I have to ask this...I work at the seashore trolley museum in kennebunkport Maine as a restoration welder/fabricator. I work with old trolleys and old steel from 1907 to the 1950's and they all tell a story. I don't know how to put this to words but let me give it a try. What happens when you cut across a Luder line, and then try and weld through it? I am replacing the bottom half of a 16 gauge sheet metal panel and joining the old steel to the new takes quite a bit of black magic and welding witchcraft. Since not all things are created equal, sometimes the welding goes smooth and then there are pockets of trouble, but then there are times when there is an area that just goes out of control and no matter what I do the area just won't play. In this particular case the steel by the doors just seems to be possessed. Could it be that I am welding through a luder line? I ended up having to replace the entire sheet after trying to heat shrink it...to the point that I ended up tearing the sheet. It just continued to warp in the same spot. I am using the Pulse TIG process. At times I have to use stainless electrodes to trap impurities in the old high carbon steel and I try to compensate for differential expansion etc. Let me know what you think on this and thanks for your input. I seem to find these spots when welding and it can make a good day go horribly wrong!