My starter on my MF 12 looked pretty bad. It was con-vexed to the point where the brushes were not making good contact & made the starter start & stop or not run at all. I don't have a lathe so I improvised a little & came up with this contraption that worked well.
See the damage to the inside area.
Make sure the drill is level with the armature & supported well to get a true flat running surface before filing. Remember this is your lathe I started out dressing up the high spots on the edges first.
Once I got the high areas down near the center I used a flat file to dress up the center. I'm 60 years young & every time to this day I use a file I can hear my metal shop teacher say " don't file backwards boys just on the forward stroke". He always drilled us about how it would ruin a file dragging it backwards on the work piece. After this I used some emery cloth, 400 grit paper & then steel wool. Make sure you blow clean it well if you use the steel wool so no metal particles give you a short later. I also went back & cleaned out the space in between the copper plates with a thin screw driver.
I'm sure this has been done before but I couldn't find anything about this repair other than using a lathe. Thanks for looking!