I used an abrasive rotary brush (and a wire brush on the dremel for the finer touches) on the truck we recently got. I cleaned everything thoroughly with alcohol, masked it, sprayed it with primer, sanded, and re-primed. Because the area is hidden by a tool box, I probably won't ever get around to finish paint, but you can get color match in a rattle can from most manufacturers along with an appropriate clear coat.
For small areas (lady bug size or smaller), be super neat with the wire brush on your dremel, remove ALL traces of rust and dribble touch up paint in with the fine point of a toothpick. If there's any loose paint around the rust, you'll want to go back to the steps in the first paragraph. If you want moral support, I'm pretty good at drinking beer and not stepping on your head while I watch.