How to clean a rusty gas/petrol tank
The best way to clean a rusty gas tank is with muriatic acid. You don't want to get it on you, so wear some gloves and a long-sleeved shirt. You also don't want to be breathing it, so do this OUTSIDE. Don't get it on any chrome/alloy pieces as it will eat them. That probably sounds worse than it is - it's an easy job - but read the label when you pick some up from a hardware store.
- Drain the tank, remove from the bike, and remove the fuel tap and cap.
- Clean the inside as much as you can with a wire brush. (And/or use a handfull of old nuts and bolts. Put in 1/2 cup gas and shake repeatedly.) Wash out with gas.
- Plug the bottom of the tank with rubber or a big ball of duct tape. Fill 1/2 way with hot water. Add 1 liter of muriatic acid, then fill to the top with hot water. Don't use the gas cap as the acid will eat the chrome. If you fill less, occasionally plug the top and turn over to get everywhere. Leave it for a few hours.
- Drain the tank, depositing the liquid with a local chemical depot if you have one. Rinse the tank out with water a few times. Clean out any remaining/semi-loose bits. Rinse out with gas.
- Add fuel tap, install, and fill. Don't leave it sitting or it will develop some surface rust.