as buckshot said, phosphoric acid.
Converts rust (red oxide) to black sumfin (VERY hard stuff).
Must be wire brushed (at least a bit) beforehand tho, as you usually wont get off the black stuff off anymore...
If you get the acid somewhere on the car: you're fooked
If it gets on the ground dilute it with water (must look up how much water if interested)
I use it on old tools, as I can put them in a small plate with ph. acid (and hydrogen peroxide, makes it a lot faster. But you have to breathe in only once to never worry about anything anymore again)
After treatment wash off with water, wire brush and coat immediately. I've never been fast enough to coat it after washing, as pure metal/steel on fresh air becomes rusty in a few minutes (light surface rust).
That's the best treatment.
I'm experimenting with zinc spray at the moment, not sure if I'm happy with it tho.
I tried waxoyl on me MG Midget once. Stopped the rust on the outside. Poked me finger thru the wing later tho... Looked fine on the outside!
