the stuff called ospho is sold in paint and harware stores it acid mixed already like 5 bucks a bottle its blue liquid i put it in a small spray bottle but you need to have the door striped or blasted clean for it to work good same with any acid and spay it with a spray bottle it will bubble up and will get a white powder and black scales after this you will need to use some kind of metal clean before primering but if you leave it in the air rust will start returning but if you have it in a shop you can get it sanded up pretty good and clean before primering
