If you require filler on your ride, try to stay away from polyester resin fillers. "bondo", and most auto part store variety fiberglass kits use polyester resins. I use thixotropic high temperature stable epoxy resins on my repairs. Hysol EA9396 and EA9394 are my favorites. They are quite expensive, but do not hold moisture, and have peel strength like you wouldn't believe. This guarantees that the filler will NEVER loosen, curl, or even sound like plastic when you "tap-test" it. If there are more affordable products of this nature, please chime in. I know a friend used to use a product called Marine tech, or something similar, that he used on his sail boat that was also impervious to mositure.
Additonaly, polyester resins are succeptable to softening by modern gasolines.