Vinegar can be used to clean rust from the inside of a fuel tank because it contains acetic acid. This can be accomplished by filling the fuel tank with water and then draining that. Next, plug the hole where the petcock goes, and fill the tank with the vinegar, leaving it in overnight to dissolve the rust. After pouring out the vinegar, rinse the tank with water. Next fill it with water and a small amount of baking soda to neutralize the acid. After 15 minutes, rinse the tank with water again and continue rinsing until the water you pour out of the tank is completely clear. At this point it is important to make sure the tank has no more water in it. This can be accomplished by getting as much water out as possible, adding WD-40 or rubbing alcohol to the tank, and rinsing it out with gasoline.