Most of the time my dad would just put in a new set of brushes. Then from a roll of 2" wide roll of commutator paper he would tear of a strip about 6" long and he would work it back and forth as if he was shining shoes.
Saying that you had to keep moving the direction from where you pulled the paper so you would remove the material evenly. He said if we had a drill press to spin the armature while holding the paper would take care of that.
My dad also said if it is not broke don't fix it. Though if your starter looks that bad you can always just clean it up.
