400-600?? Your crazy.....
800-1000, Minimum. Unless your going for that "scratchy,,,, I dont know what the hell Im doing" Look.
On aluminum, You can get away with a scuff pad even... Use actual wet/dry sand paper if you want to feather edge the surrounding paint to match. If your just doing a quick touch-up, hit it lightly with a scuff pad. Use some dish soap and hot water with the scuff pad, focus on cleaning the area as much as "sanding". Several light coats at first..3-4. Follow up with a thick "final" coat.... And you should be good to go!