What are you welding with?? Hot air? Wedge?? I have welded a lot of smooth banner...both seams and hems on our Miller weldmaster. Find that we have to turn heat and speed down to avoid as much of the warping or scalloping on the hems as we can. Never perfect...but come out pretty nice and relatively flat. Not sure of exact settings...probably in the 900° area and a speed of 250. Typically we run Cooley and UF scrim at 1050° and 425 speed.