One method that worked well for me on the rear bumper on my 2000 Silverado was to use Diet Coke (no sugar to make things sticky) and wadded up aluminum foil dipped in it. I took the rust (mainly mild pitting) off quickly and the rust did not start reappearing for about 2 years.
I had treated and painted the back surface (the side facing forward) of the bumper as well at that time.
