CLOSED COAT
Closed Coat means that close to 100% of the backing on a coated abrasive product is covered in abrasive grain. Closed Coat products are more aggressive and cut faster because the grit particles are grouped more closely together.They also provide the best finishes because there are no gaps between the grains. You receive the full benefit of the work that the abrasive grain is capable of doing.
SEMI OPEN / OPEN COAT
Semi-Open and Open Coat means that there is more space between grain particles on the abrasive product. Semi-Open Coat usually refers to about a 30% reduction in the amount of grain on the backing and Open Coat usually means that there is about 50% coverage. Semi-Open and Open Coat products have less grain on the backing, which results in less cutting power per grit designation...they are not as aggressive as a Closed Coat abrasive product. In addition, Semi-Open and Open Coat products have gaps in their grain coverage, so the finishes they provide are not quite as even and will tend to fall on the coarse end of the finishing scale for that particular grit.
So what’s the bottom line?
You should always use a Closed Coat abrasive unless there is a reason for you not to. The main reason to switch to an Open Coat abrasive is that the material you are sanding or grinding is soft or gummy. Soft materials are soft woods like Pine, Fir, Spruce, Larch, Cedar, Cypress, Redwood, Tamarack, and Yew. Soft metals include Aluminum, Brass, Bronze, Copper, Magnesium, Titanium, and Zinc. When you attempt to sand these types of materials with abrasives manufactured with a closed coat, the areas between each grain will load up with sanding swarf. Eventually it will load up to the point that it’s taller than the grain and will completely cover the grit. If you continue to sand with a “loaded” abrasive, you will burn both the belt and most likely your workpiece. This is where Semi-Open and Open Coat abrasives are most useful.
By spacing out the grains on the backing of an Open Coat material, you create room between individual grains which will help to reduce the amount of sanding swarf stuck between the grains. You can then use compressed air, a rubber cleaning stick or even the movement of the running sander to help dislodge the swarf from the abrasive. This will enable you to sand soft materials for longer periods of time and with better results.
Woodworkers that only sand cherry, oak, ash or other hardwoods usually don’t experience any problems sanding with an Open Coat abrasive, but you would benefit from using Closed Coat abrasives for your hardwoods and reserve Open Coat materials for soft woods only. You sacrifice life, speed and quality when you use an Open Coat abrasive product on a bare hardwood finish for no reason. This may not be practical for smaller woodworking shops that work with different wood varieties every day, but certainly something to consider.
