I'm currently working on a tractor tank. I was impressed with electrolysis using my batter charger and washing soda. The tank is now sitting in my garage filled with 10 gals of vinegar for the final rinse.
That's a method that would remove all the active rust. Once the active rust is gone and if the tank is in good shape I don't see the need to coat it. Steel gas tank are not coated and are just bare steel on the inside.
If the rust had caused substantial pitting then coating it to fill in the damage and help prevent leaks would be a good idea but otherwise I'd be hesitant to put anything foreign in a tank.
Here's a link to a simple battery charger rust remover ...
http://antique-engines.com/electrol.asp
