The last time I did some fibreglassing (to blend in my bonnet vents), I used a filler specially designed for fibreglass. It had fibre matting strands in the filler itself. It worked a treat and didn`t crack until my nephew decided to sit on the car bonnet :mad: (the little shit...).
Chop shop was the usual madness, lots of swearing and messing about...LOL...
Basically they were just using a Bio-diesel set up, you can use any type of used cooking oil for this, but some oils are better than others. I bet the car must have smelled gorgeous driving down the road ;) .
Regards,
Ian.

lol, yeah :illboy

LOL.... I bet used chip shop oil smells good though ;) .
Regards.
Ian.