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Often when discussing or reading about coated abrasives, the terms "open coat" and "closed coat" may be mentioned. The open/closed coat decision is one that can have significant repercussions on the life and finishing properties of abrasive materials, so it is one that must be understand well.
Closed Coat
The term "closed coat" means that whatever the backing of the belt, sheet or disc is 100% of that backing is covered in abrasive grain. Thinking about that statement in a logical fashion, two things can be deduced from it:
- Closed coat products would be the most aggressive
- Closed coat products will provide the finest finishes
They would be the most aggressive because every possible space that could be dedicated to a cutting grain IS dedicated to a cutting grain. They would provide the best finishes because as there are no gaps in the grain coverage. Whatever grit you are sanding at should provide the full benefit of the work that grain is capable of doing.
Semi Open / Open Coat
The terms semi-open and open coat mean that, to different degrees, the amount of abrasive grain on the backing of the belt, sheet or disc has been modified (lessened). Semi-open coat usually refers to a 30% reduction in the amount of grain on the backing while open coat usually means 50% has been withheld. Thinking about these statements logically, two things can be deduced from them:
- Semi-open and open coat products will cut less aggressively
- Semi-open and open coat products will finish coarser per grit than closed coat products.
Semi-open and Open coat products have less grain on the backing, which results in less cutting power per grit designation.
Semi-open and Open coat products have gaps in their grain coverage, so the finishes they provide will not be as even and will fall to the coarse end of the designated grit's finishing parameters.
Why use one over the other?
It is recommended to never go off closed coat material unless there is a reason. The main reason to do so would be that the material is soft or gummy by nature. Examples of soft/gummy materials could include soft woods (Pine, Fir, Spruce, Larch, Cedar, Cypress, Redwood, Tamarack, and Yew) or soft metals (Some Aluminums, Brass, Bronze, Copper, Magnesium, Titanium, Zinc, and Zirconium) or non-phenolic plastics/rubber.
When attempting to sand these types of materials with abrasives using closed coat grain coverage, 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 are likely to burn. This is where semi-open and open coat materials come into play.
By spacing out the grains on the backing, room is created between individual grains, which will help reduce the amount of sanding refuse trapped between grains. Use compressed air or the movement/vibration of the running abrasive to help dislodge what has become attached. This will enable the user to sand soft materials for longer periods of time with better results for both life of the abrasive and result on workpiece. It is true that the user will finish coarser per grit and if all things were otherwise equal, for a shorter period of time overall, but the soft materials will optimize both sanding time and results.
The use of open coat materials on cherry or oak leads to burning. This is because only 50-70% of the grain is trying to do 100% of the work and there has been no adjustment to the grit sequence. All of these factors are conducive to burning, not only of the abrasive but on the workpiece as well. This is overworking the open coated belt, and the excess heat results in burning.
Use closed coats on hard materials and use open coats on soft materials. Open coat materials will have an "OC" on the back ordinarily, so they aren't hard to keep separated and are readily identifiable to shop employees.
Knowing what these two terms mean and how they affect everything from the longevity of abrasives to the finishing quality of workpieces can greatly benefit both the user's bottom line and customer satisfaction.
There are usually open coat materials that have washable backings. Belt washing can be extremely simple and inexpensive or more complex and fairly expensive. Everything from solutions to soak belts in a workshop to steam cleaning machines is available. As semi-open and open coats help reduce loading, not eliminate it completely, washing can be a good way to increase longevity