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Sandpaper is fine to use to clean up sliprings or commutator pieces as long as you clean it up afterward. When I worked in a motor shop we used crocus cloth to clean them up. Sometimes we would get one that was in bad shape and would turn it down on a lathe.
Never known grit from that activity to embed in the copper, its not all that soft.
John, what would you recommend to clean up the slip rings ?
I'm sorry to say, but I don't have a good solution for a home repair. The video said to use Scotch-Brite. This would be good as long as it's not coated with abrasive. Otherwise, I would have to take it to a shop that has a lathe and they could take a light cut to true it up.