Knowing the right grains, backings and other qualities can make all the difference to your sanding.
Creating a smooth, precisely shaped product is the goal of joinery and most timber-based trades and crafts. To that end, we rely on abrasives to sand and polish our work. A few centuries ago, it was done using rough sharkskin, or equisetum reeds. These days, there is a wide range of coated abrasives to do the job. We spoke with a team from specialist manufacturer Hermes Abrasives Australia for a rundown of this vital, but often overlooked component of the trade.
Coated abrasives, more commonly known as sandpaper, are made up of several key elements. The backing, the glues, the additives and the abrasive grain all affect the performance of the sandpaper on different workpieces.
The Hermes team said, “Driven by the customer’s needs, abrasive manufacturers are continually assessing these key elements to review what can be upgraded to improve the abrasive’s performance and to reduce the overall cost for the customer. Due to this, abrasives have transformed over the years and now there is a wide variety available to suit every workpiece and industry.”
BACKINGS, GLUES AND ADDITIVES
Backings range from simple kraft paper (the cheapest and commonest type at a hardware store); through paper with a range of barrier coatings designed to resist moisture or work effectively with common timber coatings (such as animal-hide-based glues); to cloth, paper/cloth and stitch bonded cloth, which are flexible and used in belts; even rubber and vulcanised fibre, used as disc backings.
Glues/resins used to attach the abrasive to the backing also come in an extensive range. These are applied in a two-stage process, the first to bond the abrasive to the backing, the second to anchor the product and increase the physical strength of the finished product. Different options have different hardness, heat tolerance, moisture resistance and grain retention. Suppliers can advise users on the best product to meet their needs, balancing conditions, purpose and cost effectiveness.
Additives can also be combined with the grain during the attachment process to improve the final abrasive’s performance in a range of areas from durability through to anti-static and special cooling additives. Again, these come with trade-offs, usually in the area of added cost.
Some products are designed to be used in conjunction with proprietary systems built by or in conjunction with the largest abrasive manufacturers. More commonly, sheets and belts are available to work with a large range of machines and hand tools.
Added features such as backing pads, and velcro and foam backing are also available. These work to extend the life of abrasives and to make installation easier/more secure.
Providing your supplier with the most comprehensive information possible about your set-up and products will mean they can give you the most effective advice. Quite often, companies are using a good product they are happy with, but unaware that there is another available that may do the same job at a lower price, or do a better job that allows them to earn more or save money elsewhere in production.
GRAINS
The different abrasive grains available are key contributors to the variety in sandpaper. Abrasive grains are materials that are hard and tough enough to wear away another material. The most common types available in the Australian market are aluminium oxide grain, silicon carbide grain, zirconia alumina grain and ceramic grain. Hardness and toughness vary between grain types. Depending on the workpiece and required finish, different grains will perform and suit your needs better.
Abrasive grain is mostly artificially produced and there can be variations within grain types depending on the abrasive grain manufacturer, and the quality required. For example, aluminium oxide grain is generally produced by refining the naturally occurring minerals bauxite and alumina. These processes can vary between manufacturers which can result in different grades of grain. End users can trust the reliability of product type from batch to batch with reputable suppliers, but there can be differences (albeit these are often minor) between similarly described products purchased from different suppliers.
Toughness and hardness are separate qualities. Material toughness is the resistance of an object to fracturing, and material hardness is the resistance of an object to scratching or cutting. Picture hitting a block of solid metal with a hammer. Because metal has a high toughness, it can handle the weight of the impact without fracturing, but if you hit a diamond with a hammer, it would likely shatter, because its toughness is not high enough to absorb the impact. Now picture sliding a point of the solid metal block along the hammer’s face. It might scratch the hammer face very slightly, but a diamond will create a deeper scratch on the hammer face because it has a higher hardness than metal.
The two basic grains used in coated abrasives are aluminium oxide and silicon carbide. These two grains are the cheapest to produce and so are used the most commonly for general purpose applications. However, as the industry has changed from mostly softwood timbers to harder species, a desire has arisen for abrasives that last longer. Here in Australia, we have seen an increase in the use of zirconia grain products, which give greater belt life as well as quicker sanding time against that of aluminium oxide. Even more recently, demand for ceramic grain, which also gives improved belt life, has increased.
Grains are available in a range of grit types and as open or closed grains. It is also worth discussing these options with your supplier as some traditional sanding techniques have been updated, thanks to research and new materials being introduced.
For further information or to seek expert assistance, visit www.hermes.com.au
WHICH ABRASIVE GRAIN IS BEST FOR ME?
The experts at Hermes Abrasives Australia have compiled the following guide to abrasive grains. Due to the variety in applications and timber types, the information provided is intended as general advice only. Other factors will also determine what abrasives are suitable for specific applications. For example: the workpiece, machine used to sand, the required finish, abrasive budget, and the backing type of the abrasives.
Contact your local abrasives supplier for professional advice on your particular application. No matter the requirement, there is an ideal abrasive solution for every need.
Aluminium oxide
The timber industry has long used abrasives with aluminium oxide grain for general sanding and finishing. Abrasives with aluminium oxide are generally the cheapest available. Aluminium oxide grain has a higher toughness than that of silicon carbide grain and so will last longer before fracturing.
Benefits:
- Tough, long tool life
- Good for stock removal
Silicon carbide
Silicon carbide grain has a higher hardness than aluminium oxide and as a result will cut deeper and finer. This results in a fast cutting speed. The lower toughness results in faster wearing of the grain than of that seen in aluminium oxide.
Benefits:
- Sharp, fine scratch patterns ideal for a smooth finish
- Fast cutting speed
Zirconia alumina
Zirconia alumina (zirconia) was designed to grind metal but as some of our hardwood species here in Australia are so hard, zirconia seemed like a logical direction and improvement over aluminium oxide. There is no doubt the introduction of zirconia has impacted on the number of belts sold as the longevity of the product has reduced the number of belts the customer needs. Zirconia grain is tougher than aluminium oxide and so will last longer before fracturing but requires high grinding pressure to perform optimally.
Benefits:
- Good for hardwood and softwood
- Longest tool life
- Good for calibrating
Ceramic
Ceramic grain was also designed for use in the metal industry but might have found a second home in timber for the same reason as zirconia grain. Ceramic grain is harder and tougher than zirconia, however the shape of the grain is rounder and its optimal grinding pressure is lower. Due to its grain shape and durability, ceramic is better suited to hardwood applications but can be used on softwood. Interest in the market continues to grow for ceramic grain. However, due to the premium price, time will decide whether this will become a commonly used abrasive in the timber industry.
Benefits:
- Good for hardwood
- Longest tool life
- Lower optimal grinding pressure
OPEN AND CLOSED COAT
Grains are spread over the backing in a range of densities. These densities can be generally broken down into two categories, open and closed coatings. Open coated grain is used to reduce clogging of the abrasive and improves tool life. The downside to open coating is that there is less grain on the abrasive and so the stock removal rate is decreased. Closed coated grain pattern is the standard and this is used in most applications where clogging isn’t an issue.
