Anything will work.
What I do be I dip them, sandblast or put on a new skin.
After the inside is cleaned up I mix up a quart of epoxy and than reduce it 50% with urethane reducer.
With the door laying on a stand I pour the quart and ahalf inside the door with duct tape on the out side of all holes such as door handle and drain holes.
Every 30 minutes I tilt the door all different angles so the epoxy runs in all the creases. After about four hours I pull the duck tape off and let the epoxy that is left run out.
Just did the same thing to my vette doors about two months ago.
I do the same thing on the underside of hoods so the epoxy gets under all the braces and I did the same thing on the big block vette hood a couple of months ago.
Too me this is a cheap way to seal off the inside of a panel and give it
the best protection you can.
I never said I was normal!