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Abrasives 101

Abrasives - Definition & Use

Topics Covered: 

We often use common everyday products like abrasives without putting much thought into them. While abrasives are fairly simple in concept and ubiquitous in use, the multitude of different products and the definitions of various product attributes can make understanding and selecting the proper abrasives complex. The goal of this guide is to introduce you to the world of abrasives and help you select the best abrasive for your needs. When the proper abrasive product is used, you’ll be left with a finished piece that meets or exceeds expectations. 

Abrasive Basics

What Are Abrasives?

An abrasive is a material used to finish (the act of polishing or making a surface smooth) or shape a workpiece through friction. The materials used on abrasives to create friction are often minerals. Abrasives are commonly mislabeled as sandpaper by the general public, but that is a misnomer because neither sand nor glass is used to manufacture abrasive products anymore. 

Abrasives are used for many different industrial, consumer, and technological applications. Abrasives can be used for cutting, grinding, polishing, drilling, sharpening, lapping, buffing, honing, and sanding among many other things. The wide range of uses has given birth to many different types of abrasive products. 

How Do Abrasives Work?

As previously mentioned, abrasives rely on friction to remove material and smoothen out workpieces. While any two solid materials will wear each other away when repeatedly rubbed together over time, abrasive materials work well and last longer because they tend to be harder than the material that is being worked on. 

Abrasive grains (also called grit) have rough edges and when the grains come into contact with a material while in motion, the grains break away fragments of the workpiece. There are many different factors that determine how effective an abrasive will be. Factors include: 

  • The relative hardness of an abrasive compared to the material being worked on (harder abrasives will cut deeper faster)
  • Adhesion between grains (determines how quickly grains are lost)
  • Loading (worn abrasives can reduce cutting efficiency)
  • Contact force (greater force will result in faster abrasion)
  • Use of lubricant, coolant, or metalworking fluid (can help to carry away sanding debri to prevent friction and reduce heat)

How Are Abrasives Produced?

There are many different types of abrasives and each type of abrasive has its own manufacturing process. Abrasive grains are produced by heating or chemically treating minerals to produce hard materials good for abrading. After being treated, the minerals are crushed and sifted by size. The smaller a grain is, the finer the finish is. Once crushed, the grains may be washed in classifiers to remove slimes and passed through magnetic separators to remove iron-bearing material. 

For abrasive wheels, the grains are then bonded to a wheel using one of six types of bonds: 

  1. Vitrified or ceramic
  2. Resinoid
  3. Rubber
  4. Shellac
  5. Silicate of soda
  6. Oxychloride of magnesium

For coated abrasive products, the grains are bonded to a backing using resins, glues, and/or varnish. Coated abrasive products are first produced as large rolls of abrasive-coated fabric or paper. The rolls are then manufactured into a finished product. 

What Are The Different Types of Abrasives?

Different jobs call for different types of abrasives. The most common abrasive products aresanding belts,sanding discs,sanding sheets, andsanding rolls

Sanding belts are great for jobs that require a lot of material to be removed quickly. Sanding belts are often used for stock removal inknife making, sanding hardwood floors, removing paint from flat surfaces, and sharpening tools. 

Sanding discs are great for sanding large surfaces if you don’t have a wide belt sander. They are good for preparing surfaces, finishing wood, or derusting metal. 

Sanding sheets require more manual labor and are usually only used when absolutely needed. Sanding sheets allow for a lighter touch for woodworking and finishing. Sheets are also able to reach small surfaces and crevices that electric equipment can’t. Bladesmiths will often switch to hand sanding after 220 grit (using a waterproof aluminum oxide sanding sheet)  to have more precision during the finishing process. 

Sanding rolls can be cut into sanding sheets or wound around drum sanders. Drum sanders are good for efficiently putting a fine finish on materials. They are often used on wood products, but can be used to finish plastic and metal workpieces as well. If you’re interested in cutting your own drum sanding strips from a sanding roll, we’ve covered how to do that in our blog post oncutting your own drum sanding strips

Choosing The Right Abrasive For Your Needs

There are many different variations of abrasive products outside of just the type of abrasive. Abrasives are manufactured with varying grain materials, backings, coating types, weights, and grits. 

Abrasive Materials 

Characteristics

Silicon Carbide

Aluminum Oxide

Zirconia

Ceramic

Friability

Highly friable

Highly friable

Moderately friable

The least friable material

Strength

Moderately strong

The weakest material

Moderately strong

The strongest material

Price

Moderately expensive

Cheapest material option

Moderately expensive

Premium price point

Lifespan

Moderate longevity

The least longevity

Moderate longevity

The most longevity


Aluminum Oxide (A/O) 

One of the most popular abrasive materials, aluminum oxide is the industry standard for common applications and is the least expensive option on the market. The low price point comes at a cost as aluminum oxide often has the least longevity. Open coat aluminum oxide is excellent for planing, stripping, roughing, and finishing on all wood types. Closed coat aluminum oxide is great for snagging, weld removal, and grinding of metals. High quality aluminum oxide is highly friable, meaning the tips of the abrasive grains fragment as it wears, continually providing sharp cutting surfaces, and increasing working life. Aluminum oxide abrasives are highly customizable to specific applications.

Silicon Carbide (S/C)

Silicon carbide is a specialty abrasive and works well for specific applications. It’s commonly used for finish work because it delivers extremely consistent, even cutting. It is harder and sharper than aluminum oxide, so the cut rate remains the same through the entire life of the belt. Silicon carbide is more expensive than aluminum oxide, but silicon carbide abrasives deliver a level of consistency that aluminum oxide abrasives cannot, which offsets the higher cost. 

It's ability to remove material without excessive heat buildup makes it perfect for finishing stabilized wood, resins, epoxy, and other heat sensitive applications. Silicon carbide is also excellent for automotive paint preparation. Other applications include marble, garnet, glass and other solid surfaces. It is typically black in color and acceptable for applications requiring waterproof materials.

Ceramic

Ceramic is considered a premium abrasive grain, as it is extremely hard and sharp. Ceramic abrasives come with a premium price, but also have the longest lifespan over any other type of abrasive material. The higher cost can be justified as it could save you money over the long-term. 

Ceramic abrasives are ideal for aggressive cutting of metals and hard woods. The material works best when high speed and high pressure is applied. It is so aggressive, caution must be used to prevent scorching of the working material if you’re working with wood. Ceramic is friable, continuously delivering a sharp cutting surface, increasing the already impressive longevity of the product. 

Ceramic is typically available in 24 through 120 grit, with most manufacturers offering a max of 220 grit. Red Label is one of the only manufacturers to offer ceramic up to 400 grit, however, our ceramic abrasives in 180, 240, 320, and 400 grit are only available in a J weight backing. While we always recommend using ceramic for steel, bladesmiths desiring stiff backed belts over 120 grit should use closed coat aluminum oxide. 

For the best results, only use J weight belts with a rotary platen or when slack grinding. The backing on a J weight belt is thinner than the tape joint, so if it’s used against a flat platen, the seam may bump with each rotation. The seam bump can create unintended marks on the blade, which are undesirable. You can use a rotary platen to avoid this issue, but in the absence of one, use closed coat aluminum oxide with an X weight backing. Some knife makers have had success layering a surface conditioning belt under a J weight belt to absorb the bump against a flat platen. This technique works best on sanders larger than 1 x 30 due to guard constraints. 

Zirconia or Alumina Zirconia (A/Z)

Zirconia was the most aggressive and durable abrasive grain until ceramic came along. It is still an excellent choice for heavy metal grinding / polishing of metals and material removal in hardwoods. Zirconia sanding belts are less costly than ceramic and do not typically exceed 220 grit. Zirconia is the middle ground abrasive between aluminum oxide and ceramic in terms of price and longevity. 

Abrasive Backings

Paper

Paper backed abrasives typically include aluminum oxide and silicon carbide belts, discs, and shop rolls. Paper backing has some advantages, such as its light weight and lower cost compared to cloth backing. It also delivers even cutting for delicate processes and finer grits. However, paper alone does not provide the rugged durability of cloth or synthetic backings. Paper backing can be infused with material such as latex, to create an extra flexible, durable product that delivers consistent scratch patterns. Some industrial and professional users require paper wide belts for specific woodworking, stone, granite, plastics and metal finishing applications.

Cloth (Cotton, Polyester, Poly/Cotton Blend)

Cloth is a popular backing due to its cost effectiveness and strength. Premium abrasive products typically feature cloth or other synthetic backings. Polyester is a waterproof, synthetic backing that offers superior strength and longevity. Most standard sized belts are cloth backed.

Plastic/Film

Plastic film backing is excellent when users require an extremely uniform finish. It is popular in wet sanding due to its waterproof properties, strength, and flexibility.

Foam and Sponge

Foam and sponge backings are best for hand sanding moldings and veneers. These materials form fit contours and are an excellent choice when flexibility is required.

Grit

In the United States, grit size is determined by the Coated Abrasive Manufacturers Institute (CAMI). Abrasives bought at local hardware stores and lumber yards are usually measured using the CAMI scale. However, Red Label Abrasives is a global abrasive manufacturer and relies on the scale provided by the Federation of European Producers of Abrasives (FEPA). The FEPA scale is superior to CAMI due to the strict tolerance system manufacturers must adhere to in order to earn grit ratings. The FEPA scale ensures abrasives are consistent, which enhances the quality of the finished products. 

Using the FEPA scale, the grit number is preceded by the letter “P”. Grits range from P8 to P1200 and higher. The lower the number is the more coarse the grit is. You can view a breakdown of grit ranges in the table below. 


Category

Grit Range

Uses

Coarse

P8 - P80

  • Stock removal
  • Stripping of paints, varnishes, etc.

Medium 

P100 - P220

  • Prepping materials for paints, stains, etc.
  • Final shaping
  • Removal of planning marks
  • Honing 
  • Sharpening

Fine

P240 +

  • Sharpening
  • Polishing

Extra coarse abrasives in the P8 to P36 range are very tough. Extra coarse abrasives should only be used on the toughest jobs to avoid removing too much material or damaging a workpiece. 

Coating Types

Coating refers to the amount of abrasive grain applied to the backing material. In general, closed coats provide an even finish on solid materials and open coats provide better finishes on woods. You can find a detailed breakdown on the difference between coatings in our blog post onclosed and open coat abrasives

Closed Coat

Grain is applied evenly together without any voids in the coat. It delivers a longer life and finer finish. Typically used in non-woodworking applications

Open Coat

Only a portion of the backing is covered with an abrasive grain. This type of coating reduces loading in coarser grits. The backing is covered 50%-70%, allowing more room for material buildup to expel from the grain.

Semi-Open or Semi-Closed

Semi-open or semi-closed coat falls between closed and open coat. These abrasives have about a 30% grit reduction on average.

Weight

Coated abrasive products are categorized by the ‘weight’ (wgt) of the backing material used. This alphabetical system indicates the stiffness and thickness of the backing. Below is a list of commonly used weights.

Paper – A (Lightest) – F (Heaviest)

A & B wgt are common in low-end materials and/or finer grits. They are typically used for hand sanding applications. C & D wgt are the most popular for general use sandpaper products. E &  F wgt are most commonly used for belts and discs.

Cloth – J (Lightest), X, Y, YY (Heaviest)

For most applications, a good ratio of weight, flexibility and flatness are crucial for abrasive product performance. J wgt material is the lightest and most flexible. This is popular in light metal finishing applications and knife handle work. X wgt has the widest range of applications. Y wgt is typically used for heavy duty applications. Cloth materials are stiffer and used in most belt sanding applications due to their increased strength. Paper backing is used when uniform consistency is needed. Paper backed materials are used in hand sanding applications such as polishing and fine wood finishing.

Other weights (S,T,M) are seldom used, only for specific applications and products.

How To Select The Best Abrasive For Your Needs

Ultimately, there’s no one-size-fits-all solution for abrasives. The best abrasive for you will depend on the materials you’re working with and the outcome you’re trying to achieve with your materials. We’ve broken down abrasive needs for a few popular applications below. 

We know we haven’t covered every possible abrasive application in existence. If you’re having trouble finding  a solution for your application, you can speak with one of our abrasive technicians to arrive at the best product for your needs. You can call us at (844) 824-1956 or fill out acontact form to speak with a technician. 

For Woodworking 

For most woodworking applications, open coat aluminum oxide will usually get the job done. Aluminum oxide is affordable and effective when working with wood. Open coat abrasives work best with softer woods like cedar and pine. If you’re working with harder woods like African Blackwood, cocobolo, oak, or hickory, then closed coat aluminum oxide would be the better choice. You can view or woodworking products below: 

For Knife Making

Creating a knife requires working with a combination of materials as the handle or scales are often different materials from the blade. While we tend to recommend open coat aluminum oxide abrasives for wood, the wood used for handles or scales is usually either exotic wood that is naturally hard or wood that is stabilized to become harder. Therefore, closed coat aluminum oxide, zirconia, or ceramic abrasives should be used for knife making. 

Aluminum oxide abrasives are cheaper, but they won’t last as long or perform as well as ceramic or zirconia abrasives. We would highly recommend using ceramic abrasives to get the best value for your money spent. We also sell knife making kits, so that you can buy an assortment of different types of belts to fit your needs.

Coarser grits (P36 - P150) are good for grinding away stock material with ease. You can use coarse grit abrasives to shape your blade and scales. Fine grits (P180 - P800) can be used for honing or blade sharpening. Ultra fine grits (P1000 - P5000) can provide a final round of sharpening and finishing for an ultra smooth finish. 

You can view our knife making products below: 

For Metalworking 

There are two types of abrasives we generally recommend for metalworking. Those two types would be closed coat aluminum oxide and ceramic. You could also use zirconia abrasives as a middle of the line alternative. Zirconia abrasives are stronger than aluminum oxide and less expensive than ceramic, but you really do get the best bang for your buck with ceramic abrasives. You can view our metalworking products below: 

If you’re working on auto body repair, we actually have an in-depth guide onabrasives for auto body repair that you can use for reference. The guide includes how to select grits, what abrasive material is recommended and more.  

Best Practices For Abrasive Storage And Use

We cover the best practices for storage and use in our blog post onhow to store abrasives for the best performance. However, recommended storage and use guidelines can be quickly summarized. 

To give your abrasives a longer lifespan and ensure the best possible performance, the abrasives should be stored away from the ground and in an area with moderate temperatures (60°F-80°F) and low humidity. Controlling humidity is crucial to ensure good abrasive health as high humidity can damage the joint adhesive on sanding belts causing them to snap. Sanding belts should be stored on racks and sanding discs or wheels should be stored on/in boxes, racks, bins, and drawers.