Thining epoxy with alchohol can lead to problems and entirely unecessary. Just use 30 minute or longer cure time epoxy and put it on as thin as you can. Usually a stiff bristled dissposable brush works well. After putting the epoxy on the area, heat it gently with your heat gun. It will flow out very nicely and wick into all the minor crevices and open grain. The heat will also speed up the cure. Not only will this fuel proof the area, it will strengthen the bond between firewall and fuselage which is often needed in some of the ARFs. Using alchohol to thin epoxy will sooner or later leave you with an uncureable rubbery sticky mess that will be very difficult to clean up or repair. This is because alchohol will absorb water (and worst of all is the medicinal alchohol you get at the local pharmacy which already has a high water content) which gets trapped and prevents the chemical curring of the epoxy. If you must use alchohol (some do and often get away with it) be sure it is fresh denatured purchased at the local paint or hardware store.