Osborn Buffing Brushes are 80-grit brushes with 1-1/2" bristles used with either an angle grinder or car buffer (with a 5/8" arbor) to clean and polish logs. The nylon bristles are heavily impregnated with abrasives, which aid in the removal of rough bruises, pitch, and wood pulp. The brushes also work well on fuzzy or rough logs caused by power washing. You can use the 4" and 6" brushes on 7" to 9" angle grinders. If you use a 6" brush on a 4" grinder, expect to see the bearings go out of the grinder. Take care not to burn the logs when using the 4" brush. Brushes will last longer if a car buffer is used due to the slower rotational speed of the buffer. Though these brushes are rated up to 6,000 rpm, we suggest an operating speed of around 1,800 rpm. The slower speed will greatly prolong the life of the brush and also minimize the possibility of burning the logs. Not intended for heavy sanding. Not designed for use on metal.