Sungold Abrasives 3-Inch by 21-Inch 120 Grit Portable Belts Premium Industrial X-Weight Aluminum Oxide, 10 Belts/Box.   Ultra thin, bump free overlap joint. Backing is a polyester/cotton blend, a technology introduced by Sungold, that gives the strength of polyester, but employs cotton yarns to prevent the stretching that can occur with all polyester backings.  While many belts will tear or shread from nails and staples, the backing employed by Sungold resists tears.  Resin on resin bond used to adhere the grains, using high temperature resitant resins, to hold up to high heats generated by belt sanders, provides extended belt life compared to other belts.