Epoxy primer, Jotun 87 would be my choice.
Followed by a polyurethane topcoat, such as Jotun hardtop XP or zinnser allcoat if you want water based
Thanks for the suggestions, googling images now.
Spend sometime on the capillary gap between sheet and frame. Either keep water out with sealant or run wax corrosion inhibitor into the joint after painting.
Where the water sits between will be the first place to rot.