Kursed- sorry to use your case as an example because I understand that your looking for help, but this is a really good reason to not use tank sealer. It's some evil shit.
On the tanks I've re-worked I generally tig weld them with mild steel filler rod, but sometimes I'll use silicon bronze tig rod or oxy-acetylene and brazing rod. And I do alot of pressure testing to chase down any leaks. If the tanks are in good shape the work goes pretty fast, but if they're old, thin, and rusty, it can be a hell of a chore. But when they're done you have a decent leak-free tank with no sealer needed. For me, the worst part of the job is removing old tank liner .If you don't use sealer what do you do if you have a welded custom tank?