How to fix pinholes in welds?
I've been practicing welding sheet metal and have gotten to the point where I can butt weld two pieces without burning a hole in it. I am stitch welding it, but I am finding that after I grind the weld down and shine a light from the back there are pinholes in the weld. I have tried little tack welds over the pinholes and then grinding that down, but I often find there is still a pinhole there. What causes the pinhole in the first place, and how do you fix them after they occur?