brett-bike,
Use some metal filled epoxy on the leaks (JB Weld) and seal the tank with a POR15 kit and you shouldn't have future problems if you get a good seal with the POR15. Just be careful to get the tank bone dry and take your time.
Heat the tank up and let it dry for a while... a dry compressed air to help get it good and dry...
remove any scale either mechanically with nuts and bolts or electrolysis method after nuts and bolts and a good acid wash with POR15's metal prep. Definitely bone dry before coating the tank with the POR.
Otherwise the POR, a CyA adhesive type formula, will flash cure and that's a really bad thing as it will trap that moisture and rot even worse.
The POR15 works well and can be trusted to last if you follow the directions.
Otherwise you are going to need to replace the offending metal or replace the tank.
Going the JB Weld and POR15 route will cost you time and a few $ for the POR kit.
Alternatively, Eastwood company's kit could be used.
When you rinse the tank some 90% alcohol will combine with any water to help get any water out of the tank.
Provided it is safe for the etch solution and won't attack it...
The eastwood company kit uses muratic (brick) acid for the metal cleaner.
Need to start with a degreaser to remove any oil residue though...
Then move to the rust converter and etch and finally the coating.
The Eastwood kit would work or you could go with the POR-15 company kit.
Eastwood's kit is $34 and the POR15 kit is generally in $40 price bracket.
The POR15 kit includes some patching material for leaky tanks...
David