Generator armatures get the mica undercut because they have soft carbon brushes. The mica can actually wear more slowly than the copper commutator bars, and the mica ends up being higher than the copper, damaging the brushes. Powdered copper starter brushes are not as suceptible to wear problems from mica, and generally, starter armatures do not need to have the mica undercut. You've seen the result of undercutting a starter armature- - - - -the gaps between the bars get clogged with copper dust. If I was doing the buildup, I'd either replace the armature, or turn it smooth if there's enough commutator left to do that. Don't forget to replace the bushings on both ends to prevent the armature from dragging on the pole shoes. Most slow-running starters are caused by loose bushings letting the armature contact the field coil pole shoes. Any scratches or shiny spots on the armature or the field housing indicates there's been physical contact- - - -not good!
Jerry
