Go to an auto supply house and buy a selection of Scotchbrite pads.
These are used in painting cars as polishing pads.
These are similar to the green pot scrubber pads sold in grocery stores and the brown or gray polishing pads sold in the sand paper department of some hardware stores.
The auto pads come in more and finer grits and they're larger. The green grocery store pads are usually too coarse and scratch.
Select the "grit" that's the closest to the original grained finish of the gun and use the pad to rub off the rust (Yes it's rust, but not bad enough it can't be returned to original condition).
Once the metal is cleaned of all rust, "stroke" the pad in the same direction the original grained finish ran.
This prevents circular scratches where a pad is reversed.
This is how we restore the finish to a standard stainless steel gun, and remove rust, light scratches, or holster wear.
Don't use on a bead blasted area. Some guns have a bead blast finish on slide tops and sights. The only way a bead blasted surface can be restored is by bead blasting it again.
DO NOT use steel wool. It will embed tiny particles into the stainless and it can rust later, doing more damage.