Once the patch piece has been tacked into place, the battle with heat warping the metal begins. The tin on these fenders is only 22-gauge and that is some pretty thin stuff. If too much heat builds up in the surrounding metal, it will warp. The best method I found for stitch welding is taking your time! I tack welded the panel in two places then blasted the welds with compressed air. Only when the metal was cool to the touch did I use the welder again. Weld, blast with air, let cool, repeat.
When the welds were ground down on the passenger-side fender, I made the mistake of using my electric grinder with an 80-grit flapper disc. Yes, it worked, but the heat build was so intense the panel warped. I spent another hour with the hammer and dolly straightening it back out and it is still not perfect. After all that careful welding and cooling, the grinding undid all my work. Avoid using a flapper disk and grinding wheel, they will build up too much heat for this thin-gauge metal. The electric grinder is better suited to grinding welds on frames and other heavy gauge metal.