the best way is to use a small sandblaster like a gilmore or sandy-jet they don't warp the metal blast the area untill it is flat grey in color this removes all the rust, if you leave any rust at all it will return with a vengence then treat the area with a metal prep, an acid based solvent that etches the metal wash thoroughly,dry, sand and prime then use a polyester type putty to fill the pits, smooth then prime again. Now there are products out there that convert rust into something else like "rust mort" or " sem rust seal" they will do the job but I don't know if they are permanent
