You can purchase etch primer in a spray can to cover the bare metal for small areas such as you mentioned. After wet sanding wash it off with clean water and then blow dry. After sanding and washing try not to touch the surface with your bare hands before painting as oils from your skin may cause fisheyes. Use tack cloth just before beginning to spray. A clean well prepared surface is the key to a good finish as the paint seems to magnify any imperfections.
