Wash the gas tank out with soapy water and remove all valves, sending units, petcocks and internal filters. Stop all openings except the fill spout, and do not allow the tank to dry out between sealing sessions.
Mix a metal wash with 2 gallons of water and pour the mixture into the tank. Slosh the mixture around until all fuel varnish is removed and then pour it out. Pour in 1/2 gallon of muriatic acid, and slosh it around until the surface has a gray appearance. Pour out the contents into a plastic container, and then slowly add baking soda to stop the fizzing. Rinse and drain the tank once more.
Pour a bottle of rust remover into the now-empty tank and then pour it back out again. Pour in 1 quart of acetone while the tank is still wet from the rust remover, and slosh it around. Pour the acetone back into its original container, and then repeat the process again with fresh acetone. Shake the gas tank sealer bottle thoroughly before use, and then pour its contents into the tank, making sure that the entire inner surface of the tank is coated with the sealer. Let the tank stand for 10 minutes with either the fill neck or sending unit open.
Close the openings, and rotate the tank so that the gas tank sealer recoats the inner surface and then let the gas tank sit for another 10 minutes with the fill neck or sending unit open on the opposite side from the first time. Remove all the stops, and insert a compressor air line with the pressure set at 1 to 6 pounds per square inch. Keep the air line 2 inches or more away from the coated surface; it should be dry within 48 hours.