I had the same rust issue on our 85 300D,
Decided to paint the roof and rear quarters. Popped out the rear window and
found 3/4 of the lower part was rusted pretty bad. It is actually a pinch weld
where two pieces are spot welded together. water can get between the two
and start the rust problem.
I use paint stripper to remove the paint across the bottom and 1/2 way up
the sides. Wired brushed the major rusted areas, then wiped it own with
Wax and grease remover. I order it on line, pay for it and pick up in store.
https://www.oreillyauto.com/detail/b/prep-all-4074/paint---body-repair-16614/paint-prep-25200/paint---acetone---pre-paint-cleaners-19852/e04efcc6216c/klean-strip-prep-all-1-gallon-wax-and-grease-remover/sw3621/4402352
Then treat with Phosphoric acid to etch the metal and attack the rust etc...
Keep it wet for at least an hour, I put Seran wrap on it to keep it from
drying and adding to it as needed. After it dries it turns white, wipe it
down with water, let dry. Then paint on the POR and let it run between the
pinch weld.
I have some paper thin fiber glass type cloth, maybe it for model air
planes. Cut it 4", strips couple in wide, lay it over the POR and soak with paint. Over lap ea piece and this seals the pinch weld and ends the rust
problems.
YMMV...
Charlie