When welding in patches, most people use a MIG for what I call "dot" welding, skipping around to control heat, and then adding dots in between. What I would suggest, instead of adding dots in between the previous ones, is to overlap one dot to each of the previous by about 1/2 dot width. This will tend to fill in any voids that may show up in the centers. Some adjustment of the wire feed may also help.
