What type of rust is it? Is it just surface rust under and around fading paint? Does it have deep pits, or holes through?
80 Grit on a DA is a good place to start. Especially if you have the old school style that has a (thing?) on the shaft that allows you to lock it, and turn it into a grinder.
The only problem with 80 on a DA, or a soft backed Grinder Pad, is when you get into the middle of large panels. Panels like Roofs, and especially the middle of Hoods, are really vulnerable to warpage from heat. 80 strips paint, and surface rust very well but generates alot of heat. Just be sure to keep the tool moving over the work area, and dont keep it in one spot too long.
Also be ready with Zinc rich primer, or Epoxy.