Be sure the primer is bonded well and scuff it before the application of the finish coat. Your finish coat will only last as long as your primer...
Porter Paints (PPG ) Not cheap but when painting, you get what you pay for in application, coverage, longevity of paint life and appearance..
Porter's AcriShield is great paint for outside wood with their primer under it. Ask them for their direct to metal paints as they have different brands for different metals and each requiring specific primer applications. IE: steel, aluminum,galvanized ........
P.S. Sherwin Williams is good paint too, just have to remember price usually dictates quality in paints.