Inside of metal panel coating?

it sounds as though you haven't hung them on, yet. And you called them quarters, so I'm assuming rear quarters. If this is correct, they will have to be welded, yes? I can't recall right now what it is called, but I've read there is a weld-thru primer that the pros use, that protects the sheet metal but does not interfere with welding. After they are in place, then I would epoxy primer the sheet metal. After that, I would use undercoating, or rhynoliner on the backside of the broader trunk areas, like the factory did, to dampen drumming noises, and strengthen the rigidity of the sheet metal. Of course, you can top coat it with body color, as you wish.