I recently did the bottoms of my doors and found it was very easy to access them while they were cut out. With the windows out and the doors flat you can reach everywhere. Since I couldn't get a sander of grinder in there I used a wire wheel on a drill bit extension to remove most of the loose rust then, I used a phosphoric rust treatment, made sure I neutralized it well and painted the insides of the doors with rustoleum clean metal primer.(cause its inexpensive and I didn't want to worry about my epoxy primer having issues with the acid) then I applied self healing seam sealer to the panel seams for overkill.
There are a number of ways to skin this cat but be forwarned I did find a skinned cat inside one of my doors when I opened it up.
Skinned door cat
