It will not make any difference what you do on the outside. You have a
problem that MUST be solved from the inside of the tank. When diesel
gets past the surface barrier it can migrate along the glass fiber for
several feet before it shows up outside the tank. Once the diesel gets
into the layup it will find a way out. That is the main reason why
integral fuel tanks are discouraged.

Drew has the right idea for a sloshing or brushable sealing compound for
the inside. It is NOT easy to use but that is going to be your only
solution.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com