This may be an old thread but the use of 30 minute or slower epoxy (no thinning with alcohol or ??) spread on and heated with a heat gun just to point where you see it suddenly thin out and get into every nook and cranny or crack is hard to beat. An excellent second way is to paint the area with epoxy paint (think Klass Kote or similar). I recommend a light color, even white, as it makes the routing of fuel lines etc. much more visible. Another good tip, put a drain hole in the fuselage at the lowest point under the fuel tank to prevent any sudden leak or rupture to spill fuel all the way to the back of the inside of the fuselage.