Specialty Diamond PP40A 4″ Aluminum Backer for BR Triad Polisher (BRTRBACKER). Semi flexible rubber hook & loop backing pad with 5/8″-11 brass nut for Stone Sanding. This backing pad is used with different diamond grit polishing pads. Wet diamond polishing pads are used with wet polishers or wet grinders. It is flexible and can polish either contoured surface like Ogee, Full Bull nose, Half Bull nose, Bevel, Demi or flat surface. It is recommended to always use with water at rpm 2200 for best results. RPM may vary depending on stones and its hardness with a 4500 max RPM.