The BLUE ZIRCONIA ALUMINUM OXIDE Resin Fiber Discs are flat discs used for heavy grinding and weld removal. They consist of a stiff backing coated with a single layer of resin and abrasive. Our Resin Fiber Discs have grinding aids in both the A/O and Z/A products, which helps performance and keeps the disc cooler during grinding. The discs have an arbor hole of 7/8" for 4.5" discs to allow them to be attached to a backup pad