There are drains, on my MK2, more than a couple per door.
The BIG problem at least on MK2's has been the absorbent pad glued to the door skin.
Tends to hold water and encourage rusting.
Previous owner removed said pads from mine.
If it was my car, I would remove and inspect the inside of the doors.
Remove any visible rust scale.
I treat the actual rust with Rust Mort, which chemically reacts with rust, turning it hard, black and inert.
Por15 simply encases the rust, which would probably work on top of the Rust Mort once it has hardened.
Then I would paint the insides with something hard and shiny (not just primer), to shed any water.
But, then, I want mine to last, ya know?
Dave