Plug welding; I have a video of a goodmark guy demonstrating this. He starts at the outside lip of the hole, goes all the way around the outside edge, then stops in the middle to finish the center. One quick circular motion.
If you have the right heat and speed setting, the weld is almost flush. I also sometimes use a tool from Harbor Freight that looks like a type of vice grips that put a copper back plate under the piece being welded to. Then the welder can be truned way up and you get your 100% fusion without any burn through.
Make sure top an bottom pieces are always touching frimly or the top metal burns away.
