With any weld process, you will have a certain amount of shrinking. Be that MIG, TIG, O/A torch, 19 gauge sheet metal or 1" thick steel plate, it all shrinks. Just that sheet metal shows it the worst. The other Achilles heel is starting and stopping, this is where you will see the most deformity. With MIG, starting and stopping is the nature of the beast, it must be done. With either TIG or O/A, if you can do a continuous weld pass without stopping, then the panel heats up as you go and gradually cools down behind as you weld across the panel. This keeps things more in check and more consistent. I would suggest that if this door panel was your first attempt at TIG, then hopefully you did a few practice runs first to get your technique dialed in.
Fit up. Your door skin patch should have a nice tight fit, zero gap, or as tight as you can. With either TIG or OA, any gap is more likely to blow open from the heat, an absolutely tight joint will help out in the process. If your gap is cut inconsistently, then you'll have more heat trying to fill the gap, there is nothing to keep the panels from pulling together as the weld shrinks when it cools, so now you are pulling the sides together and forming low areas as the crown is pulled downward. Trimming for the panel and fit up is very much part of welding, and we need consistency throughout the process from start to finish, to include the planishing phase.
Tacking.. With either TIG or OA, you'll want to tack the panel, and start from one side and work progressively toward the other. DO NOT SKIP AROUND!!!! ….as this gives a greater chance that one side is not perfectly aligned with the other, and when you get done tacking you may notice a bulge to one side of the weld as that side had more material (length) from being locked in by skipping around. Make a tack, let it cool, and the next tack is placed where the HAZ (blue line) intersects the joint. Align the panels together, tack, let cool, repeat. When you are done with this phase you will notice the HAZ will be just about perfectly parallel to either side of the weld. This equates to more of that consistency thing, same amount of heat, for consistent results in shrinking to help keep the panel straight. Stop after tacks are done and planish as needed to restore crown where any shrinking may have occurred before attempting your continuous weld across the panel. Here is a practice I did with the Meco OA torch. Tacked first, note the very consistent HAZ. Then it was fusion welded with no filler.
Notice the consistent HAZ width from using this method. And even though I blew a hole in the fusion weld process, the finished shows a nice consistent HAZ as well..
For a good tutorial on gas welding, watch this video. Now I know that you're in TIG world, but the same tacking method, spacing method, letting things cool method, and planishing method apply...https://www.youtube.com/watch?v=fkNtUjxSf7Q
This shows tacking with the TIG without filler. https://www.youtube.com/watch?v=BO2a915SAuk