Sandpaper, also known ascoated abrasive oremery paper, is a type of material that consists of sheets ofpaper orcloth with anabrasive substance glued to one face.[1]
There are many varieties of sandpaper, with variations in the paper or backing, the material used for the grit, grit size, and the bond. It is common to use the name of the abrasive when describing the paper, e.g. "aluminium oxide cloth" or "silicon carbide paper".
Sandpaper is produced in a range of grit sizes and is used to remove material from surfaces, whether to make them smoother (for example, inpainting andwood finishing), to remove a layer of material (such as oldpaint), or sometimes to make the surface rougher (for example, as a preparation forgluing).
The first recorded instance of sandpaper was in 13th-centuryChina when crushed shells, seeds, andsand were bonded to parchment usingnatural gum.[2][3]
Shark skin (placoid scales) has also been used as an abrasive, and the rough scales ofcoelacanths are used for the same purpose by the natives ofComoros.[4] Boiled and dried, therough horsetail plant is used in Japan as a traditional polishing material, finer than sandpaper.
Cheap sandpaper was often passed off as glass paper; Stalker and Parker cautioned against it inA Treatise of Japaning and Varnishing published in 1688.[5]
Glass paper was manufactured inLondon in 1833 byJohn Oakey, whose company had developed new adhesive techniques and processes, enabling mass production. Glassfrit has sharp-edged particles and cuts well whereas sand grains are smoothed down and do not work well as an abrasive.
In the United States, a major development in the history of coated abrasives was the work ofHenry Hudson Barton. After emigrating from England in 1846, Barton married into a Philadelphia sandpaper manufacturing family. Drawing on his knowledge of gem minerals, he began experimenting withAdirondack garnet as an abrasive. Barton went on to establish mining operations atGore Mountain (New York), in 1878, creating what became the world’s oldest continuously operating garnet mine. Garnet from Barton Mines was widely adopted in coated abrasives and further developments in sandpaper and remains a significant abrasive mineral today.[6]
Depending on the desired application, different backing materials may be used for sandpaper. Some are more flexible, allowing sandpaper to follow irregular contours, while others are thicker and more suitable for heavy use.
Paper is the cheapest and the most used backing for coated abrasives, hence the common name "sandpaper". Paper backing comes in five weight categories: "A", "C", "D", "E" and "F", with A being the lightest and F the heaviest. For heavy-duty applications, such as disks for electric sanders, the paper may be reinforced with extra cloth fibers, resin or other additives.[7][8]: 23–24
In addition to paper, the most often use backing materials for coated abrasives are cloth (natural or synthetic),PET film andvulcanized fibre.[8]: 22
Cloth backing typically comes in weights "J", "X" and "Y", from lightest to heaviest. Some manufacturers use additional weight designators, like "F", "JF" or "XF" for variants with extra flexibility or "YY" for an even heavier option.[9][10] Cloth is preferred for its high durability, and used in applications such as sanding belts.[8]: 25
PET film and vulcanized fibre are other high durability options. Film is also chosen for a very even thickness across the sheet, which yields a clean sand pattern. It comes in weights from 1 to 7.[8]: 26 Fibre is a thick and extremely strong backing, generally only used for sanding disks.[7][10]
Paper backing may be coated in a layer of a waterproof material, such as latex. Those types of sandpaper, called "wet/dry"[10] or "wet and dry", allow the use of a water-based lubricant, which is required for some applications.[8]: 24 Wet/dry paper was invented by3M in 1921 and sold under the brand name "Wetordry".[11]
Wet/dry sandpaper is more effective when used wet, because the water washes particles away from the grinding surface and reduces clogging.[12]
The backing is coated in abrasive particles using an adhesive. Different parameters of the coating, such as the particle composition, size, or used adhesive can be changed to produce sandpaper suitable for different purposes.
Most sandpaper in the past usedflint as the abrasive material. However, flint particles are too soft for most kinds of sanding, so flint paper is now barely used.[13][8]: 10
Instead, most coated abrasives nowadays use synthetic grit material. The most commonly used one isaluminium oxide (commonly abbreviated asALO). Aluminium oxide paper is strong, inexpensive and suitable for a wide variety of uses for both wood and metal.Ceramic materials such asalumina zirconia are a more premium alternative, offering superior sharpness and longevity for an expensive price.Silicon carbide is another common synthetic material. It is available in very fine grit sizes and is commonly used forwet sanding.[14][15][13][8]: 11–15
Some natural materials are also still in use. For example,emery is used for metal polishing,garnet is used for wood finishing, while crocus paper (made fromiron oxide particles) is used for fine polishing of soft metals.[13][15][8]: 7–11
Some abrasive materials are designed to have a property calledfriability. Under the influence of heat produced from sanding, friable materials fracture at a controlled rate to expose new sharp edges. This allows them to last much longer compared to non-friable grits, which get dull and lose their abrasive properties with use.[8]: 13
Grit size is the size of the abrasive particles attached to the sandpaper. Sandpaper comes in a variety of grit sizes, ranging from very coarse (≈2 mm) toultrafine (<1 µm). Coarser grit materials simplify sanding of hard surfaces, but also make it easier to accidentally damage fine details.
Several standards have been established for grit size and permissible tolerances. The most prevalent standards are:
Some sandpapers may sometimes only use descriptive nomenclature such as "coarse", "medium" and "fine", without referring to any standard. Those names are not regulated in any way and may differ between manufacturers.[8]: 16–17
To achieve a uniform grit size, the crushed particles of theabrasive material must be filtered by size. Coarse grit particles are passed through a series of successively finer test sieves, with each successive layer blocking finer and finer particles. Medium grit particles are usually separated with air classification, which uses air flows to separate particles by weight. For the finest particles, water classification based onsedimentation is used, in which the particles are separated based on how quickly they settle to the bottom of the liquid tank.[16][8]: 17, 20
Sandpaper can be eitherclosed coat oropen coat. In closed coat sandpaper, approximately 90% to 95% of the surface is covered with abrasive grains. Closed coat sandpaper is good for hand sanding or working with harder materials. In comparison, 50% to 70% of the surface is covered with abrasive grains with open coat sandpaper. The separation between particles makes the sandpaper more flexible, which prevents the sandpaper from clogging. However, the gaps in grit coverage limits the sandpaper's ability to perform even polishing jobs. Open coat sandpaper is better for softer materials.[17][18][19][8]: 27
Different adhesives are used to bond the abrasive to the paper.Hide glue is still used, but this glue often cannot withstand the heat generated during machine sanding and is not waterproof. Waterproof sandpapers or wet/dry sandpapers use a resin bond and a waterproof backing.[18][8]: 26–30
Adhesives are commonly applied in two layers: themake-coat adhesive is applied before the abrasive coating and binds it to the backing, while thesize-coat adhesive is applied after the coating to further fix it in place.[8]: 6–7, 28
In traditional gravity coating process, a long ribbon of backing material sequentially receives the coating layers in the correct order. An alternative to that is electrostatic coating, which passes the adhesive-coated backing material and the grit particles through electric fields, causing the backing to receive negative charge and the grit to receive positive charge. This causes the particles to attract to the backing, sticking to the glue with higher strength than in gravity coating.[8]: 6–7
Sandpaper may be "stearated" where a dry lubricant (usuallyzinc stearate) is loaded to the abrasive. Stearated papers are useful in sanding coats of finish and paint as the stearate "soap" prevents clogging and increases the useful life of the sandpaper.[13][8]: 30
Sandpaper comes in a number of different shapes and sizes. The most typical shape is a 9 by 11 inches (23 by 28 cm) flat sheet,[8]: 89 but other variants include abrasive rolls, sponges, belts forbelt sanders and disks for disc and orbit sanders.[8]: 32–38
A quick-change system is commonly used with disc-type coated abrasives. A plastic or metal hub is bonded to one of the faces, which is threaded. This then mates directly to thesander orangle grinder or to a mandrel that can be mounted in a sander, grinder, ordrill. The advantage is that the disc can be quickly replaced when needed. Quick-change discs are available in sizes of about 50 millimetres (2.0 in) and larger.[20]
