[Steve Hamilton]

Neil
Looks like you are making progress, planishing and metal finishing is not a fast process. One to one and a half hours per foot of weld is not uncommon. Make sure that the weld bead is grind down to the thickness of the base metal before planishing. If the bead is thicker as you hammer on dolly the excess metal will create more area, which creates a high or low spot, as the metal has to go somewhere.
I try to get the fit up of the joint as tight as possible before tack welding, filling a gap requires slower weld speed and more filler, which both lead to more shrink in the heat effected area (the blue colored zone).
Some tips that might help with future welds.
All tacks and beads need to be ground to the same thickness as the sheet before planishing so the stretching is even along the weld zone.
Planishing can be done a little at a time to help keep the shrinkage under control. Weld an inch, grind off excess bead and planishing. Then weld another inch and repeat.
After tack welding if there is gap in the joint between tacks hammer on dolly the edges of the sheets and they will stretch out and fill the gap. Don’t hit the tacks as that will spread the sheets and increase the gap.
You may want to spend some time practicing with scrap material to become comfortable with the process.
The difference between a welder and a great welder is a few thousand hours of practice.
Steve