I have used POR-15 (a moisture-hardening urethane), if the surface did not have flakey paint on it. It is a bit messy in application inside the door (as any paint would be).
I sloshed it around the door insides by slowly rotating the door in all directions, ending up with the door "standing" in normal orientation.
Do this on throwaway drop cloths and/or paper.
Do this while wearing throwaway gloves.
Have old rags (and solvent) on hand to wipe off the POR that comes out of openings.
Once it has dried, you would have to grind it off, or sand it smooth for finish paint-prep, etc.
