You can buy finishes in a number of sheens, ranging from gloss to dead flat. But you aren’t limited to just the sheens provided from any given manufacturer. You can create any sheen you want just by understanding how flatting agents work. Flatting agents are the solid stuff that settles to the bottom of the can and has to be stirred into suspension before using.
Of course, you can also control sheen by rubbing the final coat of finish with abrasives such as steel wool or rubbing compounds but doing this is lot more work.
Flatting agents are composed of small particles of solid material, the nature of which makes them invisible and fairly transparent in the dried finish film. As the wet finish shrinks during drying, it pulls taught over the particles that are floating close to the surface, and this creates the microscopic roughness that gives the flatted effect.
The particles aren’t exposed; they are “shrink wrapped” by the finish film. You see the shrink wrapping occurring as you watch a flatted finish dry. At first, when you apply the finish, the film is glossy. But as the solvent or thinner reaches a certain point of evaporation, the flatting appears rather suddenly.
In most cases you buy the finish with the flatting agent already added. Manufacturers use loosely defined terms, such as semi-gloss, satin, eggshell, flat, and matte, to identify these finishes. Becoming more common, manufacturers use a more helpful numbering system, such as a 20 sheen for fairly flat or 90 sheen for gloss. You can blend any of these sheens with each other to get your own flatted effect, or you can add a separately packaged flatting agent to a finish. The more you add, the flatter the resulting appearance.
There are a number of substances that can be used as flatting agent, including synthetic amorphous silica (silicon dioxide), stearates, diatomaceous earth, waxes and talc. Manufacturers choose between these substances for cost and performance characteristics.
The best flatting agent is amorphous silica because of its high efficiency and better transparency. Most manufacturers use amorphous silica in their flatted finishes, but unfortunately, they rarely tell us on the can or in their literature, so we can’t know for sure. If you are having any trouble with the product you’re using, and the manufacturer doesn’t solve it quickly, change to a different brand.
You may have noticed that the flatting effects of a flatted finish are greater the more you thin the finish. The reason is that the finish spreads out thinner so more of the flatting agent is exposed near the surface of the film. Conversely, the thicker the finish during application the less effective the flatting agent; more of it is embedded deep within the film. On any given project you should thin your finish the same each time you refill your cup or pot, so the flatting effect will always be the same.
Because the flatting effect is created by a microscopically roughened surface, it should be obvious that a flatted sheen can be made glossy by simply leveling with fine sandpaper and polishing with very fine abrasives.
Sheen is the reflection of light when a surface is viewed at a low, or grazing, angle. In a high-sheen or high-gloss surface you see glare or a distinctly reflected image. In a low-sheen, satin, or flat surface, glare and reflection are softened to the point of non-existence.
Though the methods aren’t perfect, manufacturers have ways of measuring sheen and assigning a number when the surface is viewed from different angles. In some cases, manufacturers provide us with this number, which usually makes it easier to match existing sheens.
You may have noticed that now and then you get some white specks in your finish caused by flatting agent that has clumped. Clumping occurs when the particles of flatting agent are allowed to bond together either at the bottom of a can or by drying around the lip of the container. Once flatting agent has clumped, it cannot be effectively separated again. You should change to a fresh product.
There are two common myths about flatting agent. First, the flatting effect is cumulative either because each coat adds to the flatness of the previous coat, or because, in the case of lacquer, all the flatting agent from each coat floats to the top of the last applied coat.
Second, flatting agent weakens the scratch resistance of the film.
Because the flatted effect is created not by the particles embedded deep within each coat of finish film, but only by those particles located near the surface, there is no cumulative effect caused by applying more coats. The only way to increase the flatness of a finish is to add more flatting agent to it or to thin the finish so it spreads into a thinner film. You can continue to apply coat after coat of any given flatness and the resulting sheen will not change.
You can easily test this by applying a gloss coat over a satin or flat coat. When the finish has dried, the surface will be glossy just as if all the coats had been gloss.
Nor does flatting agent rise to the top of a coat of lacquer. If anything, the flatting agent settles, just as it does in a can. At any rate, flatting agent surely doesn’t transfer from one coat to another. This myth may have gotten started because finishers have noticed that a flatted finish can become glossy after years of use. What really has happened, of course, is that the microscopic roughness on the surface has been leveled and polished by wear, not that all the flatting agent has been removed from the film.
Because particles of amorphous-silica flatting agent are almost always coated by a thin finish film that has shrunk around them (they are rarely exposed on the surface), there is no way these particles can reduce the scratch resistance of the film. But they sometimes give this appearance because coarse objects can level them relatively easily leaving an apparent mark or scratch on the surface.
Amorphous silica is often made with a coating of wax on each particle to help resist scratching if the film is broken. This coating also aids in keeping the flatting agent from clumping at the bottom of the can. Flatting agents other than amorphous silica may, however, reduce the scratch resistance of the finish film. If you are having problems, switch to a different brand.
Bob Flexner is author of “Understanding Wood Finishing” and “Wood Finishing 101.”
This article originally appeared in the January 2020 issue.