Ok, maybe I will need a new door.
I finally took a wire wheel to the rear passenger door in the E28 today, and the rust is worse than I thought.
Definitely through in a few places. I went ahead and cleaned it with POR 15 and will coat it tomorrow, but it will need a longer term solution.
Before taking the wire wheel to it I made sure that any rust dust wouldn’t get in the car or on the door panel.
10 minutes or so with the wheel and I had it pretty smooth for cleaning with POR 15 degreaser.
I think I’ll probably get my brother in law to help me cut it out and patch it for now. In Houston I’m not worried about it getting worse, and I don’t even drive it in the rain.