the surface rust i have faced, assuming its just surface, got scrubbed off with steel wool and soapy water then when dry, primed to prevent it from coming back. if there were dimples from it, they were filled sanded then primed. elbow grease doesn't cost near as much as the "special" formulas out there. as was said above, if its removed completely it wont come back. and i totally agree to make sure what ever you use is compatible with what ever paint you'll be putting over it. nothing sucks worse then to watch your new paint job bubble and flake off because you used the wrong primer. best way to get rid of it is by going back to bare steel and starting from there.