The first step is to assess what is under the paint, primer, sealer or whatever might be on the body when you start. A piece of 80-grit strapped to the D/A will be the best way to find out. Also, when using a D/A make sure to keep it flat to the surface and let the sander do the work; you won't need to push down very hard. What you are trying to do is feather out scratches and chips to see what is underneath. If the fender is new and has the black E-coat, then all you need to do is scuff with 150, since we are working on old stuff we didn't get 150.