Primer is a good start, but it isn't weather tight. I'd sand the area with your palm sander - start with 180 and go waaay do to 400 with a hand block sander to smooth the paint and the bare metal. Do not prime over rust - get it off with the sanding.
Arnswine's suggestion for the phosphoric treatment will help retard further rusting that may be going on in tiny/hidden pits in the metal. Remember to 1) wear gloves and 2) wash the agent off thoroughly when you are done. Then, wipe the area down with a good car and truck degreasing agent. After that, you can prime.
Next, put a coat of primer sealant on the area or it will just keep rusting back through. Use the same brand primer and primer sealant.
