Hello Jordan, since you are using DC- to produce the welds you do not have the benefit of the cleaning action of the DC+ side of AC. Even though you have thoroughly cleaned everything prior to welding you will still have an immediate oxidation of the surfaces of the welding area, prior to and during the welding and you will likely also have some of the "soot" from elements of the filler rod as they are consumed/mixed in the arc column and molten puddle during the welding process. I know there is a more technically correct description of all of this, but this is a quick and "dirty" explanation. I hope that I have given some understanding to what you are seeing. Good luck and best regards, Allan
if this is a inverter type of power source, remember hearing that when welding on DC- it left a cloudy film on the weld instead of the bright clear typically seen in AC. I played with a Mileer dynasty in DC- and loved that I could get a sharper arc off the tungsten in stead of the wide arc seen with AC.