General Preparation

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 treating neutralizers.
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 Acid derivative.
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 becomes inactive. Single-pack Acid Etch primers are available in aerosol form which dry by solvent evaporation.
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, Twin-Pack, etc. 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.

General Method
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 eggs.
3. When area is well prepared and ready for primer spray on Acid 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 anti-corrosion 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 the corrosion.
Health and Safety regulations should as always be observed.

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.

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