Yes more rust, what do you expect? This rear door would be better off replaced. It has some panel issues other than rust… but I’ll have to fix it, because I have not been able to locate a replacement door.
First on the list… the door jamb area where the outer skin and the inner skin are very close together.
Cut it out, weld in a new patch.
The outer skin was stripped of all paint with 80 grit disks on my Makita polisher/grinder at low speed. Now to cut out the bad spots in the lower corners.
Corners cut out and the inside door structure treated with Picklex-20, then painted with Zero-Rust.