I agree with Vettestr; I make my own epoxy mud and I use 3M Micro Balloons and System Three Epoxy Resin, I like System Three the best because it is the most flexible; you will have a choice of hardener slow , med , fast; I like slow or medium. I keep my epoxy resins in a old refrigerator because it will not go bad or solidify. When I do any job and have some extra mud in the mixing cup I put it in the refrigerator because it will slow down the curing process and you can add to the project. I've been doing custom fiber glass work for twenty eight years and I never ever use BONDO. I also use Interlux 3000 epoxy primer marine primer; it will not reject any paint of any kind and sands like a dream and you can really build it up. I also have resins custom made to my specs. and some cost $950.00 a gallon. This link has the best products on the market.
www.fiberglasssupply.com
Check out the Divinycell.