Yes. The surfaces the brushes ride on. I have a stone I got from the Lincoln dealer many years ago which is very good. I have used sandpaper when I don't have the stone with me. This is done with the machine running. Careful. Not for the faint of heart. Copper bars should be shiny, smooth and even when you are done. Check to see that the brushes move freely in their slide tubes and that the springs are functioning as they should. Stuck brush is a common problem on these old machines. The surface of the brush that bears on the commutator should be smooth, undamaged and shaped to fit against the commutator. When the brushes wear down they will run out of travel and cease to function. It only takes one stuck or worn out brush and no welding today. When I do this I do both the main generator and the exciter, although the problem is usually with the generator. Put the brush cover back in place after to keep water, dirt and dust away from the commutator/brushes. Both brush covers.