Phosphoric acid, diluted, painted on then washed off.
http://www.dudadiesel.com/ but should be available locally. Muriatic (common pool acid) will also work but not as good of a choice. Naval Jelly contains Phosphoric as the active compound.
Disclaimer: This is strong stuff and you should study up on acids before you use it.
NEVER add water to acid, always add acid to water.
10:1 is going to be fine but wear nitrile gloves.
Good stuff to have around, eats off most any rust and converts the rest back to the black oxide configuration which ends up being rock hard and will take any paint your throw at it provided you hit it with a bit of primer/sealer first.
The brake rotors on your car have already been subjected to this stuff mainly because introducing a little phosphate into the surface of an iron/steel has been a common practice for ages since it offers such a high level of protection.
http://en.wikipedia.org/wiki/Iron(III)_oxide
http://en.wikipedia.org/wiki/Iron(II)_oxide