Sandpaper Guides

Sanding Belt Material Explained

Quick answer: A sanding belt is built from three raw materials: an abrasive mineral that does the cutting, a backing that carries it, and a resin bond that holds the two together. The mineral is usually aluminum oxide, zirconia, ceramic or silicon carbide; the backing is paper (A–F weights), cloth (J, X, Y and heavier), fiber or film; the bond is glue or resin, applied as a make coat under the grain and a size coat over it. On top of those, three manufacturing choices set how the belt behaves: closed or open coat, how the grain is applied, and how much the finished material is flexed. 1. The mineral Abrasive minerals do the grinding, finishing and polishing. Two are natural and rarely used any more; the rest are synthetic. Mineral Type Character Used for Emery Natural Dull black, hard, round and blocky Emery cloth for hand polishing; largely obsolete Garnet Natural Reddish brown, medium hardness, fractures to new edges Traditional woodworking paper Aluminum oxide Synthetic Brown, tough, durable – the most widely used grain Wood, metal, leather; every grit Silicon carbide Synthetic Shiny black, very hard and sharp, brittle Non-ferrous metals, stainless finishing, glass, stone, paint and clear coats, mill scale Alumina-zirconia Synthetic An alloy of aluminum and zirconium oxide; tough, self-sharpening Coarse stock removal on metal and wood Ceramic aluminum oxide Synthetic Micro-crystalline grain made by a sol-gel process, two to three times tougher than standard aluminum oxide Heavy grinding on steel and stainless; knife making Diamond Synthetic The hardest material; very costly Ceramics, stone, tungsten carbide – only where nothing else will cut For how the synthetic grains compare in use, see aluminum oxide vs. zirconia and ceramic belts explained. 2. The backing Backings fall into five groups: paper, cloth, fiber, film and combinations. Paper Weight Description Used for A Lightweight Medium and fine grits for hand sanding, pad sanders and finishing discs C, D Intermediate Medium grits in sheets and discs; some belts E Strong, heavy Machine sanding – wide belts, drum rolls F Extra heavy Heavy-duty machine belts, drums, cones and discs Cloth Cloth is identified by weight and by fiber. Before coating, the cloth is treated with adhesive fills that set its stiffness and flexibility – the treatment matters as much as the weave. The most common fiber is cotton; polyester is added or substituted where strength and water resistance matter. Weight Weave Description J Jeans The lightest, most flexible cloth – contours, slack belts, finishing X Drills Heavier; coated in every grit for the widest range of belt and disc uses Y Heavy drills Heavy stock removal and high-horsepower machines T Twills Heavy cloth for selected uses M Duck Extremely high-horsepower belt use A suffix tells you the fiber: F for polyester (XF, YF, MF) and E for rayon (JE, XE), which gives a firm, tear-resistant backing with good edge wear. More in J, X and Y weights explained. Fiber and film Vulcanized fiber is an extremely tough backing made from rag stock, used heavy for resin fiber discs and lighter as drum sander covers. Polyester film is the newest backing: perfectly uniform, waterproof, and used from lapping film for precision finishing up through coarse aluminum oxide and silicon carbide products where long life and a uniform scratch pattern matter. 3. The bond Two adhesive layers anchor the mineral: the make coat under the grain and the size coat over it. Five combinations are used: Bond Make / size Character Glue Hide glue / hide glue Traditional "glue bond"; flexible, low heat resistance; fine finishing papers Glue and filler Hide glue with inert heat-resistant filler Stronger, more durable glue bond Resin over glue Hide glue / synthetic resin Exceptionally heat resistant with a flexible base Resin over resin Resin / resin Heat and moisture resistant – the standard for belts Waterproof Resin / resin on a waterproof backing For wet sanding with water or water-based lubricants 4. Coating: closed or open Closed coat sets the grains adjacent to one another so the backing is completely covered – maximum cut and life on hard materials. Open coat spaces the grains at a set distance, covering 50–70% of the backing, so soft materials that load – softwood, paint, aluminum – have somewhere to shed. See open vs. closed coat. 5. How the grain is applied Gravity coating drops the grain from a hopper onto the adhesive-coated backing. It can be tuned to present the sharp edge of the grain reasonably well, but placement is random. Electrostatic coating passes the coated backing and the grain through an electric field; the grains are propelled upward and embed in the adhesive point-first, evenly spaced. This is how nearly all belts are made and why they cut so much faster than gravity-coated paper. 6. Flexing After the bond cures, coated abrasive is stiff – and the coarser the grit, the thicker the coating and the stiffer it is. Flexing is a controlled cracking of the bond in one or more directions to give the material the flexibility it needs. The rule: use no more flex than the shape being sanded requires, because the stiffer the abrasive, the more aggressive the cut and the longer the life. Flex What it is Result No flex None Stiff; standard for sheets and fine paper products Single flex Flex lines at 90° to the running direction Minimum handling flex; runs over small pulleys and contact wheels Double flex Two flexes at 45° Moderately flexible in all directions Full flex Two at 45° plus one at 90° Soft and flexible in every direction L flex Lengthwise flex lines with a minor crosswise component Crosswise flexibility so a belt conforms to contours Putting it together When you order a belt, you're choosing a combination: a ceramic grain on a YF polyester backing with a resin-over-resin bond, closed coat, single flex, for grinding steel; or an open-coat aluminum oxide on a JF cloth, full flex, for sanding a contoured handle. Tell us the material and the machine and we'll pick the construction. Every belt is made to size in Anaheim; browse sanding belts or read how sandpaper is made for the coating process itself.