A rusty gas tank is repaired by draining the gas and removing the tank from the engine, and eliminating the rust with either abrasives or acids. The cleaned tank is coated with a commercial product made to protect the inside of the tank. Some coating kits are complete with the chemicals needed to clean the tank of rust.
Abrasives sloshed around the tank to knock off rust are nuts and bolts, gravel, washers, ice or a section of chain tied to a piece of fishing line for easy removal. Acids used to clean the gas tank of rust include hydrochloric acid, phosphoric acid, vinegar or vinegar mixed with salt, cola, oxalic acid and certain brands of toilet cleaner. The acid soaks in the tank for a length of time dependent on the type of acid used. It is drained, and the tank is both flushed with water and rinsed with water and baking soda to neutralize any remaining acid. Finally, the tank is rinsed with rubbing alcohol or gas to remove any remaining water before the commercial coating is applied.
The gas tank's petcock is removed before using acid to avoid damaging the aluminum and rubber. All work on a gas tank is performed in a well-ventilated area, far from combustible materials.