How did you interpret what I wrote to that cleaning won't help? I wrote that a brush of a harder metal than what you're brushing will diminish the metal. To make it more clear: act like sandpaper and remove metal (it just takes a longer time).

I got the black "welding" spots on the plus pole pretty much from the start, that's normal and will happen as the discharge has such high amperage. Did not notice any power issues from it, cleaning the plus pole with brass would be fine as steel is harder than brass.