RE: Polyurethane?
I've used it on a couple of planes as a clear coat. It is not completely fuel proof, but it is fairly resistant. If you clean any fuel off within about 2 to 3 hours, it'll be fine. I would still go with epoxy for the engine bay/firewall. I've used both the Minwax and the Diamond Spar Urethane. I found that the spray cans can leave a bit of a bumpy texture. for a good smooth surface, I use a cheap harbor freight hvlp detail gun.
Soctt