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When discussing or reading about coated abrasives, the terms "open coat" and "closed coat" may often come up. The open versus the closed coat option can affect the life and finishing properties of abrasive materials. So, understanding and applying the proper coating to the application can lead to a longer-lasting abrasive.
Closed Coat
The term "closed coat" means that whatever the backing of the belt, sheet or disc, 100% of that backing is covered in abrasive grit. With that being the case, two things become evident:
- Closed-coat products are the most aggressive
- Closed-coat products provide the finest finishes
Every possible space that could be dedicated to a cutting grain IS occupied by a cutting grain. As there are no gaps in the grit coverage, no matter which grit size you are using, it will provide the full amount of the work that grit is capable of.
Semi Open / Open Coat
What the terms semi-open and open coat mean is that, to different degrees, the amount of abrasive grit on the backing of the belt, sheet, or disc has been lessened. The term semi-open coat usually refers to a 30% reduction in the amount of grit on the backing while the term open coat usually means 50% has been withheld. Again, two things become evident:
- Semi-open and open-coat products cut less aggressively
- Semi-open and open-coat products finish coarser per grit than closed-coat products.
Semi-open and open-coat products have less grit on the backing, which results in less cutting power per grit designation.
Semi-open and open-coat products have gaps between the abrasive grains, 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?
Closed-coat material is always recommended unless there is a specific need. The main reason to do so would be that the material you are sanding or grinding is soft or gummy by nature. Examples of soft/gummy materials could include softwoods (Pine, Fir, Spruce, Larch, Cedar, Cypress, Redwood, Tamarack, and Yew) or soft metals (Some Aluminum, Brass, Bronze, Copper, Magnesium, Titanium, Zinc, and Zirconium) or non-phenolic plastics and rubber.
When attempting to sand these types of materials with abrasives using closed-coat grit coverage, the areas between the grit tips will soon load or clog up with sanding refuse. Eventually, it will load up to the point that it is taller than the grit tops and will completely cover them. At this point, if sanding continues, 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 and vibration of the running abrasive to help dislodge what has become attached. This will enable the user to sand soft materials for longer periods with better results for both the life of the abrasive and the result on the workpiece.
The use of open-coat sandpaper on cherry or oak leads to burning. This is because only 50-70% of the grit is trying to do 100% of the work and there was no adjustment made to the grit sequence to make up for the loss of grit. 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 normally have an "OC" on the back, so they are not 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.
Some open-coat materials have washable backings. Belt washing can either be simple and inexpensive or more complex and quite expensive. Everything from soaking belts in cleaning solutions, to steam cleaning machines, to dry ice blasting is available. As semi-open and open-coats help reduce loading washing can be a good way to increase longevity.