Personally through a couple years in a body shop I would recommend WATER sanding the original paint with 320 or 500 grit wet/dry 3m sand paper depending on what the condition of the paint is. As long as there are no dents just water sanding the paint should be fine, but if there are dents or any other areas of concern I would fix prime and WATER sand primer with 320 then go back over again with 500 before painting.