Rust is a problem that makes the restoration of an older motorcycle quite difficult, since replacement parts can be nearly impossible to find. In lieu of replacing a rusted tank, restorers will often choose to remove the build-up of rust on the inside, as well as the outside, of the gas tank. While there are many products available to chemically remove the rust from the tank's lining, most require a fair amount of effort to work properly. An alternative method exists, using electricity and an industrial cleaning agent called TSP, or trisodium phosphate, to sweep away rust.
Prepare your gas tank by removing it from the motorcycle and draining any remaining fuel. Allow the tank to air dry for at least an hour. Remove the fuel valve from the tank and seal the outlet with a rubber seal.
Put on a pair of safety goggles and rubber gloves. Carefully prepare your cleaning solution by adding 1/2 cup of TSP to 5 gallons of water in a large plastic tub. Mix the solution slowly with a plastic ladle until the TSP has been absorbed completely.
Unravel your aluminium coat hanger enough to form a coil. The coil should include a short length of hanger wire to attach the battery charger's lead. Make a small hole in the middle of a plastic spray paint cap and route the hanger wire through the hole.
Fill the gas tank completely with the TSP and water solution. Insert the coiled hanger into the gas tank's filler neck until the plastic spray paint cap rests on top of the tank. Ensure that the hanger does not contact the metal fuel tank in any way.
Connect the hanger to the positive (+) lead from your automotive battery charger. Attach the battery charger's negative (-) lead to the body of the gas tank.
Turn on the charger and allow the tank to sit for at least 24 hours. When finished, turn the battery charger off and remove the leads from the fuel tank. Drain the solution and flush out the tank with clean water. Inspect the interior of the tank for any remaining rust. If rust is still present, repeat the process. If the tank is clean, flush the tank again with gasoline to remove any water from the tank before installing on your motorcycle.
You may substitute trisodium phosphate with sodium bicarbonate (baking soda) if you do not feel comfortable working with caustic chemicals.
Wear rubber gloves and safety glasses when working with trisodium phosphate. TSP is a caustic chemical that can cause injury if proper precautions are not taken. Do not allow the wire hanger or the positive (+) battery charger lead to contact the fuel tank to prevent the risk of an electrical short-circuit that could lead to serious injury.