If you need to buff off some rust off older bikes, try this.

If you've got an electric drill, take off the drill portion. Get some steel wool and wrap it around the spinning part of the drill. Leave some space so tape that you'll be wrapping around touched some of the actual drill. Cleans off rust like magic with minimal work. You can also expect less scratch marks if you use finer steel wool with this method rather than coarser steel wool by hand. Same time used, but less scratches.

However, on more expensive bikes or new parts, this isn't recommended.