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
(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.