If it's a cosmetic weld where the metallurgy isn't critical, you can also include wet rags to your tricks. Lay short beads, skip around, take breaks and do something else. The distortion is caused by heat shrinkage, so if you limit distance the heat travels into the sheet you can limit the shrinkage. I've also used silicone heat dam gels. I hate them. All of these methods are messy, and multiple starts and stops mean the final finish will be accomplished by grinding, sanding and buffing. To me this is too much like automotive body work so I have to really like the project to do this sort of work.