Some Reflections on Sheen
One of the most important qualities of a finish when it comes to appearance is sheen. A finish can vary from a gloss so high it reflects the sharp outline of an image to a sheen so dull that nothing is reflected. In determining how you want your project to look, you need to take its sheen into account.
In issue #156 of Popular Woodworking I explained how you can control sheen by rubbing it with abrasives of various grits. The coarser the grit the lower or flatter the sheen. The finer the grit the higher or glossier the sheen. The problem with rubbing is that it is a lot of work.
An easier method of controlling sheen, and the method most commonly used, is to choose a finish with the sheen already built in. Then all you have to do is brush or spray the finish onto the wood and the sheen will come about automatically.
A hundred years ago, before electric lighting, people preferred a gloss sheen because gloss reflects a lot of light and makes rooms appear brighter. Today, most people prefer a flatter sheen.
Manufacturers offer a variety of choices. Unfortunately, the words they use to describe these choices are vague: gloss, semi-gloss, satin, eggshell, rubbed effect, matte, flat and dead flat. There are no fixed definitions for these terms; so one manufacturer’s satin may be another’s flat.
To be successful in getting the sheen you want on your projects, you need to understand how the sheen-creating elements in a finish work and how you can manipulate them.
The stuff in a finish that creates all the sheens lower than gloss is called “flatting agent.” It is a very fine silica product that settles to the bottom of the can and has to be stirred into suspension before using. Gloss has no flatting agent added and therefore nothing to stir.
Though silica is a more complex material than fine sand, it’s often helpful to think of it as fine sand for the impact it creates at the surface of a finish. Here’s the way silica works.
The manufacturer adds the amount of silica necessary to create a given sheen to a polyurethane varnish, standard varnish, lacquer, catalyzed finish or water-based finish. All film-building finishes except, unfortunately, shellac are available with silica added. (It’s not true, as you sometimes hear, that lacquer is always glossy.)
Just before using the product, you need to stir the silica into an even suspension throughout the finish. You can also shake the can, but this may not be as effective unless you have a mechanical shaker like those in paint stores.
With the silica in suspension, you brush or spray the finish onto the wood. You may notice that the finish goes on glossy. That is, it has a high reflective quality. But there will come a point within a short time (depending on the drying rate of the finish) when this gloss sheen flattens quickly across the surface.
The flattening is caused by the finish “shrink-wrapping” over the particles of silica that lie at the very surface of the film. This occurs as the solvent evaporates and the finish shrinks. The shrink-wrapping creates a micro-roughness at the surface that scatters light so the gloss is reduced. The higher the density of silica at the surface, the flatter the sheen of the film finish.
It’s important to emphasize that the flattening is not caused by the silica particles embedded deep within the film as is commonly believed. Because of its nature, silica doesn’t hinder the travel of light; it’s as if the particles weren’t even there. As a result, successive coats of a satin or flat finish don’t make the finish flatter.
Because all the flattening occurs at the surface of the film, it’s easy to rub a satin finish to a gloss using fine abrasives. Begin by removing the micro-roughness with very fine sandpaper, then rub with fine abrasive compounds.