I use latex almost exclusively when painting.

I have found with latex (and most other paints also) that if you let the paint "cure" (also referred to as gassing off), 2 sprayed coats of a water base polyurethane is definitely good to 10% nitro. All I use is 10%, however, I have read several reports that people using 15% have had no problems. Latex takes an average of 7 - 10 days to cure, however you would need to check to find out how long enamel takes to gas off. A couple of pluses to the water base poly are no fumes, does not yellow with age and soap an water clean up.

You can also use Ultracote clear or LusterKote clear (I, personally, do not like the way LusterKote sprays - too much "spitting" )

You didn't mention what type of enamel you used. If you used an ACRYLIC enamel/lacquer it is fuel proof. If it doesn't say acrylic on the can, it most likely is not.

FWIW - Dupli-Color brand Truck & Van paint is USUALLY an acrylic. It will be labeled on the back of the can. 11oz spray cans are available at Wal-Mart for about $4.00 The color selection is limited though.