I don't recommend using dope to fuel-proof if one is using greater than 5% nitro. You can get by with 10% nitro, if you wipe it off of the surface right away. I learned this the hard way.

Like Tony, I used dope to fuel-proof balsa & firewalls back when I was a kid. When I returned to the hobby after many years, I doped up a firewall just like I did as a kid. After twenty flights, though, I had a fuel soaked mess. That's when I figured out the nitro limits of dope (by testing). So why did it work back when I was a kid? It dawned on me that we kids could not afford fuels with greater than 5% nitro, and the hobby shop rarely stocked the high nitro stuff!