Surely car manufacturers would be powder coating if it was feasible. Still haven't convinced me. Does powder coating actually key into the metal as an etch primer would? To my knowledge and correct me if I am wrong it wraps around the part being powder coated. Much like the plastic coating on old fridge shelves. Once the surface integrity is broken eg stone chip moisture just gets underneath the powder coating and it will eventually lift. Seam sealer also a problem as for a car bog free good luck. I am not sure if the larger flatter panel will warp at 200 degrees. Good luck if you decide to go the powder coating way.

Cheers Dave