Phosphoric acid works best on rust. Sulphuric is too strong and acetic is too weak. When the acid is done its work, neutralize the acid with three tablespoons of baking soda to 5 gallons of water. To remove the water from the tank and add a dryer, like acetone. Enough acetone is needed to coat the inside - i.e. about 1 pint. Water dissolves entirely into the acetone. To neutralize/dissolve the acetone, pour out the excess and add gasoline. Acetone also stabilizes gasohol gasolines for an added benefit.
