ORIGINAL: Rodney

A foolproof method (but time consuming) is to paint the inside area with an epoxy paint such as KlassKote. Do the painting as you build and before your cover. You may have to do some of the painting even before you finish building, paint those areas that will become inaccessible with a brush before you enclose them. Also, provide a drain hole at the lowest point in the fuselage in the tank compartment to allow that inevitable accident (if you plane lives a long time) where the tank either breaks, leaks, a fuel line comes off and fills the area with fuel.