Sandpaper is typically used in woodworking, but also has its applications in auto-body and metal finishing. Sandpaper is vital to preparing wood and metal for final finishing. Speciality sandpapers are used with water to polish metal and sometimes table tops and guitar bodies when heavy coats of nitrocellulose or acrylic lacquer are built up.
Everyday sandpaper commonly comes in 8-by-11-inch sheets that you can cut down for any use, 4-inch round discs with a sticky back that stick to orbital sanders, and smaller 3-by-11-inch strips already cut for electric handheld vibrating sanders. Other woodworking sandpaper comes in small paper belts that are 3 by 24 inches and fit on most small handheld belt sanders. Sandpaper can be just about any colour of the rainbow, ranging from orange to black to white to green. The colours may represent certain manufacturers' grit sizes, but overall the colours mean nothing when comparing grits from other brands, since they are not standardised across manufacturers
Grit is the term for the abrasive surface on the sandpaper. The abrasive is the rough, gravel-like surface that does all the work. Standard woodworking grits range from 50 to 220. The smaller the number, the larger the grit. Therefore, if you want to remove lots of wood fast, such as when shaping a piece of wood for a sculpture, you would use 50 or 60 grit. If you are sanding cabinets or furniture, start out with 100 grit on a hand block or an orbital sander. Blend and sand the project with the 100, and then, just before staining and lacquering, put a piece of 180 or 220 grit on a hand block and lightly sand the project before spraying any finish.
Wet sandpaper is used for metal and some woodworking when a glassy, polished look is required. Wet sandpaper ranges up from 220 to 600 grit. Wet sandpaper is often black in colour and almost smooth because the grit is so small. To use wet sandpaper, you can put it on a block if you want, or just palm it in your hand. For metal, apply a small amount of water to the metal after you have sprayed the primer on it. Use the wet sandpaper in the water by slowly moving the sandpaper around in small circles. When the sandpaper dries out, add more water. Start out with 220, switch to 400, and do your final polishing with 600 grit for a glasslike finish on cars, or on fine woodworking projects that have extremely heavy lacquer coats.
Sandpaper belts are used on small belt sanders. They have the same grit structure as woodworking sandpaper, but usually only run the heavy grits like 50 or 80. Some sandpaper belts even have 30-grit abrasives. These types of low grit sandpaper belts are used only to remove material fast, such as old finishes, or for sanding down decks that have been exposed to weather for a long time. Belt sanders are powerful workhorses. The belts seldom exceed 100 grit because sanding belts can actually do damage to plywood or fine furniture due to their aggressive nature.