United Abrasives- SAIT 58166 Open Coat Aluminum Oxide 3-Inch x 21-Inch AO-X 100 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.