I've been using Caswells Epoxy Sealer on several fuel tanks since I did my '84 tank a few years ago. I'm happy with the results and the epoxy does an excellent job of bonding to the interior of the tank rusted or not. It's tough stuff.
http://www.caswellplating.com/restoration-aids/epoxy-gas-tank-sealer.html
I just got an updates from Caswells on a new product, another epoxy sealer, but with a new feature. They have added a red color to the product so you can see the coverage on the inside of the tank, (for what you can see without an inspection camera). It's the same quality product from the original kit so I wouldn't be afraid to use it for any major repair.
http://www.caswellplating.com/restoration-aids/epoxy-gas-tank-sealer/dragons-blood-gas-tank-sealer.html?utm_source=MailingList&utm_medium=email&utm_campaign=May2012
So it's worth a look if you're in the market for a tank sealer.
Mike