- whatever you do to it will depend on what condition its is now-any rust will need to be removed...i have friends that do this with diluted lemon juice, believe it or not, phosphoric acid is a good rust remover-just make sure you flush it out well...what i do is strap the tank over the mouth of a concrete mixer, put some small washers and nuts into it with some diluted phosph acid and switch her on for a couple of hours