The Secret Life of Sandpaper
You may know it by its variety of uses, its grit count, and/or its uncanny ability to make you cry the night before your project is due. (Yes, we can all see those scratch marks.)
Sandpaper, a coated abrasive on flexible backing, is used to remove various amounts of material from all kinds of surfaces. With the help of friction, it can remove entire layers of paint, roughen smooth surfaces to allow for better adhesion, and create smooth finishes. Its story began in 13th century China where crushed seeds, shells, and sand were glued to paper with tree gum. Nowadays, sandpaper isn’t backed by paper at all. Poly-cotton cloth backs allow for more flexibility and resilience. There also isn’t any sand in sandpaper these days either. Commercial abrasive surfaces, or grits, are made up of more robust materials.
It’s A Hard Rock Life
The most common is aluminum oxide. It’s a highly friable material with the widest variety of grit count. Friability is a kind of self-healing quality that renews its sharpness as they deteriorate and extends the life of the sheet. The properties in aluminum oxide allow for low heat retention and contain durability comparable to industrial diamond. It can be extensively used on iron, steel, and wood. Garnet grit is a lightweight champion composed of reddish-brown rocks that leaves a better finish than aluminum oxide, but requires more patience, since it removes material at a much slower pace. It’s a fantastic grit for finish sanding and polishing metal. Other abrasives include alumina-zirconia, ceramic aluminum oxide, and silicon carbide. Silicon carbine, known as ‘wet and dry’ paper, can be used for both dry and wet sanding.
But how do these minerals make it into the cloth backing, anyway? What makes them stick? Enormous sheets of poly-cotton cloth are rolled into a hot and humid electrostatic pit. Buckets of grains are poured onto a conveyor belt that moves the abrasive particles under the passing poly-cotton cloth, between electrodes and ground plates. This creates an electrical field that allows for flying grain to become evenly distributed and embedded into the cloth. Later the sandpaper is baked at three increasingly hotter temperatures and a layer of resin is applied and then cured to the gritty surface to complete the binding. On the back of each sheet of sandpaper, an important number can be found. This number is indicates the grade.
All types of abrasives are produced in different grades. With the United States CAMI standard, each grade is determined by the number of abrasive particles per square inch. At the top of the macrogrit level is the 24 grit, an extremely coarse abrasive that can easily peel back a layer or two of hard material quickly. At the top of the microgrit category lies the ultrafine 1000 grit.
Coarse (40-60 grit) is great for fast surface removal.
Medium (60-80) is a great prep and stain remover.
Fine (100-120) is no reason to stop. Keep on sanding!
Very fine (150-240) is around the time when you may be about done. These grits are great for sanding between stain coats.
Extra fine (320-360) can touch-up all kinds of paint finishes.
Super fine (400-600). Feeling fancy, huh? These grits can give wood a glassy touch.
Depending on the material, polished finishes appear around 1000 grit, mirror finishes appear around 1500-2000.
Sanding is one of the most essential steps in the furniture finishing process. A poor sand job can ruin a great design and your ability of finding love. (Kidding!) Luckily, sandpaper comes in various forms that can make it easier for you to get those small corners and cover larger areas without burning your hands out. Perhaps the coolest sanders are orbital disc sanders. The direction of the wood grain is irrelevant as the orbital disc sander, well, orbits in circular disc motions, and allows for large surfaces like floors and cabinets to be handled quickly. Where sanding sheets fail, sanding sponges can save the day. They are extremely malleable, able to sand even the curviest of contours.
Great things take time and sanding is no exception. If you’re beginning with a rough material, jumping to a fine grit will not quicken the process as much as it will quicken your disappointment. Sanding is best done in a succession of at least four steps from coarse to very fine.
Seriously hurting yourself can also put a damper on the last stages of your project. Remember to always keep your materials secure, your eyes goggled, and your breathing holes covered with a mask. Look at a used sheet of sandpaper, think of your lungs, now look at the sheet again. Breathing in dust can clog up your lung tissues and lead to respiratory problems and even lung cancer. You may not always see it, but it’s always there. (The dust, I mean. Not impending doom.) So keep your windows open, spirits up, and always give yourself plenty of time and space to relish in the joy of sanding.