I've used that Por15 on many different metals over the years and have had very good results with it retaining minor to moderate rust. *Paper thin metal that peels off with slight movement should be replaced. Ain't no paint that'll help that
When painting (if possible), try to coat both inner & outer sides so it completely encapsulates and stops it. Then scuff, clean and apply your basecoat/clearcoat over the por15 paint. That should buy you much more time and keep any remaining rust from pushing through.
Water gets in the door, if the drains are plugged the water sits a rots the metal...one tactic is to spray atf inside the door.
I save all my drained atf, i fill spray bottles with it and keep them handy,..whenever im working on soething that looks dry or corroded,..under carriage,..calipers,..spray away. Atf is a good rust inhibitor,..and drained atf is free.
You would think that with this area rusting on so many of our rides that someone would make a replacement section so we can cut the rust out and weld in a new section. I saw a guy somewhere on youtube cut his off and make a new piece that he was able to weld back in after prepping the inside of the door skin. This is where an auto body friend would come in real handy. Used doors are $300 plus and then add shipping too.
And it would also seem that they're quick to respond too! Under an hour.
As a long time GM truck fan and owner, I agree with you. Especially for all the Trailblazer SS type guys that are building these trucks. I will certainly pass this along to my R&D head and maybe that can be something we’ll offer in the future. We are just dipping our toes in with some more parts for 1999-2007 Classic Silverados so who knows?