What is a sandpaper?
Sandpaper is a coated abrasive made of paper or another type of backing with abrasive material glued to one face. The old name for sandpaper was glasspaper. Despite the name, sand or glass as an abrasive material are no longer used. They have been replaced by modern abrasives like aluminum oxide and silicon carbide. Sandpapers are produced from very coarse to extremely fine grades. The grit size of the abrasive material defines the way how the sandpaper cuts and where it is used.
Sandpaper is engineered for specific uses, depending on the material to be abraded and whether the paper is designed for hand- or machine sanding.
Sandpaper is really a cutting tool. When looking up close the grains of abrasive look like sharp, tiny, irregular shaped teeth. The sharp edges of the abrasive grains cut out wood shavings but simply on the microscopic scale. Shavings are so small that it is generally labeled as a fine dust but magnified it looks like a shaving produced any cutting tool. In industry jargon, the sandpaper shaving is called swarf.
What is a sandpaper used for?
Sandpaper is used to remove mill marks and other flaws such as dents and scratches from the surface, to smooth out or to roughen the surface for finishing or gluing, or to remove layers of material from the surface.
What is a sandpaper made of?
Sandpaper consist of backing, abrasive, and an adhesive material to bond the backing and abrasive grains together. Some types of sandpaper are covered with stearates to prevent clogging the pores.
Backing material of the sandpaper varies depending on the intended usage. For light, hand sanding thin and lightweight backing is used. Heavy duty and fast material removal require thick and durable backing. Backing materials are made from paper, cloth (cotton, rayon, and polyester), PET, fiber, and rubber
Paper backing for sandpapers are rated in five weights: A, C, D, E, and F, where A is the lightest weight for light sanding and F is in the other end for toughest material removal jobs.
Grades of the paper backing are:
- A-weight (70g / 40 lbs) -Light and flexible. A weight is primarily used for sheet products used on hand sanding operations in grits 80 and finer.
- C-weight (120g / 70 lbs) -Stronger and less flexible than A weight. This backing is chosen for hand sanding and for use on small, portable power sanders. Used for intermediate sanding in grits 36 through 80.
- D-weight (150g / 90 lbs) -Stronger and less flexible than C weight. This backing is also used for hand sanding and small, portable power sanders on coarse and intermediate grits 24-80.
- E-weight (220g / 130 lbs) -Stronger and less flexible than D-weight, this backing is primarily used on a roll, belt and disc applications where high resistance to tearing is needed.
- F-weight (300g / 165 lbs) -The strongest, least flexible paper backing utilized. Used primarily for heavy-duty sanding discs, sheet goods and sanding belts used in the floor sanding market and paper wide belts used in woodworking applications.
Cloth backing – for more durability
Cloth backings are used in applications requiring more durability, resistance for tearing and tolerance for continual bending and flexing. Sanding belts and discs are usually made with the cloth backed material. Cloth backing is also graded by thickness.
Grades of the cloth backing are:
- J-weight (Jeans) is lightest and most flexible cloth backing. Ideal for finishing and where flexibility of abrasive material are required, for example, contours and curved surfaces.
- X-weight (Drills) is stronger and stiffer material for coarse grits. The thicker backing material gives longer product life.
- Y-weight (Heavy duty) is the strongest of the cloth materials designed to withstand extreme pressures and heavy-duty stock removal application. A long life abrasive belt material.
Fibre backing – for greatest strength
Fibre or fiber backing has the greatest strength of any coated abrasives. It is hard and strong. Fibre backings are made of many layers of impregnated paper which still provides sufficient flexibility for an intended application. Used on resin bonded fibre discs.
Film backing – for micro grades and finest finishes
Polyester film backings are used in micro finishing wet and dry sandpapers. Film backing has a good resistance to chemicals while being tear resistant.
Combination backing – for special applications
Combination backings are designed for special applications. It is constructed by laminating cloth and paper backings together to get suitable properties.
Types of abrasive materials
The abrasive material is the “blade” of the sandpaper. Its features define the quality of the sandpaper and the speed and quality of the cut. Abrasive grains are sharp “shards” of hard material that cut wood fibers. The great abrasive material is suitably hard, durable and sharp but at the same time friable to maintain the sharp edges by its tendency to fracture and reveal fresh sharp edges.
Aluminum oxide – the most common abrasive material
The most common sandpaper abrasive material used is the synthetic aluminum oxide. In the production of aluminum oxide, the crystalline structure and its chemistry can be modified, allowing the production of differently behaving abrasive grains. The versatility of aluminum oxide makes it the most commonly used abrasive. It works well on bare wood, paints, lacquers, and metal. When sanding, the aluminum oxide grains constantly cracks and breaks of revealing new sharp edges which helps the aluminum oxide sandpaper to last longer. Comes with the widest variety of grits and is relatively cheap to manufacture.
- Aluminum oxide is a very tough and durable
- Works well on hardwoods, carbon steel, alloy steels, and bronze. Premium heat-treated variants of aluminum oxide offer extended life.
- Higher grades feature friable grains and have a significantly longer life
- Low friability in lower and cheaper grades. The absence of clearly defined crystal structure which causes the sharp edges of the grain to blunt and round over instead of fracturing and revealing fresh sharp edges.
Other widely used abrasive is silicon carbide. It is sharper and harder than regular aluminum oxide. Grains are more needle like that resemble broken glass. It is too friable for bare wood use but it is an excellent choice for smoothing finishes between coats and rubbing out film finishes. Works also well with plastics and fiberglass. When used with waterproof backings and with water or oil its durability and life is extended considerably. Silicon carbide is offered from very coarse to very fine grits.
- Can be used in wet or dry applications
- Silicon carbide grains are among the hardest synthetic abrasives
- Excels in sanding between coats of hard finishes like lacquer, epoxy, and polyurethane
- Useful also with plastics and some metals.
- If used on bare wood, the dark, brittle nature of grains can discolor the light colored wood surface.
The glass was used historically as an abrasive but it is almost completely replaced by modern materials. Still used in French polisher circles for cutting back cured finish surfaces. Available in sheets.
- Inexpensive to produce
- Works well with traditional French polished shellac finish.
- Cuts very slowly
- Wears and loads easily
- Useful for hand sanding only
Flint is a natural abrasive for very special use. It is used in flammable environments where sparks from abrasives must be eliminated. Performance of flint abrasive is similar to garnet.
- Doesn’t produce sparks when used with metals
- Wears out fast.
- Limited availability
Garnet is also a natural abrasive made by crushing the garnet gemstone. It is still used in light, hand sanding applications in woodworking. Its friability is not good and it tends to wear out faster than aluminum oxide. Because of this garner produces a smoother surface than other abrasives of the same grit numbers by burnishing and sealing off wood grain. It is an excellent choice as a final sanding abrasive.
- Leaves smoother surface than similar grit aluminum oxide when sanding by hand.
- Great abrasive for softer woods.
- Abrasive particles wear out fast under pressure.
- Limited availability
Emery is a mixture of corundum and magnetite. It is a commonly used to abrade and polish metals. Available in sheets, strips, and rolls.
- Work quite well with metal finishing and glass grinding applications
- Slow cutting
- Scratch pattern is not suitable for wood finishing.
- Can discolor wood.
High performance abrasive for machine grinding applications. Zirconia alumina is a mixture of zirconium dioxide and aluminium oxide.
- Extremely sharp and durable self-sharpening grains with very good friability
- Great heavy-duty stock removal properties
- Expensive to produce
- Limited availability of grit sizes. Grits ranging from extra coarse p24 to medium fine p180
Diamond is used for finishing and polishing hard metals, ceramics, and glass.
- Diamond is extremely durable abrasive with really long service life when used on a variety of hard and tough materials
- Gives superior finish quality due to an excellent uniformity of grit sizes.
- Best abrasive for sanding stone and glass
Ceramic aluminum oxide
Modern high performance abrasive with long life and fine micro-grain structure. Used in high-pressure applications. Longer life than alumina-zirconia.
- Superbly hard-wearing abrasive. Lasts 5-7 times longer than common abrasives when used on wood
- Good friability due to fine micro-grain structure
- The best abrasive for heavy shaping and the production sanding of wood
- Grit sizes are limited to 80-220
Sandpaper grit sizes
What is a sandpaper grit size?
The grit size is an indication of the abrasive particle size embedded in the sandpaper or other abrasive mediums. The grit size of the abrasive is stated as an inversely related number to the particle size. The higher the grit number (1200 for example), the finer the abrasive and the surface produced. Lower grit numbers, like 24 or 40 represent bigger and coarser particles which remove and cuts material faster resulting rougher surface.
Grit size standards
Several methods and standards have been established for grading the grit size and allowed variation from the average size. All of the standards use the size as an indication of how aggressive the abrasive coating will be. In the United States, CAMI standard dominated the abrasive field for decades. In Europe, manufacturers used their own standard called FEPA. In recent years more and more manufacturers around the globe are started to grade their abrasives with FEPA standard and it is nowadays the most widely used standard everywhere. There is also a couple of other grading standards used including JIS, MICRON and MM for Micromesh products. Grit sizes are also divided into macro and micro grits.
Grit sizes graded with FEPA standard are identified with prefix “P” in front of grit number (P240 for example). FEPA stands for (Fédération Européenne des Fabricants de Produits Abrasifs) Federation of European Producers of Abrasives. FEPA Grit size scale ranges from extra coarse P12 to ultrafine P5000. The FEPA system is also the ISO 6344 standard. Compared to CAMI, FEPA grading requires that a higher percentage of the abrasive particles on the paper have to be close in size to the stated grit.
CAMI stands for “Coated Abrasives Manufacturing Institute”
JIS standard is used in Japan and is similar to the FEPA standard. JIS stands for “Japanese Industrial Standard” issued by the Japanese Standards Association.
is the letter “micron” in the Greek alphabet. As measuring unit it describes 1 thousandth of a millimeter (1 μ = 0.001 mm). The particles are measured in micrometers. A micrometer, also known as a micron, is one-millionth of a meter, or one-thousandth of a millimeter.
MM (Micro mesh)
Micromesh products are graded with MM rating.
Abrasive grains are bonded to the backing by two layers of adhesive or resin. The backing is primed with the first layer, called the make coat, over which the abrasive particles are applied. Another coat called, the size bond is spread over the particles to secure them permanently to the substrate. The thickness of the first and second coat varies depending on the intended use of the sandpaper. The thinner the coats, the more abrasive is left exposed and the more aggressively the sandpaper cuts.
Hide glue was used traditionally as an adhesive to bond the abrasive to paper backing but it has been replaced by modern adhesives and resins.
Sanding soft and resinous woods, soft metals, and paints and lacquers with common sandpaper are troublesome because of its tendency to clog easily. Within a short period of time, all the pores are stuffed full of sanding debris. To make the sandpaper less prone to clogging and to prolong its life, sandpaper manufacturers use special stearates or dry lubricant to make sandpaper less sticky.
Use of started sandpaper with water-based finishes is not recommended. The residues of the lubricant on the wood can cause fisheyes or other flaws in the finished surface.
What is stearated sandpaper?
Stearated sandpaper is an abrasive which is used to sand resinous woods, paint, and soft metals that build-up and clog easily in the pores of non-stearated sandpapers. The stearated lubricant used prevents clogging and extends the life of the sandpaper.
Sandpaper types – open or closed abrasive coating?
The abrasive coating can be either open or closed coat. In an open coated sandpaper abrasive particles are distributed to cover 40% to 70% of the total surface area leaving empty spaces for swarf or sanding dust to gather into. Open coated sandpapers tend to clog slower than closed coat papers. Open coat sandpapers are commonly used in woodworking. Open coated sandpaper is particularly good with soft and resinous woods like pine, fir, spruce, larch, cedar, cypress and redwood and soft metals such as aluminum, brass, bronze, and copper.
In closed coat sandpaper, the abrasive covers the entire surface. Closed coat sandpaper cuts faster due to more cutting particles per area but clogs faster. Closed coat sandpaper is best used on hardwoods such ash, cherry, oak and walnut, and harder metals like iron and steel.
What is the difference between an open coat and closed coat sandpaper?
Difference between open and closed coat sandpaper is in the coverage area of the abrasive particles. On an open coat, the abrasive particles cover only a 40 to 70% of the total area whereas closed coat is totally covered.
Wet and dry sandpaper
Wet-and-dry sandpaper is special waterproof coated abrasive that can be used with lubricants such as water or oil. Dust and debris are held under control when sanding and easily washed away. Wet and dry sandpapers tend to last longer than common abrasives.
Wet-and-dry sandpapers are widely used in car finishing and refinishing shops, guitar making, furniture finishing shops etc.
With ultra-fine wet or dry sandpapers (up to 12000 grit) surfaces can be polished to high shine easily.
Sandpaper Formats and sizes
Sandpapers and coated abrasives are available in a wide variety of formats and sizes. It is manufactured in wide widths and roll lengths that are cut to suitable sizes and processed further for example when making sanding belts, sleeves, flap wheels etc.
A common format for sandpaper is a sheet. The standard sheet size of sandpaper/coated abrasives is 230x280mm (9 by 11 inches). Sandpapers are also available with a fractional size such as quarter, one-third and half sheets. Sheets are available in all manner of abrasive materials and backings.
Sandpaper rolls is an economical way to get sandpaper. It is usually cheaper to get per sqm/sqft over precut sandpaper sheets. Sandpaper can be cut to the desired length and the remaining can be saved. They come in a wide variety of compositions and product types. They are available in almost all backings and abrasive materials.
Sanding disks are available in many sizes and forms for every grinding and sanding task. Types of the sanding and grinding disk are Fiber-disc and Flap-disc for angle grinders and drills, perforated and non-perforated paper discs for ROS-sanders, cloth and film disks for special applications and surface treatment and heavy-duty floor sanding discs for wood and concrete. They are made in the round, triangles, and rectangles. For random orbit sanders, the discs are backed with a hook and loop fabric for easy mounting.
Sandpaper / abrasive sleeves
Sandpaper sleeves are cylinders that fit over rubber drums used, for example, on drill presses or stationary oscillating sanding machines. They are made with various grit, abrasive materials and backings depending on the intended usage.
Sandpaper flap wheels
Flap wheels are special flexible sanding tool designed to conform to the shape of the object being sanded. They sand both the high and low areas with the same effectiveness. They are made from small drill mounted sizes to large sanding machine wheels.
Sandpaper / abrasive belts
Sanding belts are used in woodworking and metalworking sanding machines. They are manufactured from very narrow and short file belts used in hand band files / mini belt sanders to wide and big belts used in wide belt sanding machines.
The sanding sponge is a useful hand sanding tool made of foam core and abrasive grains on two or more sides. The main advantage of the sanding sponge is its ability to conform to profiled and rounded surface. It is good for detailing small areas and can be used either wet or dry. Sanding sponges are available in couple different sizes and grits. It is also available with diamond abrasive for sanding edges of tiles and stone, repairing glass chips and edges, as well as composites.
Read more about best sandpapers for wood: BEST SANDPAPER FOR WOOD – MONEY AND TIMESAVERS REVEALED