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.

I was just looking at Klass Kote white primer on Klass Kote's website (never used the stuff but have known about it) when you replied. I am thinking of ordering some up. I like that it can be reduced for airbrushing or even simply brushed on.
Thanks for the suggestions, Todd.

No reason not to use the laminating epoxy. It will seal the wood as well as anything else and is already thin enough to brush on and soak in. Weight gain is minimal (no worse than fiberglassing your bird) and its where most planes need the weight, in the nose. 30-minute epoxy thinned 50% with acetone is just as good, and quicker drying.

Thanks, I think I will go that route as I have it on hand.

So, I'm going to use thinned epoxy. .. my question, do I paint it before or after epoxy?

Epoxy first. Or use an 2-part epoxy paint instead or any fuel proof paint for that matter. You can even add color to the plain epoxy although it won't end up matching any other color on the plane.

As has been mentioned I use polyester resin, as it is pretty thin, soaks in well, and I normally have some around unlike epoxy. I use it for hot rods.