The MIG welding setup is an interesting compromise. It is possible to slow down the feed such that when the metal melts there is not enough filler to prevent a hole forming. Sometimes with a little more wire speed it builds up faster than you want.
For butt welds I use the small spot welds at intervals along the length. I then put welds midway between them. I continue the process until the spacing gets close. I then place each subsequent weld so that it run into the weld on the left. Eventually there is only enough space left at each spot to join the two sides. If everything goes perfectly this technique yields a seam with no holes. However I don't often score 100% on this!
