My choice would be to butt weld.

Warping is an issue so I would start by tack the two parts at six inch intervals starting at either end, then the middle, then keep dividing until you have the tacks roughly six inches apart.

After the tacks are in place then you can start stitching the door. Weld one inch stitches starting from each tack point until each tack has a one inch stitch. Then add another one inch stitch to the previous stitches. Keep this up until you've completely butt welded the seam. The idea here is to control heat buildup that warps the panel. Only so much heat will build up in a one inch stitch. When you move to the next location the previous one cools off. You're trying to keep the heat down and uniform across the whole panel.

I would also prefer to weld from the inside. This way it is easier to finish the outer door surface. Leaving a weld bead on the inside will give the seam strength. If you place the bead on the outside you will have to grind most of it off which will probably warp the panel and leave the seam weak. That's why I prefer welding on the inside. Of course the very edges will have to be welded from the outside and ground off but you get the idea.

Is butt welding difficult?
 

Hmm didn't think of working from this inside but it makes sense now that you mention it. I was under the impression butt welding was really tricky?

Rolls, the pros will butt weld, hammer and dolly flat (mig weld is hard and difficult to hammer though).

Here's a link to my resto where I did a door corner... about half way down http://forums.pelicanparts.com/paint-bodywork-discussion-forum/545166-restoration-my-69-911-a-11.html

If you're good with a welder, butt weld it.

If not put a lip on the remaining metal on the door tack, plug or seam weld, much less warp to deal with, and it will stop you from grinding your door or new section.

Weld through primer, epoxy prime inner and outer, then seam sealer, filler to smooth out then paint.

Either do it on the car or refit the door a bunch of times to make sure you don't weld a warp into the door.

Lip
 

Thanks for the information....

What's the best way to create a lip? Do I need a joddeler tool?

Just to clarify. I was referring to doing a butt weld with a MIG welder as opposed to using bailing wire, torch and hammering afterward like some of the old pros do. I wouldn't hammer a MIG weld. They are too hard and brittle. Using a sanding disc is best for smoothing over the peaks.

Here's a sample of what I was talking about. This was done with a MIG welder. Welding from the back side.

http://forums.pelicanparts.com/uploa...1197749100.jpg

http://forums.pelicanparts.com/uploa...1197749241.jpg

http://forums.pelicanparts.com/uploa...1197749691.jpg

I think it's called a flanging tool? The one I have is from Bursons.

I butt welded everything on my resto. Tried to on the bottom 6" of the doors and failed, I could have just bogged it, but compared to the work i'd accomplished it would bug me.

I ended up cutting it out and put a lip on the bottom of the door, I ended up seaming welding, rather then plug, as I have a thing about rust. Dressed the weld with a flap disk, Evercoat all metal, which is a fiberglass based filler, then epoxy seam sealed the inside, and used quick sand filler on the outside, then another coat of epoxy, guide coat and a long board, and handed it to my painter.

If you've never done this type of work, or aren't committed to doing the steps take it to someone who is, it requires a lot of equipment, which may exceed the costs of getting someone to do it for you.

I'm not trying to put you off doing it yourself, just giving you a heads up

Just in case you haven't seen DarrylD's outstanding Porsche 912 restoration website...here is a link to his door rust repair. He certainly inspired me to buy a mig welder and jump in to body work.

DarrylD's Porsche 912 Project Page - Phase 7: Right Door Skin Bottom Edge Replacement