Primers and surface coatings

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Different types of primer.
There are many different types of primer on sale, each with its’ own specific purpose. Below is a brief overview of their uses, including good points and bad points, to help decide which type to use for any particular job.

Etch primers
Used as the first coat on any bare metal. Air drying like cellulose, but must usually be activated before use. Spray mist contains phosphoric acid, so adequate spray mask must be used.
** Etch Primers which contain zinc chromate are known to cause cancer and should be avoided **

Cellulose primers/fillers
The most well known for diy work. Low-moderate filling properties. Fast drying, easy too use. Spray putties are basically just very thick cellulose primer filler. Normal spraymist precautions suffice.Tends to sink slightly over a longer period due to solvents leaching out. May cause old paints to lift or wrinkle.

Acrylic primers
Very similar to cellulose but uses different resins. Slightly better resistance to sinkage.

2-pack Primer Fillers
The best choice for quality. Must be activated before use. Excellent filling properties. Easy to apply with excellent resistance to sinkage when cured. Does not usually react with old paint films. Slower drying than cellulose. Isocyanate activators means that proper respirator cartridges or air-fed masks should be used when spraying.

Synthetic Primers/Fillers
Not now in common use, except coachbuilders and commercial vehicles. Can be applied over virtually any prepared surface without problems. Fillers have excellent filling properties and very little sinkage when dry. Very slow drying, (up to 24hrs), can react with topcoats if not hard, thick coats may wrinkle.

Etch primers for bare metal

Etch Primers
Supplied as primer base plus activator, to be mixed before use. Provides maximum adhesion to bare metal and better corrosion resistance. Absolutely essential for aluminium or galvanized steel. The activator is usually phosphoric acid based, therefore do not store activator or mixed primer in metal tins. The useful life once mixed is about 48hrs.

There are two general types available;

Etch Primer-Fillers
have moderate filling properties similar to cellulose primers, and are applied in two or three wet coats, leaving 10 - 20 mins between coats to allow solvent to evaporate. Suitable for minor filling uses, or can be overcoated with cellulose or 2-pack primers.

Wash Primers
that is, with no filling properties at all, are usually applied as one single wet coat over all bare metal areas. These are used simply for maximum adhesion and must be overcoated with cellulose, 2-pack or synthetic primer filler to achieve the required build and moisture protection.

If any etch primer is used under a conventional primer filler, then one single wet coat is sufficient The etch coat must be dry (resistant to a light fingernail scratch) before continuing with either primer filler or paint. Etch primers are sensitive to moisture, so must NOT be left for more than a couple of hours without either primer-filler or paint being applied.

Cellulose primers and fillers

Primer Fillers/Surfacers/Spray Putty
This type of product has a higher solids content (usually talc or similar) which is used to provide filling properties. `Filling' and `Surfacing' in terms of primer and other sprayable products means the ability to fill small surface scratches and defects left by previous preparation, not dents, gouges and deep scratches.

Air dry primer fillers are available from factors and in aerosol form, and are primarily cellulose or acrylic products. Their main advantages are fast initial drying times, the disadvantages being lower filling power, reactions with underlying paint products and sinkage if not dried properly. If bought from a trade factor, most of these products will need to be thinned, usually 50/50 with the appropriate thinners. Follow the particular products guidelines.

Application is normally two or three coats with adequate drying time between coats to allow solvent to evaporate. Particularly heavy coats will need additional time to dry out, otherwise solvent will remain trapped below the surface which dries rapidly. A greater number of thin coats - well dried between coats - is preferable to fewer thick, heavy coats.

2- component (2K) Primers and fillers

2-pack primer fillers have a much higher solids content, which together with their particular type of resin base gives much better filling power plus less problems with sinkage than air dry products. Most are activated immediately before use, as once mixed their usable life is limited, depending on temperature and speed of activator used.

Please note that nearly all rely on isocyanate activators which have their own health and safety risks over and above normal spray-mist concerns.

Follow the manufacturers directions for activator and thinner quantities, as these can vary widely between brands, from 1:1 by volume up to 10:1 for some fillers.

Application is normally one to three coats, depending on the surface build required. Allow adequate drying time between coats to prevent solvent build -up in the lower layers. Most two pack primer fillers are very thick and heavy even when activated and thinned, therefore a suitable spraygun set-up, i.e. large nozzle capacity, is needed to apply them at anything like a reasonable speed.

Some brands of two pack primers can, when activated and thinned appropriately, be applied as non-sand surfacers. This means simply that providing the previous preparation work has been carried out with care, and final flatting done with fine grade papers, the finishing coats of paint can be applied directly on to the primer without any further flatting.

This method is normally only recommended for use in properly controlled spray booths to limit problems with dust, etc.