Fiberglass repair - for the most part I use fiberglass cloth and polyester resin.
I prefer fiberglass cloth to mat because it is usually easier to work with and results in a stronger repair. Mat does conform to complex shapes better, so it has its use sometimes.
The cheap polyester fiberglass resin sold at auto parts stores and hardware stores will work fine for most repairs. Use as little resin as possible that will fully wet the fabric. Don't use a body filler with the cloth, it is much to thick to wet the cloth and the repair will have no strength. You can also get high dollar epxoy resins, but the polyester will work just fine.
As far as body filler (Bondo) goes, use it only after the structural fiberglass repair to make it look pretty. The filler should not add strength. Use non-reinforced filler and keep it thin for flexibility. I only use the cat hair (fiberglass filled filler) on a rigid piece, such as filling a dent on a tank.
Hey Steve, if you have any fiberglass questions let me know. I've done all too much of it. Elaine even made the bellypan on the MZ from scratch.