You may have noticed the terms “open coat,” “semi-open” or “closed coat” when browsing our products and wondered what that was about. In this article, we’ll explore why the coating matters and which option might be best for your application.
Why Sandpaper Loads
Before we get into discussing coatings, let’s get back to basics and discuss one of the most frustrating problems we face in sanding: loading.
Why does sandpaper load?
Well, when you move the sandpaper over your work surface, you are creating friction, or resistance between the abrasive and the work piece. This action facilitates the cutting/scratching that you desire, but, there may also be some undesired consequences… namely: heat, static electricity and dust.
When heat is produced in the sanding process, this can cause the resin in the wood or chemicals in the finish to melt, which leads to a sticky residue that can get clogged, or loaded, into the sandpaper.
It’s not just annoying to have to replace your sandpaper, but this problem can also ruin your ability to create the finish you desire. It can lead to an uneven scratch pattern and blotchy finish when staining, as a result of sanding with grains that are not sharp… due to the residue and dust clogging your sandpaper.
While no sandpaper will last forever, the type of coating of the sandpaper has will help to increase the life of the product and achieve a better finish.
What are “Coatings”
So, what does coating mean, which is right for you and why should you care?
Well, when it comes to sanding, there’s actually a science to maximizing efficiency in the process, increasing the lifespan of your sandpaper and achieving the highest quality finish. The type of coating is one of the technical aspects that helps get these results.
The coating refers to the overall coverage of abrasive grain particles (such as aluminum oxide or silicon carbide) which are affixed to the backing material of the product, or rather, how much space is filled or not filled with abrasive grains.
While it could seem like you wouldn’t be getting the most for your money by purchasing a product that doesn’t have as much abrasive grit as will fit on a particular area of a backing, this is actually where the science comes in, and is somewhat counter-intuitive… By controlling the amount of space left “open” or “closed” (filled), you are actually controlling how quickly (or slowly) your paper will become clogged, or rather, how long it can remain usable.
The more open space there is between grains, combined with the constant motion from sanding, can lead to a longer abrasive life. As the sandpaper is used, a more open coat allows the residue to fall off, thus making it less likely to get stuck in between the grains. Not to mention, the extra space allows more room for air to flow, thereby cooling down the sandpaper during the sanding process. And, ultimately, with less heat, there will be less opportunity for the wood resins and finish to soften in the first place.
So, what are the different coatings you’ll see, and what should they be used for?
There are generally three types of coating options available: open, semi-open and closed – each with a different ratio of grain and each with a different optimal application. There can be some variation in the terminology used between different manufacturers, but, for now, we’ll focus on these.
A closed coat offers the most grain coverage, coming in around 90-95% coverage on the product.
This type of coating is best for sanding in a raw wood state, meaning, sanding with coarser grains. This works well because the rough sanding produces larger pieces, and with harder woods, there is also less pitch (wood resin), so heat has less of an impact. This means there is less need for space between grains because it will not load as much in the first place. Additionally, with coarser grains, there is naturally more space between the tips of the grain, simply because the larger grains take up more space and fewer of them can fit on any particular size of surface area.
For example, think of a mountain range with very sharp peaks and deep valleys, with the base of each mountain touching. Now, pretend that there is water in the valleys. So, underneath the water, there is not much space between the grains, but on the surface, there is quite a bit of space between the peaks. This is what happens on sandpaper. On sandpaper, the mountains are your grains and water would be the resin securing the grain to the backing.
Closed coats products are also used for sealers, lacquers and in other finishing applications to obtain the most uniform scratch pattern and highest quality finish. If you’re grinding hard metals, you’ll want to stick with closed coats as well, to maximize the amount of grain cutting the hard material.
The benefit here is that the closed coats have more grains, and the harder woods have less pitch, so the paper will not load as quickly anyway.
A semi-open coat offers 70—75% coverage of grains, a nice happy medium.
This level of coating is useful for hard woods, such as oak, cherry and maple, in the raw wood stage.
Similar to the closed coat, with harder woods, there is less resin present in the wood, so less opportunity for sticky residue.
With the most open space of the three coatings, coming in around 60-65% coverage of grains, this type is best for softer materials, including soft woods like pine and poplar, as well as softer metals, both for coarse sanding and polishing. With the extra space between the grains, there is more likelihood for the waste material to simply fall off, thereby taking a longer time to load the paper.
When it comes to sanding, apart from tearing and other breakage, there are two main reasons for changing your belt, disc, sheet or sponge: either, the product is completely clogged, or the grains have dulled, rendering the abrasive useless. We always want to maximize the life of our abrasives, so making sure to choose the appropriate product based on the application is essential – that means understanding the mechanics of sanding, loading and prevention and the technical aspects of the products. For example, coatings, whether open, semi-open or closed, refer to the amount of grain coverage on a particular product. Understanding this can help to avoid common issues.
Quickly clogged sandpaper is one of the most common and frustrating problems known to anyone who sands. While it’s difficult to avoid completely, particularly when sanding soft woods and lower melting point metals, choosing an open coat product for these applications can help minimize the effects of loading, due to the additional space between grains. When sanding harder materials, or rough sanding, closed or semi-open coats can be used.
Ultimately, choosing the right product for the application will help you get the best finish in the most efficient way – whether you’re considering coatings, minimizing loading, grain type, or backing.
If you are looking to get a better finish, sand more efficiently, or maximize the life of your abrasives, contact our sales team to get personalized recommendations.