (weather permitting). It may be necessary to apply more than one coat of primer, This will depend on the paints overall thickness after application and it should completely obliterate any bare metal or wood AFTER wet flatting, If not, reapply and wet flat again.
Primer is the single most important layer of paint on any vehicle because it provides the initial seal from the elements and in conjunction with a finishing gloss can do so for many years. Primers and undercoats are porous and must be protected by an additional gloss layer.
An undercoat provides build up layers and should also obliterate the primer, It is common practice to match the undercoat colour to the final gloss colour because this can have the advantage of only needing to apply one coat of gloss if the gloss is a particularly good coverer, Applying one coat of gloss is not always recommended when two coats are usually required.
Undercoat should also be wet flatted and may require several coats to completely obliterate and build up the underneath layer of colour whether it be primer or undercoat.
Unthinned undercoat can often leave heavy brush-marks just like primer does so it is customary to mix undercoat with gloss 50-50 to help with the flow out process, This leaves a smoother finish with little or no brush-marks which makes the surface easier to wet flat prior to further applications of undercoat or gloss.
It was a popular procedure in the old days to apply several layers of undercoat each one being a different colour to show imperfections and each layer was wet flatted with Pumice stone to create exceptional depth on high quality work.
Gloss is the finishing layer that takes the brunt of the weather so it has the tougher formulation so it should be durable and remain relatively colourfast.
Varnish can be added to or painted over the top of gloss to provide a glass like finish maximising durability, The gloss must first be completely wet flatted to obtain a perfectly smooth surface which will also provide sufficient key for varnish to adhere to.
Varnish should be applied over an existing newly painted surface within a week to obtain absolute maximum adhesion otherwise the finish is likely to just sit on top which would eventually lead to premature flaking through lack of adhesion.
It's not unknown for the coach painter to go to extream lengths in order to obtain the cleanest possible finish by wearing lint free overalls or simply just shorts and covering the human body with Linseed oil, this will stop human body fallout illiminating skin dust, hair, fluff etc from falling and contaminating freshly varnished/painted panels.
Paint shops generally have central heating which provides the perfect environment to apply paint even in the depths of winter, for the best possible results paints and varnishes should always be applied at room temperature. Room temperature is around 68°-72° Fahrenheit 21°-23° Celsius.
During the varnishing process in particular no other person was allowed to walk into the paint shop and that included the boss. A coach painter was authoritative when it came to protecting his paint work from human contamination or intrusion.