Ah, you got to love the Internet! Post a simple question, and within minutes you have a dozen mostly contradictory answers.

I'm one of those opposed to thinning epoxy. First, I tried it and it was a gooey mess (yes, I know others have a different experience). Second, the manufacturers recommend against it (but what do they know?).

I used to use epoxy resin exclusively. It seemed like the right stuff for the job. I had a big can, and it lasted for years. But I "loaned" the can to somebody, and it never came back, so I'm now in the same camp as Wayne Miller. Polyester resin is much cheaper than epoxy resin, and people say it's just as fuel proof.

The main problem with most epoxy (and some epoxy resins, I found out), is they don't like to be mixed in big quantities. Say you mix up 2 oz or so, and you're happily glassing the wing joint, and you look over, and the epoxy is smoking. The heck? You pick it up, burn your fingers, and realize the whole mixing tub is about 5 seconds away from being 1 super-hot solid mass of hardened epoxy. Trust me on this one, you're going to have a crappy glassing job. Use the right material for the job, I say.