I would never put any abrasive on the bushings, they are probably oillite bushings made of sintered bronze which has lots of pores to hold oil and you will not be able to clean out? the abrasive. If you think the shaft is too tight on the bushings then there is possibly something in the surface of the shaft which is causing it to bind. I would start with adding drops of light oil to the bushings since they are porous and made to retain oil and let the motor run for a while. If that doesn't work, chuck the armature into a drill and use very fine sandpaper (like 5000 wet dry sandpaper) and lightly clean and polish the shafts.
