Rust Master, you are referring to phosphoric acid. Turns the rust into iron phosphate. It comes in various concentrations and a few auto body suppliers still stock it. There are safer products out there though.
A word of caution that if you do decide to use it, you have to be real careful with it. Acid resistant gloves and an air purifying respirator rated for acid fumes is a must. Along with good ventilation.
And be sure to neutralize it with a baking soda and a clear water wash. Any left over acid or soda can play havoc with anything you use to coat over it.
I've also heard of some navy guys that swear by Naval Jelly. I tried it a long time ago. It is basically the same product, but in a gel form. In my opinion it makes more of a mess.
