Acid Etch primer contains a chemical
similar to Sulphuric Acid which burns into the Aluminium to provide an
excellent bond. Phosphoric is a milder form of Acid more commonly found in rust
Top Tip Acid etch primer can be mixed and
applied to small areas of bare Aluminium using a lint free wad of cotton
wool or gorse cloth.
Sulphuric Acid diluted can also be used to treat severe cases of corrosion on
Aluminium or Steel but you really need to know what you're doing when using any
Aluminium double glazed units are anodised which is a form of protection
by introducing corrosion to the surface of the metal that will cease further
corrosion. Vauxhall Motors used to allow bare body shells to become anodised
before treating and eventually painting.
AJS Motor cycles used to leave all the
barrels on the roof of the building
in Plumstead Road London to weather before machining. Apparently some metals
require seasoning similar to hardwood.
New bare Aluminium should be degreased with panelwipe or
genuine turpentine and sanded down with wet-or-dry or a scotch cloth to provide
a key, Then painted with an acid etch primer to bond or etch into the aluminium
and provide a base for additional primers Synthetic, Cellulose, Twin-Pack etc.
Etch Primer is commonly a twin-pack component
consisting of primer and an additive or thinner mixed at a ratio of 50-50.
Two-pack etch primer has a short life when mixed, typically 6 hours after which it
Single-pack Acid Etch primers are available in aerosol form which dry by solvent
Acid Etch primer can also be used on steel although the acid properties are more
suited to Aluminium.
Steel should be prepared in the same way
as aluminium and can be painted with any suitable primer Synthetic, Cellulose,
Various proprietary products can be used to prolong the life of rusted
metal or corroded aluminium and they all work in the same way by attempting
to seal or neutralise the metal from further corrosion or oxidization.
Rust or corrosion should be removed completely from the area first but in
practice this approach is not always possible.
On corroded Aluminium or Steel use a 3M Scotch-Brite "clean & strip" fibre
disc fitted to electric drill, The action removes paint and loose deposits
without damaging metal surfaces. Preparing aluminium ready for painting you'll
come across four probable cases.
- Many layers of old thick paint.
- Blistered or bubbles in paint finish.
- Signs of previous repairs or filler work including dents.
- Actual holes in alloy with powdery corrosion deposits.
1. Sand areas with an Orbital sander and remove all traces of
old damaged paint using 80 grade sanding discs on stubborn parts and
finishing with 120 or 150 grade.
2. Create a key for the filler on areas that require filling
using 80 grade paper then fill dents with a suitable polyester filler and rub
with 80 grit
paper (fill and rub repeatedly if necessary) to a smooth finish just
above the surface of the body, Then rub with 180 wet-or-dry paper using
water and a rubbing block (depending on repair) with a little Fairy Liquid
(to ease lubrication and clean the paper to prevent clogging) to a level
just below the panel surface feathering across the alloy as you go.
(Some fillers require dry rubbing only, Check on filler tin for details.
Body filler should always be applied on bare metal or aluminium for maximum
adhesion and not over paint of any description, Otherwise when painting with
primer or top coats the solvent or thinners may penetrate the old paint
underneath and could create problems later on like lifting, bubbling or fried
3. When area is well prepared and ready for primer spray on
Etch or rub on bare areas if very small and allow to dry.
Spray one light coat of primer (if using cellulose) allow to flash off then
spray a heavier coat allowing to flash off and continue until at least three
heavy coats of primer are applied to overall body shell remembering to apply
slightly more primer to repaired areas building the level of primer slightly
above the repaired body surface.
Spray a guide coat any colour except black (black has a higher density pigment
than any other colour and flatting is generally harder) over the repaired areas
and when dry wet flat with 800 grade wet-or-dry paper again with a little Fairy
liquid using a rubbing block on flat repairs. To flat the remainder of the
shell use a quarter section of wet-or-dry wrapped around a large wet piece of
mutton cloth which is ideally suited in following the contoured shape of panels.
Use a four inch grinder with rubber backing disc and grade 80 card disc
on severely corroded areas stroking the panel to avoid causing deep scratches.
4. Corrosion can be treated with various chemical
neutralises to help prevent further corrosion usually by sealing the metal from
oxidisation. I personally use a diluted mixture of Sulphuric acid that in my
opinion attacks the powdery substance adequately enough burning away most of
Health and Safety regulations should as always
Working with coach enamel
You can wet flat with any combination of wet-or-dry, 1000, 800, 600, 400, 320
or sometimes 180 in order to remove brush-marks, the higher the number the
finer the grade of grit.
What grade of wet-or-dry to use will also depend on how smooth or clean the
finish is when dry before choosing a particular grade of wet-or-dry.
The wet flatting process may also need to be applied between all coats,
particularly undercoats to keep the surface absolutely smooth and flat.
All prestigious work will need wet flatting to produce that extra smooth
surface ready for gloss, the gloss when applied will (or should)
flow-out completely free from brush-marks because it contains more
oil, undercoats and primers contain more pigment than oil and
therefore require more flatting to remove the brush-marks that heavy
pigmented paints often leave behind.
Flatting is the term used when rubbing down paint work, cutting back is
a term used when polishing back to a shine either by hand or machine,
using rubbing and or cutting compounds.
240, 320 and 400 are ideal grades for dry scuffing or wet flatting, 600
might be considered too fine a grade to finish off with, particularly when
using synthetic paints, because synthetic paint is a thicker material and
is less likely to sink, it will not show flatting marks when using a coarser
grade of paper.
120 grade would be considered too coarse for flatting between coats, 120
or 180 are more suited to rubbing down severe defects prior to using primers
or indeed body fillers.