For the frame, we used black Endura top finish coat over epoxy primer. The same stuff we used for our boat trailer which was regularly dipped in salt water so we know it's tough.
https://www.endurapaint.com/I tried POR15 and didn't like it. Found it expensive and it had a poor shelf life. My guess is that the two part paints like Endura have a longer shelf life since you only mix up as much as you need when you need it.
