I would total disagree with the use of JB weld or other Metal filler's, while they will fill the hole temporally. Nothing is being done to fix the problem RUSTING FROM THE INSIDE OUT.
Even if your jb weld holds (Or some other filler) in that particular spot It doesn't stop or slow down other pin holes from opening up a short while later.

Proper repair would be to remove door skin clean, treat, repair the inner structure removing metal too far gone for metal repairs and treatment.

Is a Mig the correct tool? NO! Gas welding or a Tig would be preferred in my opinion.

Stripping to Bare metal like that shows commitment to wanting to do the job right so why half **** the rust repair?

Actually I don't think this rusted from the inside out. This section of the decklid was heavily pitted on the outside. I think it may have had pine needles or something pressed up against it at some point that caused it to rust.

This decklid is from a 1964 Skylark. It's not reproduced and not easy to find a replacement so I'm not sure that pulling the skin would be a good idea.

Decided to just go ahead and try to weld them up. I got it done without much trouble. The surrounding metal is still pretty solid so I just turned the heat down and fine tuned the wire speed. A skim coat of glaze or some poly primer should be enough to smooth it all out.