Open Coat Abrasives VS Closed Coat Abrasives
To determine the difference between the two main types of coating, you have to look at the amount of abrasive grains that are covering the surface of your material -- whether that is aluminum oxide or ceramic, etc. It is important to chose the right coating due to the fact that your belt will last longer and overall give you a better finish.
First we will talk about open coat abrasives and their function. Open coat allows for dust and grain debris to not get stuck or pile up on your belt or surface. The reason it does that is because the grains only cover 50-70% of the material. The open coat not only gives your material a longer life, but it also allows for air flow to prevent over heating or burning. Open coat abrasives can be used on many different types of surfaces ; for example, soft woods such as pine or cedar, or soft metals like zinc, brass, or aluminum. A good tip is if your woodworker you should use aluminum oxide and if you are working with knives, ceramic is probably best for you. There is also a semi-open coat that can be used if you are looking for more of an aggressive cut , compared to the open coat. Semi- open, however, is still not as aggressive as a closed coating belt because they still have about 30% less grit.
If you tend to work with your hands or harder metals and woods, closed coat abrasives may be best for you because the grit surface coverage ranges from 90 to 95%. This will ensure a harsher cut if you are removing a lot of stock, as well as a smooth finish due to the material having no/or fewer spaces between the grains. The reason the material has less spaces between the grits is because the abrasive will not load as much as the open coat. Closed coat abrasives are great for finishes on the surface you are working on because there as an even scratch pattern. Using closed coat abrasives on softer metals or woods can cause overheating and burning your project.
Both types of coatings are great, but make sure to consider the factors in order to make your life easier, ensure longer belt life, and to get your preferred finish.