RUST-OLEUM has a automotive primer that I like. It is a spray can, gray in color, and their #2081. It dries very fast, sands easy, and allows wet sanding as well. It will fill minor scratches with several coats. I highly recommend always having a scrap to test spray, this allows you to spray the top coat on the test piece to assure compatibility.
One thing I have learned is to read the label on rattle spray cans for RECOAT instructions. Many colors state, MUST SPRAY ADDITIONAL COATS WITHING AN HOUR, OR AFTER 24-48 HOURS. Here again a test piece is needed. I try to buy only spray paints that the RECOAT instructions state, RECOAT ANYTIME.