This is that single curved hammer face I made years ago. Using it in the corners to hammer outwards onto a flat faced dolly to stretch it outwards. Due to the gap I was left with, the quickest way to weld this up was using the mig with 0.6mm-0.23" wire. This was welded non stop from one end to the other putting one tack on the next while it was still red.
Hasn't pulled in much at all. This straight after welding and no hammering. Just took the top off the weld but leaving about a razor blade thickness left. You want some extra thickness to stretch the weld area back out. If you go flush this early you end up with thinner metal all the way along the weld seam which could lead to cracking. I really get into it with the grinder heating it up until blue. I find this softens the weld ready for planishing. Do the same on the inside if you need to.
Hammering from the inside out with my arm through the window holding the flat face of the dolly against the weld and heat affected area. Doesn't need much if you have kept your weld size down with the thinner mig wire. Don't use flux core! Can see the weld is at the right height but a little dip above it. That was there before welding too so will hammer out that area.
I check the tightness of the panel as I go along and as soon as I can feel some movement in it, I switch to hammering against this lead filled rubber dolly. What this allows is still moving the panel but without stretching it. If you keep hammering on a steel dolly you will end up with an oil can or loose panel. Working off dolly I also do at this stage with a steel one. Just depends on how much and where it needs to move. If I see a high spot near by, I will press the steel dolly with a slight dome shape to it, against that high spot while hammering the low spot on the opposite side. The joint has now been taken down to flush with a flap disc once the panel was very close to flat. Just remember if may move with the sanding a little. Can be could to tighten a slightly loose panel without using a shrinking disc. Will use one at this stage if needed.
Now done with a strip disc and checked again to make sure it is still all flat. Have finished welding the A pillar and mirror screw holes.