Painting Metal Lintels

[DIYTIMJ]

I have a metal lintel over the garage door and about two years ago I needed to repaint it after I replaced all of the weather stripping & trim around the garage door. I was in a rush to get everything done before a party and it was getting cold out so I sanded it down to bare metal and painted it with a latex paint/primer. Early last year it was showing rust spots through the paint but the paint is still well bonded to the metal. I would prefer not to have to scrape down the entire lintel again but any ideas on the best stain blocker to use before recoating it?