You might want to consider using this stuff:
Located at
Gas Tank Sealer - Large Motorcycle Tank Sealer Kit - Better then Kreem - KBS Coatings
This kit contains AquaKlean (a water-based, heavy-duty, cleaner/degreaser) and KBS RustBlast (a powerful rust remover/metal etch) to obtain a permanently sealed, rust-free fuel tank. These KBS prep products are formulated with a unique Oxygen-Block Technology which reduces the occurrence of flash rust by over 80% and extends rust-free storage times by leaving a temporary protective coating in addition to a zinc phosphate film.
There are others but I have personally used this kit and have not looked back. If you follow the directions the inside will look like new. It did not hurt my paint and is much safer than the acid. That acid is nasty stuff and would only use as a last resort.