the best way i've found to remove all the paint and primer and get to bare metal is using 36 or 40 grit sandpaper and an air powered sander...electric sander will work also
there are composite discs made for use with grinders and you can also get them for use with a drill, for removing rust, even better than heavy sandpaper...but be carreful not to stay in one place too long because they heat up FAST and could distort the metal
i don't like to use the composite discs on paint because of the heat factor...it has a tendency to heat the paint, roll it off the metal, and clog the disc...
i get to bare metal with the sandpaper then use the composite disc to hit the rusty spots...then use a wax & grease removing metal cleaner to wipe the dust off then hit the panel with some rust inhibitor/reformer and then use primer