Aside from removing the undercoating from the door skin, I removed as much rust as I could from both the inner and outer skins. Then I sprayed the panels down with some rust Mort. After letting that do its thing, I wiped the panels down with some soap and water to neutralize the product.
Removing rust from inner skin
Sprayed inner panel with rust mort
Removed and patched the lower corner on the outer skin that was rusted through
Both the inner and outer skins after applying the rust mort. I really like that product.
And just to be sure I won't have any future rust issues with the door, I gave it a real good coating of Por15.