Sandpaper and glasspaper are names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to one face.
There are many varieties of sandpaper, with variations in the paper or backing, the material used for the grit, grit size, and the bond.
In the modern manufacture of these products, sand and glass have been replaced by other abrasives such as aluminium oxide or silicon carbide. It is common to use the name of the abrasive when describing the paper, e.g. "aluminium oxide paper", or "silicon carbide paper".
Sandpaper is produced in a range of grit sizes and is used to remove material from surfaces, either to make them smoother (for example, in painting and wood finishing), to remove a layer of material (such as old paint), or sometimes to make the surface rougher (for example, as a preparation for gluing). The grit size of sandpaper is usually stated as a number that is inversely related to the particle size. A small number such as 20 or 40 indicates a coarse grit, while a large number such as 1500 indicates a fine grit.
The first recorded instance of sandpaper was in 13th-century China when crushed shells, seeds, and sand were bonded to parchment using natural gum. In the Bible, King Solomon is mentioned to have used a mysterious abrasive called shamir allowing the king to build his temple (e.g. cut huge blocks of stone) without using iron tools, since the temple was meant to be a place of peace and iron was used in war. Shamir was also held in Hebrew lore as being a magical worm capable of cracking glass when resting on it.
Shark skin (placoid scales) has also been used as an abrasive and the rough scales of the living fossil, Coelacanth are used for the same purpose by the natives of Comoros. Boiled and dried, the rough horsetail plant is used in Japan as a traditional polishing material, finer than sandpaper.
Glass paper was manufactured in London in 1833 by John Oakey, whose company had developed new adhesive techniques and processes, enabling mass production. Glass frit has sharp-edged particles and cuts well whereas sand grains are smoothed down and do not work well as an abrasive. Cheap sandpaper was often passed off as glass paper; Stalker and Parker cautioned against it in A Treatise of Japaning and Varnishing published in 1688.
In 1921, 3M invented a sandpaper with silicon carbide grit and a waterproof adhesive and backing, known as Wet and dry. This allowed use with water, which would serve as a lubricant to carry away particles that would otherwise clog the grit. Its first application was in automotive paint refinishing.
In addition to paper, backing for sandpaper includes cloth (cotton, polyester, rayon), PET film, "fibre", and rubber. Cloth backing is used for sandpaper discs and belts, while mylar is used as backing for extremely fine grits. Fibre or vulcanized fibre is a strong backing material consisting of many layers of polymer impregnated paper. The weight of the backing is usually designated by a letter. For paper backings, the weight ratings range from "A" to "F", with A designating the lightest and F the heaviest. Letter nomenclature follows a different system for cloth backings, with the weight of the backing rated J, X, Y, T, and M, from lightest to heaviest. A flexible backing allows sandpaper to follow irregular contours of a workpiece; relatively inflexible backing is optimal for regular rounded or flat surfaces. Sandpaper backings may be glued to the paper or form a separate support structure for moving sandpaper, such as used in sanding belts and discs. Stronger paper or backing increases the ease of sanding wood. The harder the backing material, the faster the sanding, the faster the wear of the paper and the rougher the sanded surface.
Types of abrasive materials include:
Sandpaper may be "stearated" where a dry lubricant 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.
The harder the grit material, the easier the sanding of harder surfaces like hardwoods such as hickory, pecan, or wenge. The grit material for polishing granite must be harder than granite.
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.
Sandpaper can be either closed coat or open coat. Approximately 90% to 95% of the surface is covered with abrasive grains with a closed coat. 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.
Wet and dry sandpaper is more effective used wet because clogging is reduced by particles washing away from the grinding surface.
Sandpaper comes in a number of different shapes and sizes:
Grit size refers to the size of the particles of abrading materials embedded in the sandpaper. These measurements are determined by the amount of the abrasive material that can fit through a square inch filter. Several standards have been established for grit size. These standards establish not only the average grit size, but also the allowable variation from the average. The two most common are the United States CAMI (Coated Abrasive Manufacturers Institute, now part of the Unified Abrasives Manufacturer's Association) and the European FEPA (Federation of European Producers of Abrasives) "P" grade. The FEPA system is the same as the ISO 6344 standard. Other systems used in sandpaper include the Japanese Industrial Standards Committee (JIS), the micron grade (generally used for very fine grits). Cheaper sandpapers may sometimes only use descriptive nomenclature such as "coarse", "medium" and "fine" without referring to any standard.
The following table, compiled from the references at the bottom, compares the CAMI and "P" designations with the average grit size in micrometres (µm).
|ISO/FEPA Grit designation||CAMI Grit designation||Average particle diameter (µm)|
|Extra Coarse (Very fast removal of material, hardwood flooring initial sanding)||P12||1815|
|Coarse (Rapid removal of material)||P40||40||425|
|Medium (sanding bare wood in preparation for finishing, for gentle removal of varnish, also used for skateboard grip tape)||60||265|
|Fine (sanding bare wood in preparation for finishing, not suitable for removing varnish or paint from wood, use for cleaning plaster and water stain from wood)||P100||162|
|Very Fine (sanding of bare wood)||P150||100|
|Very Fine (sanding finishes between coats)||P240||58.5|
|Extra fine, start polishing of wood||320||36.0|
|Super fine (final sanding of finishes, final sanding of wood)||400||23.0|
|Ultra fine (final sanding and polishing of thick finishes)||P1500||800||12.6|