Open, Semi-Open and Closed Coat Abrasives
When determining which type of abrasive is most suitable for a particular project you may have encountered the terms open coat, semi-open coat and closed coat.
Closed Coat Abrasives
An abrasive that has a closed coat means that the backing of the abrasive, whether this is a belt, sheet, disc, or other, is covered completely in abrasive grain and resin. As a result of 100% of the abrasive being covered in abrasive grain, closed coat products are usually more aggressive and produce the finest finishes. This is because every possible space on the abrasive surface is covered with cutting grain, therefore leaving no gaps in the grain coverage, producing a fine finish.
Semi-Open and Open Coat Abrasives
The other terms which you may have read when purchasing abrasives are semi-open coat and open coat. These terms mean that the amount of abrasive grain situated on the backing of the abrasive products has been modified. When referring to products that have a semi-open coat, this usually denotes approximately a 30% reduction in the amount of abrasive grain on the backing. The term open coat usually indicates an approximate 50% reduction in the abrasive grain. As a result semi-open and open coat abrasives generally cut less aggressively and will finish coarser per grit than closed coat products. This is because semi-open and open coat abrasive products have a reduction in the grain situated on the abrasive product when compared to closed coat abrasives. This results in less cutting power per grit designation. Semi-open and open coat abrasive products have spaces in their grain coverage which produces a more open configuration that helps the abrasive to clog less.
Why would you use one over the other?
Generally, you should always use abrasives that feature a closed coat due to the advantage that 100% is covered in abrasive, allowing you to maximise the life of your sandpaper products and the results it achieves. However, there are instances where semi-open or open coat abrasives may be more suitable for particular sanding applications, especially where clogging may be an issue. The primary reason for choosing a semi-open or open coat abrasive is if you are sanding a material that is soft by nature. This may include softwoods (pine, fir, spruce, larch, cedar, cypress, redwood, tamarack, and yew) or soft metals (some aluminiums, brass, bronze, copper, magnesium, titanium, zinc, and zirconium) or non-phenolic plastics/rubber. When sanding these materials with closed coat abrasives, the areas between the grain tips will soon load up with sanding refuse. Eventually, it will load up to the point it's taller than the grain tops and will completely cover them. If sanding is continued, both the belt and workpiece may experience excessive heat and burn. This is where semi-open and open coat abrasives can be most useful.
As the abrasive grain on semi-open and open coat sandpaper is spaced out, distance is created between individual grains, which helps reduce the amount of sanding refuse trapped between grains. This enables the sanding of softer materials for longer periods of time and additionally produces better results in terms of the life of the abrasive and the resultant finished product.
It is important to understand the difference between the terms closed coat, semi-open coat, and open coat and the effect the backings may have on the longevity of abrasives and the finishing quality of workpieces. A greater understanding can increase the benefit and satisfaction that is obtained from the use of sandpaper products.