Sandpaper

Sheets of sandpaper with different grits (40, 80, 150, 240, 600).

Sandpaper is a form of paper where an abrasive material has been fixed to its surface.

Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother (painting and wood finishing), to remove a layer of material (e.g. old paint), or sometimes to make the surface rougher (e.g. as a preparation to gluing).

History

The first recorded instance of sandpaper was in 13th century China when crushed shells, seeds, and sand were bonded to parchment using natural gum.

Shark skin was also used as a sandpaper. The rough scales of the living fossil Coelacanth are used by the natives of Comoros as sandpaper.

Sandpaper was originally known as glass paper, as it used particles of glass. Glass frit has sharp-edged particles and cuts well, sand grains are smoothed down and work less well. Cheap counterfeit sandpaper has long been passed off as true glass paper; Stalker and Parker cautioned against it as far back as the 17th century.

Glass paper was manufactured by John Oakey's company in London by 1833, who had developed new adhesive techniques and processes that could be mass-produced. A process for making sandpaper was patented in the United States on June 14 1834 by Isaac Fischer, Jr., of Springfield, Vermont.

In 1916, 3M invented a type of sandpaper with a waterproof backing, known as Wetordry. This allowed use with water as a lubricant, and to carry about particles that would otherwise clog the finest grades. Its first application was for automotive paint refinishing.

Sandpaper has occasionally been used as a surface for painting, as by Joan Miró. Sandpaper was even used as a musical instrument, in Leroy Anderson's Sandpaper Ballet.

Types of sandpaper

320 grit silicon carbide sandpaper, with close-up view.

There are countless varieties of sandpaper, with variations in the paper or backing, the material used for the grit, grit size, and the bond.

Backing

In addition to paper, backing for sandpaper includes cloth (cotton, polyester, rayon), PET film, and "fibre". Cloth backing is used for sandpaper discs and belts, while mylar is used as backing with 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 rounded contours of a given workpiece; relatively inflexible backing is optimal for regular rounded or plane 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.

Material

Materials used for the abrading particles are:

• flint — no longer commonly used
• garnet — commonly used in woodworking
• emery — commonly used to abrade or polish metal
• aluminium oxide — perhaps most common in widest variety of grits; can be used on metal (i.e. body shops) or wood
• silicon carbide — available in very coarse grits all the way through to microgrits, common in wet applications
• alumina-zirconia — (an aluminium oxide - zirconium oxide alloy), used for machine grinding applications
• chromium oxide — used in extremely fine micron grit (micrometre level) papers
• ceramic aluminum oxide — used in high pressure applications, commonly known as Cubitron by 3M, who invented sol gel ceramic grains. Used in both coated abrasives, as well as in bonded abrasives. But the most significant development was made by Norton which patented "Seeded Gel" (SG) grain in 1986 which use seed grain to control crystallite size which is harder than Cubitron, but the affect for the applications still difficult to make comparison between. This bring the 2 companies lead to some several law action regarding the matter.

As well, 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. Aluminium Oxide with stearate is also known as PS33, a Klingspor product.

Innovative abrading surfaces now include long-life stainless steel sanding discs.

Bonds

Different adhesives are used to bond the abrasive to the paper. Hide glue is still used, but this paper often cannot withstand the heat generated when machine sanding and is not waterproof. Waterproof or wet/dry sandpapers use a resin bond and a waterproof backing.

Sandpapers can also be open coat, where the particles are separated from each other and the sandpaper is more flexible. This helps prevent clogging of the sandpaper. The wet and dry sandpaper is best used when wet and when using material like acrylic where it leaves a nice smooth feel afterwards.

Shapes

Sandpaper comes in a number of different shapes and sizes.

• sheet — usually 9 by 11 inches, but other sizes may be available
• belt — usually cloth backed, comes in different sizes to fit different belt sanders.
• disk — made to fit different models of disc and random orbit sanders. May be perforated for some models of sanders. Attachment includes Pressure sensitive adhesive (PSA) and "hook-and-loop" (similar to velcro).
• rolls

Grit sizes

Grit size refers to the size of the particles of abrading materials embedded in the sandpaper. A number of different 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 Manufacturers' 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 Japan Industrial Standards Committee (JIS), the micron grade (generally used for very fine grits). The "ought" system was used in the past in the United States. Also, cheaper sandpapers sometimes are sold with nomenclature such as "Coarse", "Medium" and "Fine", but it is not clear to what standards these names refer.

Grit size table

The following table, compiled from the references at the bottom, compares the CAMI and "P" designations with the average grit size in micrometres (µm).

Grit size table

	ISO/FEPA Grit designation	CAMI Grit designation	Average particle diameter (µm)
MACROGRITS
Extra Coarse (Very fast removal of material)	P12		1815
	P16		1324
	P20		1000
	P24		764
		24	708
	P30		642
		30	632
		36	530
	P36		538
Coarse (Rapid removal of material)	P40	40	425
		50	348
	P50		336
Medium (sanding bare wood in preparation for finishing)		60	265
	P60		269
	P80		201
		80	190
Fine (sanding bare wood in preparation for finishing)	P100		162
		100	140
	P120		125
		120	115
Very Fine (final sanding of bare wood)	P150		100
		150	92
	P180	180	82
	P220	220	68
MICROGRITS
Very Fine (sanding finishes between coats)	P240		58.5
		240	53.0
	P280		52.2
	P320		46.2
	P360		40.5
Extra fine		320	36.0
	P400		35.0
	P500		30.2
		360	28.0
	P600		25.8
Super fine (final sanding of finishes)		400	23.0
	P800		21.8
		500	20.0
	P1000		18.3
		600	16.0
	P1200		15.3
Ultra fine (final sanding of finishes)	P1500	800	12.6
	P2000	1000	10.3
	P2500		8.4

See also

• Coated abrasive‎
• Steel wool
• Emery (mineral)
• Polishing
• Belt sander
• Dremel
• Grinding machine
• Sander

References

Bibliography

• Michael Dresdner (1992). The Woodfinishing Book. Taunton Press. ISBN 1-56158-037-6
• Federation of European Producers of Abrasives
• sizes.com on sandpaper