Is Polyurethane fuel-proof enough for firewalls? I will be running up to 20% nitro. I have both water-based and oil-based Minwax.

A good polyurethane varnish will be as fuel-proof as epoxy resin (if not more ?).
I only use G4 polyurethane varnish to protect my airframes. This is the best, by far.

http://www.soloplast.fr/generale.htm

Minwax water based is not fuel proof, it turns to goo after a while...

I am assuming you mean you want to use poly glue for fuel proofing, not attaching the firewall to the plane. I would really only suggest using a good 30 minute epoxy for attaching the firewall to the fuse, and the angle braces behind as well. But, for fuel proofing the firewall face, then yes, poly should work just fine.
Although one little trick I do, which really helps reinforce the firewall, is I like to use epoxy on the face as well, and add on a layer of 0.75 oz glass cloth covering the whole firewall and wrapping around the edges with about an inch of extra glass helping to reinforce it to the fuse. When done around the whole firewall, the lightweight 0.75 cloth really adds alot of strength, but is still very light. I bet it doesn't even add 0.5 ozs when done right. Just remember, squeegie away excess epoxy with an old credit card, as it just isn't needed.

Patriot