If the spots aren't too bad I'd use something like naval jelly to dissolve the rust first then work with it. It's the same stuff used on rust spots on many ships before they're repainted. It will leave the metal sort of black, and it won't be nice and smooth since the rust tends to pit the metal unevenly. It shouldn't damage the metal at all, and you may be able to get away without nearly as much sanding that way. Plus you can see what's left unrusted before you start sloughing off metal sanding it.
Brad