I agree with the other guys. I have never heard of applying epoxy, thinned or otherwise, over a painted surface on a plane, except the firewall area. You can get away with this area because it really needs heavy-duty fuelproofing, and it's not a large area and won't really adversely affect overall weight or balance.
If you've used Monokote on the rest of the plane, I'd strongly recommend using it on the tail surfaces as well. It is fuel proof by itself, way lighter than any paint system you could apply as a beginner, and as long as you take reasonable care to clean the day's oil accumulation out of the hinge areas and along seams, it will stay looking good and stuck securely for a long time.
What kind of paint were you planning to use? Is it enamel, polyurethane, lacquer, or what? All of these are already fuelproof by themselves. Enamel is not as resistant because it dries kind of soft. IMHO, auto parts store spray cans are not the way to go for glow powered planes anyway. If you HAVE to use them, a coat of clear polyurethane would work better and be lighter as a fuel proofer coat.
Stay with the Monokote. Anything else would be screwing up majorly.

Rick