What is the Difference Between Open and Closed Coat Abrasives?

Topics Covered:

• What Are Open Coat Abrasives and When Should They Be Used?
• What Are Closed Coat Abrasives and When Should They Be Used? 

With abrasives, there is no ‘one size fits all’ solution. Not only is it available in a variety of grain (grit) sizes and types, but there are also two primary types of coating.

The coating of an abrasive refers to how densely the product is covered with abrasive grains like aluminum oxide, or ceramic. In this blog, we’ll explain the difference between open and closed-coat abrasives and describe their ideal applications. Choosing the right one for your project will make your abrasives last longer and help ensure a high-quality finish.

Open Coat Abrasives

With open coat abrasives, the abrasive grains cover approximately 50 to 70% of the surface. At first glance, it seems like you would want as much abrasive grit as the surface can hold, but more space between the grains can actually extend the life of your abrasive. 

As you use the abrasive, an open coat lets more sanding residue fall off and reduces the likelihood of it getting stuck between the grains. If any material does become wedged, the movement of the running sander will usually dislodge it, but stubborn debris can be easily removed with a cleaning stick or compressed air. The open space also encourages air flow, which cools down the abrasive and helps prevent the wood resins and finish from softening due to the heat.

When you’re shopping for open coat abrasives, you may come across products labeled as semi-open. Semi-open coat is used to describe abrasives with about a 30% grit reduction. While it has slightly more cutting power than open coat surfaces, it still does a less aggressive cutting job than closed coat abrasives. 

Open coat abrasives are frequently used with soft woods like cedar, pine, fir, and spruce, as well as softer metals like aluminum, brass, and zinc. It also does a good job of coarser sanding and polishing, but gaps in grit coverage make it unsuitable for finer, more even polishing jobs. 

Most abrasive grains are available with open coatings, but open coating is usually reserved for aluminum oxide abrasives. At Red Label, our open coat abrasives are aluminum oxide. You can find our open coat products by clicking the links below: 

• Open coat aluminum oxide sanding belts
• Open coat aluminum oxide sanding discs

Generally speaking, we recommend using open coat aluminum oxide products if you’re a woodworker or you’re looking for a general abrasive to use on house projects. Outside of those two groups, we typically recommend ceramic abrasives, especially for knife makers. 

Often knife makers will buy aluminum oxide belts, because they are cheaper, but the woods used in knife making are either naturally hard (cocobolo, African blackwood, desert ironwood, etc.) or stabilized to be harder. In these cases, you’ll get better performance by using closed coat ceramic abrasives. 

Closed Coat Abrasives

Closed coat abrasives have 90 to 95% of their surface covered by abrasive material and are more suitable for hand-sanding or working with harder wood and metal surfaces. Due to the higher grit-to-surface ratio, closed coat cuts more aggressively and provides a smoother finish due to fewer gaps between the grains.

This type of abrasive is ideal when you’re working with harder woods because the rougher sanding process creates larger pieces, which aren’t going to get caught between the grains. In short, there is less need for more space between grains because the abrasive will not load as much.

Closed coat abrasives are also recommended for:

• Grinding hard metals, which calls for maximum grain cutting capability.
• Finishing applications like lacquers and sealers, as it provides a more uniform scratch pattern and a better-quality finish.

Some of our closed coat products can be found below: 

• Closed coat aluminum oxide sanding belts
• Closed coat ceramic sanding belts
• Closed coat zirconia sanding belts
• Closed coat aluminum oxide sanding discs

Closed coat abrasives are not an ideal choice for softer materials because the few spaces between the grain will quickly load up with sanding swarf (debris). Eventually the swarf will cover the grit entirely and an overloaded abrasive can end up burning your belt and the workpiece. For these applications, use open coat abrasives instead.

Contact an Industrial Abrasive Specialist

Selecting the right coat for your sanding application will help you get the best results in the most efficient manner. At Red Label Abrasives, we take pride in producing quality open and closed coat abrasives for everything from individual woodworking and knife making operations to industrial-sized and specialized applications. We’re a family-owned business that has been serving hobbyists and manufacturers for over 35 years. We work hard to provide exceptional customer service and unrivaled technical support. For help matching a sanding solution to your individual needs, please call 844-824-1956 or fill out a contact form.