Sandpapers use different amounts of abrasive particles, known as grit, to remove layers of old paint, to smooth surfaces for painting or finishing or to make surfaces rougher for better gluing. The type of project and type of surface determine how fine or coarse the grit should be.
As sandpaper is repeatedly rubbed across or around surfaces, the abrasive side loosens layers of paint or finish and smooths the surface. Grit refers to the number of abrasive particles on each square inch of sandpaper, according to the WoodZone website. The lower the grit number, the rougher the sandpaper surface will be. Coarse, low-number grits generally are used for heavy sanding and stripping or adding roughness to a surface. Finer grits, with higher numbers, gently finish woods.
Coarse sandpaper with 40 to 60 grit easily strips and roughs up surfaces. Medium sandpaper with 80 to 120 grit is best for removing small imperfections and marks on surfaces. Final sanding before finishing wood requires 150 to 180 grit fine sandpaper.
Very light sanding projects should be undertaken with one of several varieties of increasingly fine-grit papers. Sandpaper with a very fine grit of between 220 and 240 particles per inch gently smooths surfaces between coats of stain, sealer or paint. Extra-fine paper with 280 to 320 grit lightly removes marks between finish coats. Super-fine 360 to 600 grit paper removes blemishes or scratches on finished surfaces and can be used to reduce lustre on shiny surfaces, according to the WoodZone website.
Sandpaper is sold in sheets, belts, discs and rolls of varying sizes for personal, commercial and industrial use, according to the Abrasive Oasis website. Uses for each type of sandpaper and grit may vary for different woods or different surfaces.
Sandpaper consists of three components: the grit, the paper or fabric backing and the adhesive attaching the grit to the paper. Industrial-grade sandpapers generally last longer than commercial-grade papers, according to the WoodZone website. While the name sandpaper implies using sand as an abrasive, contemporary sandpaper uses aluminum oxide, garnet, silicon carbide or ceramic abrasives.
Sandpaper use can be traced back to 13th century China, when it was made of crushed shells, seeds and sand, according to the Abrasive Oasis website. Issac Fisher, Jr. patented the first mass-production process for making sandpaper in 1834. Glass was used as grit into the 19th century, so the product was known as glasspaper. The 3M company in 1916 started developing new abrasives for different types of sanding projects, including metal and automotive refinishing.
Stronger, longer-lasting backings also were developed throughout the 20th century, including a durable cloth made of polyester, cotton or rayon. Plastic films such as Mylar also are used as sandpaper backing, according to the Abrasive Oasis website. Backing ratings range from A to F, with A being the lightest and F being the heaviest. Flexible cloth or plastic backings facilitated the use of sanding belts.
The sanding process releases tiny particles into the air that can be inhaled, so dust masks and safety goggles should be worn during any sanding project, according to the Abrasive Oasis website. Because sanding metal surfaces can cause sparks, cotton fabrics should be worn. Polyester fabric can melt and burn or stick to skin. Safety gloves protect hands from abrasion during sanding projects.