Leaking wing tank

So, in the ongoing process of slowly making my poor previously abused 'coupe airworthy, my mechanic noted leakage from the left wing tank (old style 9 gal aluminum with groove on back for gear leg), removed it, found a crack, welded it, and re-installed after overnight leak test (not pressurized). Now there's dampness on the underside of the wing again 3 weeks later. Vernon at Precision Services says he might have one ($575) but added that attempts to weld the older tanks usually results in more leaks occurring adjacent to the repaired area. We're gonna pull the tank again and have a look but I have a bad feeling and think I'll end up replacing it or at least sending it to a professional shop for professional work. Anyone have one lying around or more advice/experience on the subject?

Thanks,
Chris
Sioux Falls, SD