I had parts of my engine case polished and buffed to a mirror finish last December, Took me at least three days worth of time, some 1200 and 2000 grit sandpaper, metal polish, aluminum brightener, water, and lots of patience. Actually, polishing it up to a good shine is easy; maintaining the shine and polish is really the tough part.
I'd stop that rust before it starts doing damage to the frame, engine and bolts.