Backings are the base for the abrasive minerals and, combined with the adhesive bond, support and anchor the abrasive mineral. The backings used in the manufacture of coated abrasives are:

Paper
Due to the fine surface of paper, a consistent finish is produced. Paper weights include A, B, C, D, E and F weights with A being the lightest and most flexible and F being the heaviest and least flexible. A, B, C and D weight papers are used for hand sanding and light mechanical operations in the form of sheets, Speed-Grip and PSA discs and sheet rolls. E and F weight papers are primarily used for more aggressive mechanical operations in the form of belts and discs;

Cotton
Very flexible, heat resistant, and not waterproof. Most widely used in off-hand sanding in wood or light metals;

Rayon
Flexible, stronger & firmer then cotton, resistant to edge wear, not waterproof, will stretch with heat. Used mainly in off-hand or slack-of-the-belt operations for wood mould sanding or metal working;

Polyester
High strength, water resistant, low stretch, not very flexible. Used for heavy stock removal or high-horsepower grinding applications, can also be used for wet-or-dry applications;

Special Polyester
Polyester/Nylon: Improved adhesion, water resistant;
Stitch-bond Polyester: Very low stretch–Higher strength, water resistant, less stretch, high stock removal. Exceptional for heavy stock removal, usually has the premium mineral coating for optimum stock removal in wood or metal working

Vulcanized fibre
Cotton fibres which are chemically treated and then pressed under temperature and pressure to form a very durable backing (Vulcanized, hardened and condensed for strong body). Is used exclusively as the backing for resin fibre discs.

Film
The newest type of backing. Serves as the base for extremely fine mineral grains. Used for precision finishing in computer and electronic industry. Extremely flat surface, resists tearing. Produced in sheets, discs, and belts. Two types of film:
• Polyester (clear or opaque)
• Re-enforced Polyester (mylar – silver)

Various thickness:
• 0.5, 1, 1.5 & 2 mils – Applications: most commonly used for precision finishing;
• 1, 1.5, 2, & 3 mils – Applications: most commonly used for standard finishing;
• 2.5 & 3 mils – Applications: most commonly used for disc sanding;
• 3 & 5 mils – Applications: most commonly used in belts for wood and solid surfacing sanding.

Combinations
• Paper and cloth lamination - Is sturdy and shock resistant. Primarily used for coarse grade floor sanding, high speed drum sanding, and light disc applications;
• Paper and Film lamination - Is sturdy and smooth for exceptional finish.•Applications: Primarily used for medium-to-fine grade sanding on veneers, stroke or wide belt sanding where cupping is a concern.

source: 3M Abrasive Systems - Bob Stanek - Senior Technical Service Engineer & NORTON ABRASIVES

Lap Splice (5% of all splices, mostly paper)
• Unskived - Overlap splice retains full coat of mineral on top lap. Usually used on unsupported or contact wheel work. Used on unsupported belt applications or contact wheels where resiliency will off set any bump caused by the thickness of the splice.
• Skived - Overlap splice has all or part of mineral removed from the top lap to provide a thin, smooth construction.

Butt Splices/Film-Lok (95% of all splices, cloth or paper)
Two ends of the belt are angle cut, butted together and anchored with a plastic film material pressed on the back side of the belt. Recommended primarily for “low-power” operations or those where flex resistance is not required. Can be placed on top of the abrasive when applications of severe backside media wear primarily in medium or coarse belts.

Butt Fabri/Sine-Lok
• Fabri-Lok - Identical to Film-Lok except that a synthetic cloth is used for the splicing media. This splice media possesses high strength, low stretch, and high flex resistance characteristics. Recommended primarily Roll Grinding, Center less Grinding, and Drawer Sanding applications. NOTE: Because of its greater splice thickness than Film-Lok material, splice marking may be encountered with fine grades.
• Sine-Lok - Ends are cut in a sine-wave or dove-tail pattern. Primarily designed for applications requiring rigidity at the splice area to offset hinging or snagging. Also helps offset chatter marks. Gives added strength to the spice while retaining the advantages of film and fabric spices. Used when accounts wash the belts.

source: 3M Abrasive Systems - Bob Stanek - Senior Technical Service Engineer

• No-Flex (no mechanical flex) – Properties: This is standard for sheets in fine grade paper products where flexing is not desirable or necessary;
• Single (perpendicular to length of the web) – Properties: Mechanical flex that creates flex lines at 90°angle to the edge or run direction of the material for conformability (most common flex);
• Double (crisscross flexing of web) – Properties: Two mechanical flexes at 45°angle to the edge (third most common flex);
• Full Flex (flexed both length-wise and cross-wise on the web) – Two 45°angle flexes and a 90°angle flex resulting in a soft and flexible product regardless of direction of bend (2ndmost common flex);
• Longitude Flex (severest flex for conformability) – Mechanical crosswise directional flex allowing conformance to contours.

source: 3M Abrasive Systems - Bob Stanek - Senior Technical Service Engineer

An adhesive bond system is required to secure the abrasive mineral to the backing. All coated abrasive products are made with a two stage bonding process, the make coat and the size coat.

The make coat is the first layer of bond applied to the backing. The make coat provides the adhesive base between the abrasive mineral and the backing. The size coat is the second coat, which is applied over the abrasive mineral and make coat to anchor the abrasive mineral and provide the desired physical strength of the finished product. Glue, urea resin and phenolic resin are the three basic bonding agents most commonly used. There are many size coat and make coat combinations, such as glue over glue, urea over glue, and resin over resin. Glue over glue is the most flexible bond while resin over resin bond is moisture resistant, harder, less flexible, heat-resistant and has superior grain retention.

• Glue and Filler - Gluebond – Animal hide glue for both the make and size coat.
Provides sharp, flexible product, but is moisture and heat sensitive
Applications: Paper product, mainly disc form and in sheets;

• Resin over Glue - Resinite (RN) – Animal hide glue make coat and synthetic resin size coat
Exceptionally heat resistant, slightly moisture resistant (longer life then a Gluebond product)
Applications: Mainly paper products in disc, sheets or light-weight belts;

• Resin over Resin - Resin bond – Synthetic resin used for both the make and size coats
Exceptional heat resistant and has the strongest mineral anchorage. Some formulations have special grinding aids which are added for greater efficiency. Longest life bond.
Applications: Cloth and paper products, mainly belt form;

• Waterproof - Waterproof – Synthetic resin used in the make and size coat on a waterproof backing
Permits use of water, oil, or other lubricants to reduce heat and abrasive wear.
Applications: Cloth products for cut and paper products for finishing.

source: 3M Abrasive Systems - Bob Stanek - Senior Technical Service Engineer & NORTON ABRASIVES

EMERY - Emery is a dark gray, round-shaped grain, naturally occurring mineral, which tends to polish rather than abrade a work surface. Largely replaced by synthetic minerals
• for polishing and cleaning metal only (Metal & Jewelry Polishing);

GARNET - Garnet is reddish brown in color. This natural abrasive is medium hard and relatively sharp, has a tendency to break or re-fracture when in use, thus creating new cutting edges but not as durable as synthetic abrasives. Is being replaced by newer AL OX as mineral of choice for woodworking.
• for use on wood only
• particularly good for soft woods such as pine
• produces an excellent finish;

SILICON CARBIDE - Silicon carbide is the hardest and sharpest of the manufactured abrasives. Because of its extreme sharpness, this bluish-black abrasive grain permits fast stock removal and cool cut.
• cast iron
• non-ferrous metals, i.e. brass, aluminum and bronze
• non-metallic's, i.e. glass, rubber, plastic and stone
• final finish on wood and stainless steel
• abrasive planing particleboard;

LIGHT BROWN ALUMINUM OXIDE - Light brown aluminum oxide is a tough, yet sharp, synthetic abrasive characterized by cool cut, long life and the ability to break down under pressure producing new cutting edges.
• production wood sanding
• non-ferrous metal finishing;

BROWN ALUMINUM OXIDE - Brown aluminum oxide is a tough, durable, synthetic abrasive characterized by the long life and wear resistance of its cutting edges. It offers enormous penetrating strength, even at high speeds.
• ferrous metals
• aluminum
• hardwood;

HEAT-TREATED ALUMINUM OXIDE - Heat-treated aluminum oxide is a tough but cool cutting abrasive which gives both long life and freeness of cut on a wide range of materials.
• ferrous metals
• Wood sanding;

ZIRCONIA ALUMINA - Zirconia alumina is an ultra-tough, synthetic abrasive which provides a free, cool cut for high stock removal applications. It is tougher and sharper than aluminum oxide. It has a micro-crystalline structure which allows for controlled breakdown and self-sharpening.
• heavy duty snagging and grinding of all ferrous and non-ferrous metals
• abrasive planing of wood, plywood and particleboard
• grinding fiberglass, rubber and plastics;

CERAMIC ALUMINA - The sub-micron structure of ceramic alumina allows each grain to continually expose sharp cutting points, resulting in a cooler cutting action and an extended life.
• all ferrous/non-ferrous metals, carbon steel and exotic alloys;

DIAMONDS (Natural & Synthetic) – Hardest known natural or man-made substance.–Green-grayish in color and very blocky shape.–Capable of grinding very hard materials.
• Ceramics, stone, concrete, glass, and very hardmetal alloys such as tungsten carbide.

source: 3M Abrasive Systems - Bob Stanek - Senior Technical Service Engineer & NORTON ABRASIVES

For those unfamiliar with abrasives (i.e., sandpaper, stones, pastes, etc.) they’re typically spec’ed using a “grit” rating. Historically, grit size is equivalent to the “size” of the wire mesh screen used to sieve the abrasive particles, size being the number of wires in a linear inch of the screen. (E.g., 60 mesh has 60 wires each direction in a square inch and is the size commonly used in kitchen faucets, an 80 mesh has an opening which will barely pass a human hair.) All abrasives
are made up of varying sized particles that fall within a size range… for example, a 100 grit stone might be made up of abrasive particles that pass through a size 100 mesh but do not pass through a 120 mesh (remember, the smaller the number the larger the hole in the mesh). For large grit sizes (e.g., for mesh numbers <240), the sieving process actually uses a wire mesh screen-like process. But for the grit sizes we’re interested in (>1000), the sieving process is much more
exotic since we’re dealing with micron-sized particles.

source: Joe? - why diamond pastes have conflicting grit sizes.pdf

ANSI (American National Standard Institute – 1956) - uses three screens made with silk strings to filter the mineral before the final grading.
• Macro Grits 12, 16, 20, 24, 36, 40, 50, 60, 80, 100, 120, 150, 180 & 220
• Micro Grits 240, 280, 320, 360, 400, 500, 600, 800, 1000 & 1200;

FEPA (Federation of European Producers of Abrasive Products - late 60’s) - uses five screens made with brass wires to filter the mineral before final grading. This results in a “tighter” grading system.
• Macro Grits P12, P16, P20, P24, P36, P40, P50, P60, P80, P100, P120, P150, P180 & P220
• Micro Grits P240, P280, P320, P360, P400, P500, P600, P800, P1000 & P1200;

JIS (Japanese Industrial Standard – mid-70’s) - also uses five screens made with brass wires to filter the mineral before final grading. JIS tends to be “tighter” then even FEPA.
• Macro Grits 8, 10, 12, 14, 16, 20, 24, 36, 46, 54, 60, 80, 90, 100, 120, 150, 180 & 220
• Micro Grits 240, 280, 320, 360, 400, 500, 600, 700, 800, 1000, 1200, 1500, 2000, 2500, 3000, 4000, 6000 & 8000;

Micron Particle-size Grading - 'Micron' is based on the mean particle size for the grit. Micron is more focused around the finer grades. Opposite numbering system vs ANSI, FEPA & JIS in that the smaller the grit number, then the smaller the mineral size for finer finish. Micron grading is used mainly on light-weight paper or film products. Micron numbers will have Greek letter “µ” after the grit number.
• Macro Grits 180µ, 150µ, 120µ, 100µ, 80µ & 60µ
• Micro Grits 50µ, 40µ, 30µ, 20µ, 15µ, 12µ, 9µ, 5µ, 3µ, 2µ, 1µ, 0.5µ, 0.3µ, 0.1µ & 0.05µ;

Trizact™ Grading - Trizact is a 3M specific micro-replication based system. Minerals are embedded in pyramid like structures. Trizact grading is similar to micron in that the smaller the number, the smaller the mineral size. Trizact system will have letter “A” in front of the grit number. The process of applying a predetermined…or engineered …surface microstructure or topography with precise dimensions and configurations to various materials.
• Macro Grits A160, A130, A110, A100, A90, & A80
• Micro Grits A65, A60, A45, A35, A30, A25, A20, A16, A6 & A5.

source: 3M Abrasive Systems - Bob Stanek - Senior Technical Service Engineer