For anyone interested in learning how not to do it...
I wanted to make a body panel to fill in the hole in the front of my car. I wanted to do a foam core fiberglass build. I did some quick research and determined (wrongly) that the foam I had would remain structurally sound when in contact with poly filler and resins..
So I went to work. First making a backing plate of cardboard and tape.
Then I filled up the empty place with the foam.
Then carved and sanded to get the new panels shape.
I filled in the air bubble holes in the foam and fill out low areas.
After lots of Filling and sanding I got the panel to where I wanted it.
So I removed it and shaved up the backside, taking care to maintain the mounting points. Then I got everything lined up again and drilled the fastener holes.
I felt that now was the time to fiberglass the backside to give it some rigidity. Then I would do any final adjustments to the front and then fiberglass the front.
When I came back the next morning the panel looked like a potato chip as a result if its reaction with the resin. I was heartbroken, but I don't have time to redo it right now, so I decided to just make it work until I get some more time to try again with a better plan.
Lots more filler and shaping. Not as nice as the first time, but It will be close enough.. I made sure there will be no foam exposed to the resin and then I laid fiberglass on the face.
Lots of work and plenty of fail, but it will do the job for now...
I hope yours turns out better.