In another post, JohnT mentioned he thought the AC portion of the aluminum TIG arc would clean the aluminum. I also harbored that same thought-so my question is, when we see example welds on videos at WT&T, for instance, the explanation of AC balance makes it a point to show the frosty cathodic cleaning zone. What is the difference between those welds, which break up the aluminum oxide layer and produce the frosty clean zone, and my welds, where I have to use a flap disk to get to bare metal? Is it the alloy of aluminum, or maybe the amount of oxide that has built up?