mold release between fiberglass and bondo

I'm creating a mold for a custom interior piece.  It'll be similar to the original but had to be stretched.  I made a prototype by cutting two originals at different locations and joining them.  My seams are pretty good but not nearly good enough.  The piece was originally plastic and I figure I'll have to use bondo or some other type of filler to smooth out the seams and imperfections.  What makes a good mold release between these surfaces and the fiberglass?  I've heard some cooking sprays work on plastic to fiberglass but don't know about bondo to fiberglass.  I appreciate any help you can provide.

if you want you can use a can a spray paint if you dont need the original part, use a high gloss paint and when it is hard use mold release or reg car wax works too.

The surface of the mold should be sealed first especially if it is pourous. Paint, shellac, and polyurethane (for wooden molds) are all good sealers. For the release agent I've used carnuba wax. I made a reverse mold out of fiberglass for fiberglass with no release problems at all. I have also heard that PVA was a very good release agent for but could not locate any locally. I have seen gallon jugs of it on the internet though.