I ran into that problem when I put the sheeting on my shop. Running the coarse thread self-tappers into the 14-gauge girts and purlins went swimmingly, but then I ran one into a 3/16" or 1/4" weld plate at the end of a girt, and "snap!" I was able to screw it out with a vice-grip, and I think I then pre-drilled those slightly oversize to avoid the snapping. I had no idea that switching to a fine-thread screw would work, but it makes sense. For the Adel clamps I installed on the LJ, I drilled and tapped the frame for 1/4-28 screws.