GRP panel repaint over crazing results in bubbling

[Tempest_5]

Hello,

The bonnet on my Westfield is badly crazed due to two mishaps with the bonnet catches issues and the bonnet parting company with the car. Following damage repair to a few small sections the bonnet needs a repaint. Due to the number of cracks in the gel coat it is not viable to gouge each crack out and re gelcoat. Therefore I just want to respray the bonnet so it is all the same colour even though crazing is still visible. I can't really afford £467 for a new bonnet.

Given the amount of crazing meaning I would have to re gelcoat approx 2/3rds of the bonnet. I did consider building a mould using the old bonnet as a pattern but I estimate that ultimately I wouldn't save much, if anything, over buying a new one from Westfield.

I tried painting last summer but I got bubbling of the paint along some of the cracks. I had used wet & dry to sand back the primer (which didn't bubble) so I assumed that the bubbles were caused by moisture retained in the cracks. I waited a while then tried using just dry sand paper but still got similar results. Strangely after sanding down the bubbled areas the primer appears to have lifted with the basecoat even though that appears to be the layer suffering the bubbling.

So am I not waiting long enough for the moisture to evapourate or will there always be some moisture in the cracks unless I bake out the bonnet for a few days/weeks? This could be an excuse for prolonged evening drives, or is there some strange chemical reaction as well between the primer and the basecoat? Both appear to be acrylic.

The primer is Halfords and the basecoat I got made. I will have to confirm it's actual composition.

[LargeRed]

there is a new (ish) filler for this type of repair. it's a carbon fibre filler. search internet.

[Tempest_5]

Thanks, I'll have a look for that.

I should have added the bubbling seems to occur when the paint gets hot in sunlight or with the engine running before it has fully hardened.

[bobtail4x4]

I would grind it back, fix the cracks and repaint,

you know the panel fits

[Happy Jim]

You are pissing in the wind trying to paint over crazing, sadly there isn't a shortcut on this, either grind out the crazing and fill/sand/paint - or buy a new bonnet

Jim

[magpies]

As you are doing the work yourself then the only outlay is on materials.

Moisture has already penetrated the cracks so you need to remove all traces of them.

Use a 'dremmel' style tool with a round bit and gouge out every crack - cut at least 3mm deep and 5mm wide and at least 10mm passed the end of each crack. Use a chopped glass filler rather than a smooth easy sand filler to as close to but just under the surrounding surface. Finnish with smooth filler. Then spray. It may not be as good as a professional finish but should be good enough.

That should last 4 or 5 years then as the filler has sunk slightly re prepare and paint again.

[gtmdriver]

The other thing you need to do is to reinforce the area under the crazing with more glass fibre and resin. The crazing was caused by the GRP flexing on impact and once this has happened the panel is weaker than it was. It may be that any cracks you repair will re-appear later on unless you restore the original strength to the panel.

[LargeRed]

this is why I would, and did, recommend the CARBON FIBRE filler, see post above. NO under-work required.

see https://www.option1sportscars.co.uk/composite-repa...

Edited by LargeRed on Thursday 4th June 09:00

[LimSlip]

LargeRed said:

this is why I would, and did, recommend the CARBON FIBRE filler, see post above. NO under-work required.

see https://www.option1sportscars.co.uk/composite-repa...

Edited by LargeRed on Thursday 4th June 09:00

Can't see anything about special carbon filler in that link? However it does say "We maintain that a fibreglass panel should be repaired with the materials it was originally made of. "