I successfully repaired my right wing tank (aluminum) some years back externally. It was leaking at the tank seam when I got it in 2004, right there below the gas gauge. I cleaned the surface, covered it with tank repair putty (two-part looks like a tootsie roll) and then covered that with a layer of fiberglass and two-part resin for gas tank repair. BTW, while leaking, with wings folded, the gas ran down the wing to the second flap hinge and into the flaperon, dissolving the Styrofoam core within. I got it that way.

I have read most of the threads about sloshing tanks. Curiously, they gravitate towards Kreem as the preferred slosh, even though it is the only one that is known to flake and peel away. (Honorable mentions are Por 15, Caswell, and the plastic replacement tank from AirDale.) Mine has aluminum tanks that were coated with Kreem. When I noticed that failing about four years ago I sloshed it with MEK to remove most of it. More recently I noticed my left tank seeping, but only with a full tank and I conclude that the source is at the top of the tank. I was going to rip into the fabric to do an external repair, but have decided first to re-slosh the inside. Why should I use Kreem?