Weld-thru primer, in general, is not designed to actually weld to or thru. Its more like a high heat primer that does not flake off when the surrounding metal heats up. I mean think about it this way, do you need primer under your weld? No, but the surrounding area is where it is needed. I generally spray the area surrounding where my weld will be and then sand off the portion directly where the welds are going to be.