All phosphoric acids are not the same. The safest you can buy is Picklex20. $40 for 1 pt. You might do a search and find it cheaper. That is not expensive because you can do several tractors. Use it as a rust preventative after removing the rust. I do not consider it to be a true rust converter, but it will kill the rust in pits if you can't get to it. You DO NOT wash it off, thus it is not an "acid wash" as described on this forum and elsewhere. Simply put it on clean sheet metal and keep it dry, it will last for months. When ready to paint simply rub off the white residue with a scotchbrite pad and then use wax and grease remover. I have used it on cast once, but I do not recommend that because I think it might get stuck in the pores and lift the paint later. Picklex20 is a relatively mild H3PO4 and Zn combination. I wouldn't use anything else.