I'm sure I'll get a host of various replies but let's give it a shot.
Double Garage door Metal Suspect original factory paint from 10-20 years ago. Faded and chalked. No peeling, cracking, etc. (why I suspect original factory paint) No bare exposed metal In NC, - temps exceed 100° - temps 90°+ constantly July & Aug Exposed to direct sun all afternoon till sunset
Obvious scrub with TSP, rinse well & let dry first. I'm hearing latex everything including primer. More flexible than oils. I'm hearing 100% acrylics for both the primer and paint. "Some manufactures use modified resins that are cheaper and inferior, commonly called latex primer. The best primers use 100% acrylic resins and will cost more."