anyplace your scuffing, use a grey scotch and the scuffing goo that comes in a tube. wonderfull results
anyplace thats scratched / has alot of orange peel / is going to get abused by road debris use 400 or 600 grit sand paper
no reason to reprime, paint is a great subsurface for more paint
IN SOME AREAS- depending on how many coats of paint are already on the car - it might not be a bad idea to off a few coats of paint. like the back edges of the doors or other areas that are easy to chip. throw some 120 grit on a DA and take off a few layers (not to bare metal) and then resand with 400 wet
thicker paint chips easier, which is why (for exapmle) extra coats of clear is a waste of money. the thicker the paint the deper and wider the rock chips will eventualy be