I haven't tried it on my bike, but Bar Keepers Friend powdered cleaner works really well to take surface rust off stuff around the house. (Probably because of the oxalic acid in it.) Put some on a damp sponge, wet it down to a paste, rub it on the rust spot gently for a few seconds, and voila. Wipe down the spot with a wet paper towel or rinse to clean off the BKF, then dry.

Target and a number of other stores carry the stuff:
https://www.target.com/p/bar-keepers...z/-/A-13304297