My local Advance Auto Parts sells a tank sealer and tank cleaner.
I clean my tanks with a double handful of clean rocks and about a gallon and a half of fresh gas.. shake the hell out of it for about a half an hour and dump it all out.. the rocks knock loose most of the rust.. but if your tank is in real rusty bad shape, you'll probably want to follow that procedure up with the 'proper' tank cleaning kit.
I've been told that Muriatic acid works great for cleaning gas tanks.. and should be available at any swimming pool supply company.. but word around the campfire is that stuff just etches the metal and doesn't inhibit 'new' rusting.
Phosphoric acid is supposed to 'convert' the rust back to metal, and then leaves a coating on the 'unrusted' metal that inhibits future rusting.
Here\'s a site with some information . While it's a motorbike site, the principles are still the same.
