If the part is not peeling or anything like that, just grab a red scotchbrite, a bucket of water and some Ajax or Comet. Wet the scotchbrite, put some Comet and scrub until no shiny spots are left. The comet is to help in the sanding process. After you are done, you should have a even dull finish. Rinse and wax and grease, tack rag then shoot.
If the part is severely chipped or peeling, it will need to be primed. If this is the case, back up, turn around and call the nearest proffesional.