United Abrasives- SAIT 63247 Open Coat Aluminum Oxide 4-Inch x 36-Inch AO-X 120 Grit Sanding Belt, 10-Pack AO-X: Aluminum oxide, open coat X weight cotton. For general purpose sanding and grinding where an open coat structure is needed. Best choice for soft woods, non-ferrous metals, aluminum, leather, rubber and other materials that have the tendency to load.