Next, clean the surface with solvent (avoid touching it after) and mask the area (the tape should bisect the area on the existing, sanded paint). Then, using a good brush that won't shed, apply an aluminum-compatible primer, preferably from the same manufacturer as the paint you intend to use. Follow the directions, which may call for sanding between primer and topcoat. Then apply the topcoat; use either a high-quality brush that's designed for the type of paint you're using or a one-time spray bottle, such as those from Preval. If you use the latter, masking will be much more important and will need to cover a larger area; never attempt this job on a breezy day. Depending on coverage, several coats may be required.