I've done the inside of door scuffing on a few first generation camaros, there's a few areas that are tough to get at but most areas can be reached, even with gloves I usually cut my hands up a little, not fun. For used doors I do the epoxy thing then follow up with 3M rustfighter after all the paint work is done. Transtar also makes a good coating and cosmoline has worked good for years.
If you've got brand spankin new doors that were e-coated I would just scuff the inside up with a red scotchbright, epoxy prime, then put a nice bead of urethane sealer around the perimeter inside the shell leaving only the drain holes open, use your finger to tool a nice radius. Epoxy the outside of the sean and just apply enough sealer to seal the joint without any buildup for a factory look - I do my doorskins this way and have never ever seen one start rusting. With old doors cleanliness before your primer is sometimes a problem and this is where the cavity wax rust fighter spray helps, it's a self healing spray coating that will leach it's way into voids and will not harden. Just my 2 cents.