If it were me i would sand off all the surface rust till its clean bare metal. if you have any pitting i would get all of the loose flacky corrosion off with a wire brush/wheel mini sand blaster and apply a rust encapsulator (theeastwoodcompany.com). this can be scuffed and painted over with primer surfacer. as for your primer issue i would consider using a good urathane primer(not hard to spray or you could get a local shop to shoot it. by the time you spend all that money on can primer you could have paid for the materials for a good long lasting primer base for your project. hope this helps