mopguy wrote: Click on the ARTICLES icon (Riding and Wrenching) above, and then click on technical tips, now look for the how to clean Rusty Tank articles.
Just read through this article and strongly disagree with the idea of using muriatic (HCl) acid. The tank sealing kits you can purchase include phosphoric acid, which is much safer and doesn't need to be neutralized after derusting.
Home Depot sells phosphoric acid in gallon jugs in the Paint department. Etch and Prep it's called. Plug the tank openings, after removing things like the petcock and fuel gauge or similar, and dump in the Etch and Prep. Rotate and shake the tank every 10 mins or so and the tank will be rust free in an hour or so. Recycle the sauce for reuse and rinse the tank with clean water. There will be some minor flash rust while the tank is drying so consider some WD-40 inside to reduce the impact. Blow dry the inside of the tank with compressed air or a heat gun to avoid rust reformation.
If you need to line the tank get Por-15. They sell a complete kit which makes the project easy. They suggest that the rust doesn't need to be fully removed but don't believe them. Rust is like cancer and needs to be removed for best long term success. If the metal in the tank is thin, or if you have rust through spots, get Caswell's tank sealing epoxy. This stuff is uber thick and will plug holes better than the Por-15 stuff. Whatever you do stay away from Kreem. That stuff is junk. I've stripped out old tank liners and the Kreem is attacked by chemicals MUCH faster than Por-15's sealer. It's best to avoid sealers all together if possible, but go with Por-15's or Caswell's if you have no other choice.
Good luck and hope this helps.