Painting the underside of Fibreglass
 

I see a lot of builders do this.

I am guessing they use something like an underbody seal, something that remains a little flexible and durable against things like stonechips.

Am I right?

Helps against cracking?

Anyway, what is the best thing to use?

And where from?

the "pros" who painted my car used underseal under the front grill and lower body edges behind the wheels, but not in the wheel arches (single skinned front!), but apparently a basic flat black elsewhere. Would've been a good idea to paint all reverse sides with an underseal like material cos star crazing looks UGLY, and amateurish

very late reply.... but I used a simple satin black on my Ultima.... main reason to make it look more finished but also to seal in the fibreglass smell that will linger otherwise.

A rubber stone chip in the arches is an extremelly good idea if the pannel is not heavily reinforced in this area.

I used the stuff CBS sell in the front wheel arch area on my Corse. Painted over it afterward with some tyrewall black to match the rest of the underside which was done in satin black. Like VOJX, my 'Pro' painters did nothing under there other than a thin coat of body schutz on the bit that was double skinned. No way that would have been up to the task of keeping the stones at bay. The Marcos we had years ago had lots of nasty star crazing at the back when we got it. Nothing had been done to protect the inner wheel arches.

Spray on Bedliner!
 

In the US people have started using spray on truck bed liner. It is urethane and can be mixed in different colors. It doesn't matter how thick you spray it. Most put it on about 1/8" (3mm). It really helps with sound proofing too. Lots of the "mini truckers" use it on the interior metal to keep the vibration down. They spray it, let it cure then reinstall the interior.

Bill Gullatt
Harvest, AL - USA

The one I have heard about is Wunderseal flexible stone resistant coating

http://www.nfauto.co.uk/wunderseal.htm

Just one source Try the trade stands at stoneleigh if your going

Edited to say I dont work here Just the first hit When I googled "wunderseal"