For pirmer, I like the polyester based sandable stuff. Uses liquid hardner just loike Bondo, builds great, goes on smooth and can be block sanded to a grat base for color. I would avoid the rattle can primers. Many are enamel based and don't sand well, some are HOT lacquer based and burn enamel. Duplicolor is an exception, very good spot primer. Just better to squirt it on w/ a spray gun for a quality job on panel painting.
HK mentioned hardner. I save the drippings from my harnder cans and use it in all my enamel paints. Great for getting Testors model paint to harden on models!