I had a friend that had a new KX250 and a friend of his sold him a new FMF pipe,well the pipe was so close to the tank it melted a hole in it (turns out we were at Washougal national MX race so FMF looked at it, told us the pipe had the mounts welded in a jig that was out of alighnment and gave him a new pipe but wouldnt fix the tank.Turned out his "friend" sold the pipe because it had melted a hole in his tank but didnt mention it untill after the fact,nice!)My father inlaw owned a body shop and gave me some epoxy(dont know the name of it,was back in'95)after my buddy had tried every thing to fix that hole,well the epoxy I got held for years untill he sold the bike and still is as far as I know.So you may want to try a body shop or look for some epoxy that works because some where out there there's some that works.Ty-Davis had a custom aluminum tank on his KX250 years ago,but I hear youll always have leaks popping up no matter how good the welds are,let us know how it goes and sorry I couldnt give a brand name for the epoxy.