Sandpaper Guides

Sandpaper and How It Is Made

Quick answer: Sandpaper is made in a continuous process on a coating line: a backing (paper, cloth or film) is coated with a resin "maker coat", abrasive grain is applied – usually electrostatically so the sharp points stand up – then a second resin "size coat" locks the grain in place and the whole roll is cured in an oven and flexed. The result is a jumbo roll of coated abrasive. Converters like us then slit, cut and splice that roll into belts, discs, sheets and rolls of every size. A short history The first recorded "sandpaper" comes from 13th-century China, where carpenters glued crushed shells, seeds and sand to parchment to smooth wood. The idea hasn't changed – hard particles stuck to a flexible backing, used with friction to remove material – but everything about how it's made has. Modern coated abrasives are engineered products with precisely graded synthetic grain, and the way the grain is placed on the backing matters as much as the grain itself. Step 1: The backing Everything starts with the backing, which decides how strong, flexible and waterproof the finished product is. Backing Weights Used for Paper A (lightest), C, D, E, F (heaviest) A and C: sheets and finishing discs. D: heavier discs. E and F: wide belts and drum sander rolls. Cloth J (flexible), X (standard), Y (heavy) J: contour and finishing belts. X: most belts and discs. Y: coarse-grit, high-pressure belts. Film Polyester, various thicknesses Fine finishing and wet sanding discs; very flat, waterproof. Fiber Vulcanized fiber Resin fiber discs for grinders. Cloth backings are cotton, polyester or a poly-cotton blend. Cotton is more flexible; polyester is stronger and more water-resistant, which is why poly-blend X and Y weights are what you'll find under most heavy-duty and wet-use belts. Before coating, the cloth is treated – filled, stretched and sealed – so it doesn't soak up the resin and so it keeps its shape under tension. Paper is sized for the same reason, and waterproof papers get a latex treatment so they can be used wet. See our guide to J, X and Y weights. Step 2: The maker coat The backing is unwound and run through a coater that applies a thin, even layer of resin – the maker coat. Its job is to anchor the grain. Most modern abrasives use a phenolic resin bond; glue bonds still exist for some fine finishing papers. The coat has to be perfectly uniform, because an uneven maker coat means uneven grain, which means a belt that leaves a mark. Step 3: Applying the grain This is the step that makes or breaks the product. The abrasive grain – aluminum oxide, zirconia, ceramic or silicon carbide – has already been crushed and screened into precise grit sizes. It's applied to the wet maker coat one of two ways: Electrostatic coating. The backing passes, resin side down, over a bed of grain in an electric field. The charged particles jump up and embed in the resin sharp end up, evenly spaced. Nearly all belts and quality discs are made this way – it's why a modern belt cuts so much faster than one with grain simply poured on. Gravity coating. Grain is sprinkled onto the resin from above and lands however it falls. Cheaper, less consistent, used for some low-cost papers and very fine grits. How much grain goes on is the coat density. A closed coat covers nearly the whole surface for maximum cut on hardwood and metal. An open coat leaves 30–50% of the backing bare so softwood dust and paint have somewhere to go instead of loading the belt. Step 4: Drying and the size coat The coated web runs through a drying oven in a long festoon – hanging in loops – so the maker coat sets and the grain can't move. Then a second layer of resin, the size coat, is applied over the top of the grain. The size coat locks the grain in, gives the product its strength, and can carry additives: grinding aids that keep the belt cool on stainless, or stearate anti-load coatings that stop finish and paint sticking – that's what makes a gold disc gold. Step 5: Curing and flexing The web is cured at high temperature for hours to fully harden the resins, then flexed – run over bars at controlled angles that crack the brittle resin bond into a fine pattern so the product can bend around a contact wheel without shattering. Single, double or triple flexing sets how flexible the finished belt is. The finished coated abrasive is wound into jumbo rolls, typically 50 inches or more wide and thousands of feet long. Step 6: Converting – where we come in Coating mills make jumbo rolls; converters turn them into product. In our plant in Anaheim, California we take jumbo rolls of the materials we've selected and: Slit them to width on precision slitters – anything from 1/4" for file belts up to 62" wide belts. Cut the strip to length and at the splice angle for belts, or die-cut discs, sheets and shapes. Splice belts – skiving and joining the ends with heat-cured tape into a butt, lap or sine-lock joint, then inspecting every joint. Print the grit, material and direction arrow on the back, and package. That's why we can make any size belt in about three days: the coated abrasive already exists on the roll, and converting is what we do all day. Order custom sanding belts, or browse sanding belts and discs. Why this matters when you buy Electrostatic grain placement is why a good belt cuts fast; gravity-coated bargain paper doesn't. The size coat additives are why some discs resist loading and some belts run cool on stainless. The backing weight is why an X-weight belt survives a knife grinder and an A-weight sheet doesn't. Grit numbers are the size of the screened grain: the lower the number, the larger the particle and the coarser the cut. See US vs. European grading.