Open vs Closed Coat Abrasives

With abrasives, there is no one-size-fits-all solution. Not only are there a wide variety of grit sizes and types, but also two main types of coating.

The coating of an abrasive refers to the density with which the product is covered with abrasive grains such as zirconia or ceramic. In this article, we'll explain the difference between open-coat and closed-coat abrasives, and describe their ideal applications.

The difference between open and closed coat abrasives lies in the proportion of the surface covered by abrasive grains. Open coat abrasives are less grain-dense than closed coat abrasives. Open coat abrasives are better suited to sanding softer materials such as cedar, pine, aluminum and brass. Closed coat abrasives are better suited to sanding harder materials such as oak, hickory and steel.

Open Coat Abrasives

With open coat abrasives, the abrasive grains cover around 50-70% of the surface. As the abrasive is used, an open coat allows more sanding residue to fall away, reducing the likelihood of it getting caught between the grains. If a material does get stuck, the motion of the sander in operation will usually dislodge it, but stubborn debris can be easily removed with a cleaning stick or compressed air. The open space also promotes air circulation, which cools down the abrasive and prevents wood resins and finishes from softening under the effect of heat.

Open coat abrasives are commonly used with soft woods such as cedar, pine, fir, and spruce, as well as softer metals like aluminum, brass, and zinc. They are also suitable for coarser sanding and polishing jobs, but gaps in grit coverage make them unsuitable for finer, more uniform polishing work.

Most abrasive grains are available with open coatings, but this is generally reserved for aluminum oxide abrasives.

Closed Coat Abrasives

Closed coat abrasives have 90-95% of their surface covered by the 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 gives a smoother finish due to fewer gaps between the grains.

This type of abrasive is ideal when working with harder woods, as the rougher sanding process creates larger pieces that won't get caught between the grains. In short, there's less need to increase the spacing between grains because the abrasive won't load up as much.

Closed coat abrasives are not an ideal choice for softer materials, as the few spaces between the grains will quickly become loaded with sanding swarf (debris). The swarf will eventually 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.

Closed coat abrasives are also recommended for:

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