I use polyurethane for fuel proofing, but it's not fuel-proof. It's exhaust proof, but raw fuel will eat it up. But that's not a problem because I don't pour raw fuel on my planes.

If a tank breaks, I know about it soon enough that I can clean it up before it breaks through the polyurethane. However, if you want something that is truly fuel-proof, use fiberglass resin, epoxy or epoxy paint.