Whatever little benefit of coating the metal is not worth hacking up the car. As long as you're not parking it all the time under a tree in wet weather, it's not going to be an issue. I would just strip as much old paint as possible through the vent slits, tape up the slits and hit it with primer. Remember that there was a gray sealer primer applied to the entire exterior, so go with that as a base primer, followed by paint/over-spray from when the car is painted.