Sandpaper

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

Sandpaper or glasspaper are generic names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to one face. Despite the use of the names neither sand nor glass are now used in the manufacture of these products as they have been replaced by other abrasives. 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). It is common to use the name of the abrasive when describing the paper, e.g. "aluminium oxide paper", or "silicon carbide" paper.

History

The first recorded use of sandpaper was in 1st-century China when crushed shells, seeds, and sand were bonded to parchment using natural gum. 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 by 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.

Types

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

There are many 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", or 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.

Type of abrasive

Types of abrasive materials include:

• glass: no longer commonly used
• flint: no longer commonly used
• garnet: commonly used in woodworking
• emery: commonly used to abrade or polish metals
• aluminium oxide: The most common in modern use, with the widest variety of grits, lowest unit cost; 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(III) oxide: used in extremely fine micron grit (micrometre level) papers
• diamond: used for finishing and polishing hard metals, ceramics and glass

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 surfaces like wood. The grit material for polishing granite slab must be harder than granite.

Bonds

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 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. Wet and dry sandpaper is more effective used wet because clogging is reduced by particles washing away from the grinding surface. Arguably there are also benefits due to lubrication and cooling.

Shapes

Sandpaper comes in a number of different shapes and sizes:

• sheet: usually 9 by 11 inches (280x230 mm), 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: known as "shag rolls" by many contractors
• sponge: for tight places

Grit sizes

Grit size refers to the size of the particles of abrading materials embedded in the sandpaper. Several 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 Japanese Industrial Standards Committee (JIS), the micron grade (generally used for very fine grits). The "ought" system ({0, 00, 000, ...} aka {1/0, 2/0, 3/0, ...}) was used in the past in the US. Cheaper sandpapers sometimes use nomenclature such as "Coarse", "Medium" and "Fine", but it is unclear 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, hardwood flooring initial sanding)	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, for gentle removal of varnish, also used for skateboard grip tape)		60	265
	P60		269
	P80		201
		80	190
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
		100	140
	P120		125
		120	115
Very Fine (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, start polishing of wood		320	36.0
	P400		35.0
	P500		30.2
		360	28.0
	P600		25.8
Super fine (final sanding of finishes, final sanding of wood)		400	23.0
	P800		21.8
		500	20.0
	P1000		18.3
		600	16.0
	P1200		15.3
Ultra fine (final sanding and polishing of thick finishes)	P1500	800	12.6
	P2000	1000	10.3
	P2500		8.4
	P3000		6
	P6000		4

See also

Similar to sandpaper

• Emery cloth
• Crocus cloth

Tools used with sandpaper

• Sanding block
• Sander
• Belt sander

Related processes and tools

• Polishing
• Grinding
• Grinding machine
• Rotary tool

Miscellaneous

• Sanding sealer
• Emery (mineral)
• Particle size (grain size)

Further reading

• Michael Dresdner (1992). The Woodfinishing Book. Taunton Press. ISBN 1-56158-037-6

External links

• Federation of European Producers of Abrasives
• sizes.com on sandpaper
• Conversion Chart Abrasives - Grit Sizes