Try 70S 6 wire, more oxidizers=cleaner welds.
I just wrote a nice little guide for you, but a power fluctuation hit and lost it.

I'lll hit the high spots again of what I BELIEVE to be the problem. Likely you have a couple of issues compounding the problem.

You are moving slowly from the appearance of your welds, and likely overheating them (the twisted collet is definitely a sign) by welding at too high an amperage. What is resulting is you are putting a lot of heat into the metal, and the weld is cooling slowly after the argon is gone. Likely its still a red or dull red color after the torch has passed by and the gas has dissipated. This will cause rapid oxidation at the weld surface, since the metal is cooling in the presence of oxygen. Torch angle and gas flow can be a cause as well...but I think from the look of the weld you are putting too much heat into it and moving too slowly. Work your torch side to side to fuse the weld, just carry enough heat with you to keep the immediate area melted. If you have a big puddle trailing behind you, it is too hot.

You won't remove much mill scale with a scotch bright very easily. Don't be sissy with it. Grind it.