I've restored several older bikes (early 70's) in the last couple of years that needed the tanks cleaned and sealed. There is a company on the web that you can get a great kit that cleans, preps, and then seals the tank, even if it has small pinhole leaks. Go to this link,
CYCLE TANK REPAIR KIT-POR-15 Inc. . As Bunky said above, the company is Por-15, and they specialize in chemicals for this type of applications. This link takes you to there cycle tank repair kit.
Good Luck,
GeorgiaRider