In addition to the usual dents and the bottom corners getting sprung out due to the oversize sills that had been riveted on top of the originals, the doors also had rust issues in the rear lower corner. You can see the problem in this picture where I have already marked where I plan to cut.

I made the cut with a Dremel, taking care not to go all the way through the door skin. The plan was to cut out the rotted section, fold back the edge of the door panel, de-rust, prime and weld in a repair piece.

Here's the cut:

And here's the rust:

I used the piece that I had cut out as a template to make the repair section

and after cleaning up and priming the door I welded in the new piece and folded back the door skin edges.