Depends on where...

Epoxy (I use 2 hour, which goes on thin and soaks into the wood very well.) for fuelproofing firewalls is a great idea. The minor weight penalty of the heavy epoxy isn't a big deal due to the small area. Its hard to get a better fuel barrier than the epoxy.

You can use butyrate Dope for fuel proofing some areas where weight saving is important. You can also use Lusterkote or several other paints. (check compatibility with your covering and be careful of solvents whenyou have foam cores or plastic parts...)