Door bottoms of most cars built when this one was usually don't hold up well as many of you know. I can't tell you how many cars I've seen with fully rusted out bottoms and they're a pain to replace the metal in because of the curves & folds. Luckily, although mine had some rust they were repairable without having to replace metal. The passenger side door is usually worse than the drivers and mine's no exception so I'm using that to show how to do a door that's got some rust but is still structurally sound without welding all new metal in.
In this case there's no visible rust in the interior of the door, if there was, new metal would have been installed. When I painted the car 20+ years ago the outside rust was there and I cleaned it with wire brushes/wheels, etc. and put rust converter on it before paint. Granted the car has rarely gotten wet since the first time but the rust had not popped through the paint or progressed. This time it's getting a better repair because I have a blaster.
Sandblast edge of door & get the rusty parts really good. Blow out all send & debris and then coat all the rusty areas with panel bond, let sit 24 hours. Because this isn't a place where the panel bond would get hot from exhaust or rapidly change temperature by heat from the sun it makes a great filler to seal the pits and stop new rust from forming after blasting. Inside will be sealed later.
Sand down panel bond till it's just filling the rust pits and open up the drain slots, and gasket holes. Then skim coat with regular filler, sand, and prep for primer.
