During the lock down last year, I finally got around to cleaning out the dust and sand from the inside of the door. And replaced the waterproof barrier .
The first time I pulled the door panels off my 60 I found 5 inches of dirt in both doors and a big steel workers wrench in the drivers door. Also under the removable bed panel had over 12 inches of dirt in it...........SS
Once the inside of the door is clean, stabilize the surface with Rustoleum Rust Reformer, then brush on or spray in bed liner. Make sure the weep hole is still open. It will last another 50 years.
Yep I've done all that plus replaced all mechanical parts and such. This car is definitely gonna live longer that I will.
At my bodyshop we use 3M Rust Fighter I, 3M 08892 (similar to Ziebart cavity wax) . About 4 or 5 inches up is all you need. It will seep down into the seams at the bottom of the door where rust starts before the extra will drain out at the drain holes. After a day wipe off what gets on the finished side. Undercoating and bedliner will just bridge across between the skin and the door shell leaving the seam prone to rusting below it. I haven't bought a can of undercoat for at least 3 years, mostly to patch up where some one else has sprayed it.
