Pimples??

[bulldog 1]

Hi, i'm just in the process of having my car sprayed and would like some backround on why the fibreglass has developed these 'pimples' in certain areas of the car(worst effected area on rear wing).
I know a good body shop operator will know the answer to this, but it seems that this answer does vary from person to person and comes down to opinion as well.

The car has been sprayed a couple of times, one colour on top of the other.

What is the scientific answer to these pimples and is there a 100% pre-paint prep that would stop the pimples from re-occuring?

[Pb3]

Do you mean pimple as in raised or a crater as in a sort of hole, where the paint does not take? If it is the later it is most likely down to contaminates, and silicone is the number one suspect. I had my wing sprayed a month back and the guy said it is a problem he sees with carbon more, but a lot of polishes have silicone in, he hates Mer!!!

[60mph]

When I've sprayed cars with silicone contamination lt usually leaves craters as when the paint is wet it runs off the silicone radially , increasing the depth of paint around the particle of silicone . Number 1 suspect for blisters is water or moisture in the work underneath . It doesn't take much and leaves hollow "microblisters" . Both conditions are very depressing ! You can buy silicone remover to wipe the work before paint but remember that if any rubbing down has taken place the whole workshop is contaminated with silicone dust . To avoid microblisters the work should obviously be dried out , a hot day in the sun is best , then taken inside to cool down and paint . [ All this is without a proper body shop ! ].

Frank

[738 driver]

Full removal of moulding release agent is your first hurdle when overpainting laminates.
Release agents and silicones cause 'fisheye' the trade name for the dimples people refer to.
Lumps and blisters (raised areas in the surface/gelcoat) are either surface contaminate or delamination. Trapped moisture or solvents (even trapped in air) combined with rapid heating expansion is the usual recipie for delamination/microblister.

There are some resin/layup methods which hardly suffer these problems at all.

One method to help reduce this is to reseal, with the appropriate resin, any area/edge which has been cut. Body edges, arches, vents etc.

Good luck.