One of the most important qualities of a finish when it comes to appearance is sheen. There are many finishes that can be applied to your project. They could have a high gloss that reflects the image’s outline or a dull sheen that reflects nothing. In determining how you want your project to look, you need to take its sheen into account.
In issue #156, I discussed how to control sheen using abrasives with different grits. The coarser the grit the lower or flatter the sheen. The sheen will be higher or more glossy if the grit is finer. Rubbing is hard 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. Most people prefer a flat sheen today.
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 manufacturers satin may be anothers 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.
Flatting Agent
The stuff in a finish that creates all the sheens lower than gloss is called flatting agent. Flatting agent is a fine silica product. It settles to bottom of can and must be stirred into suspension prior to use. Gloss does not contain any flatting agents and is therefore easy to stir.
Although silica can be more complex than fine sand it is still useful to consider it fine sand because of the impact it has on the surface of a finish. This is how silica works.
The manufacturer will add the required amount of silica to create the desired sheen to a polyurethane varnish or standard varnish, lacquer finish, water-based finish, catazed finish, or lacquer. All film-building finishes except, unfortunately, shellac are available with silica added. (Its not true, as you sometimes hear, that lacquer is always glossy.)
Before you use the product, stir the silica to ensure a uniform suspension. The can can be shaken, but it may not work as well if you don’t have a mechanical shaking device such as those found in paint shops.
You can spray or brush the finish onto the wood using the silica suspension. You may notice that the finish goes on glossy. That is, it has a high reflective quality. However, depending on how dry the finish is, the gloss sheen will eventually flatten quickly.
Flattening occurs because the finish shrink-wraps over silica particles that are at the film’s surface. This occurs as the solvent evaporates and the finish shrinks. Shrink-wrapping causes a micro-roughness on the surface which scatters light, reducing the gloss. The film’s sheen will be flatter if there is more silica on the surface.
It is important to stress that flattening is not due to silica particles embedded in the film. This is contrary of what is often believed. Because silica is light-sensitive, it doesn’t slow down the movement of light. The result is that successive coats of satin or flat finishes don’t make the finish less flatter.
Because all the flattening occurs at the surface of the film, its 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.
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