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When our thoughts turn to Delta maintenance, we often think of repainting the car's components. Powder coating has become the refinishing choice, here's an opinion.

Powder coating

An opinion

These powder coated leisure items are in a hostile seaside environment, but are only 3 years old

Powder coating is a method of colour coating metal, using electrostatically charged polymer particles sprayed from a 'Corona gun'. The particles are propelled from the gun by compressed air, and are attracted to the item by its own opposite, or earthed, electrical charge. The coated item is then heated and the powder film melts into a liquid coating, when this is cooled the film forms a hard layer. The advantages are near zero volatile compound emissions, original style finish, no runs, also special surface effects can be created.

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Metal surface preparation for powder coating should include, removal of all surface oil, dirt, grease, metal oxides, finger marks and welding scale. Chemical pre-treatment should include degreasing, desmutting, etching and phosphating of the metal substrate. This involves the use of volatile compounds, which sort of negates the low emissions of the powder coating process.

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So a powder coated metal item will have a hard, glossy, plasticised, even coating.

There are problems with powder coating, one is the 'Faraday Cage Effect'.

Briefly, this is an electrostatic effect that can shield an enclosed object from an electrostatic charge. The edges of a hole in a piece of metal could be shielded from the charge required for powder coating to adhere to it.

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Also a problem are the variables in the coating process. Colours and finish can change during the curing process, powder mixing and curing temperature must be carefully controlled.

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Another problem is coating thickness. On a sharp edge the coating will be thinner, here it only needs a microscopic hole to allow moisture in contact with the metal underneath. This will start rust forming under the coating, eventually lifting the coating off.

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Lastly, because powder coating is so hard, it doesn't flex. Metal does flex, this causes the coating to crack at points of movement, allowing moisture in, etc. etc. So with controlled industrial application powder coating is good, used on items not exposed to damp or stress, like a gas cooker for instance.

Let's take an Integrale suspension wishbone. You get it sandblasted then take it to your local powder coater, who paints it black. It looks good, with an as-new finish and it was easy to do. You re-assemble the car, drive around a bit, then 2 years later the coating is flaking off as rust appears. This is annoying.

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As previously stated, powder coating needs precise, controlled conditions to work properly. Without careful controls, it's doomed to fail. Watch Wheeler Dealers replacing their TVR's chassis, this shows how even well applied powder coating is a brittle finish, and will fail eventually.

So my component painting choice would be blast - etch - paint. Softer mineral based paint finishes take longer to dry, but are flexible and with good preparation have better adhesion. The softer paint resists chipping, and to a certain degree is self healing.

My choice of component refinishing is more traditional. Sandblast, clean, etch prime and paint. First the component should be bead or grit blasted to remove all rust, grease and old paint. Then blow off the dust and wipe with an oil free solvent, the blasting roughens the surface to give a good key for the paint.

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Next spray with an etch primer. This is a liquid coating containing an acid that etches into the metal substrate, forming a chemical bond. Etch primer doesn't activate until it dries, so allow a day in the warm to dry. This also lets the solvents evaporate.

 

Then apply 2 or 3 coats of spray topcoat. Aerosol spray can paint is very thin, so several light coats are required. You can brush paint over the primer with Hammerite or similar, but this should be left for a week for the paint to harden before refitting. Because brush applied paint is much thicker, I would mask off areas that have a bolt fixing, touching in after tightening up if required.

This is a simplified essay! There are many variables with both powder and paint finishes.

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Galvanised steel and aluminium require a 2pack activated etch primer to bond paint to the substrate. Powder coating can, and is, applied to these metals, but it will sit on the surface, just waiting for the day it can jump off, and that won't take long.

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