[Daves55Sedan]

HRR, since you have the interior all stripped off the inside of the door, why not just go ahead and pop out the window weatherstrip to make the job easier and more thorough. This would require the window glass to be removed first though. Stick a flathead screwdriver between the top of the weatherstrip clips and the door structure and carefully pry off. Locate each clip and repeat. At the very bottom on the inside of the door there is one hex bolt to remove. After pulling the weatherstip out though the window opening you now have full access to the door surface.
I suggest sanding that rust down to bare metal and also inspect on the backside of the door structure to see if additional sanding is needed there. Then clean and paint with a good acid-etch primer.