The doors tend to rot in the corners because Ford in their infinite wisdom put drain slots that don't go all the way to the front or the rear. As a car sits to some incline water (and dust) builds up in the corners. It keeps the area wet and promotes rust.
I formed and welded a new exterior piece (about a 4" X 4" patch). There was some inner damaged, but nothing that I felt was a major structural weakness. The stamping of the inner corners isn't pretty to begin with. And, who looks there anyway. So, I used POR-15 and their thin, fiberglass mat. For things like this I like the product. I also drilled holes at the extreme front and rear of the inner panel. That way the area does drain. I also POR-15-ed the bottom 1" of the inner door. It may not be their product on bare steel like they say it should be, but it is still a stronger barrier to water migrating into the seam.