A woodworker friend called me with a problem. He had just built a bookcase and was trying to match the approximate sheen of a factory-finished piece of furniture he had and couldn’t find a polyurethane that was flat enough. What should he do? He had tried rubbing it with coarse abrasives to match the sheen but the scratches showed too much.
His problem was that most stores that sell to amateur woodworkers just carry gloss and satin finishes. Few sell flat finishes. But creating one himself is not hard.
First, I explained to him how satin, semi-gloss and flat finishes are made and work. A flatting agent, usually silica, which is similar to fine sand, is added to a gloss finish by the manufacturer. You have to stir this flatting agent into suspension before use because it settles to the bottom.
When you brush or spray the finish, some of the flatting agent is, of course, near or at the top of the layer. It creates very fine bumps, too fine to see, that reflect light randomly. The more flatting agent in the finish the more light is reflected randomly and the flatter the effect when the finish dries.
You can see the flattening happening if you watch a satin or flat finish dry. For a while after application the finish is glossy. Then very quickly, as the solvent is evaporating, the sheen develops. Thinking about this, it should be obvious that it’s the last, or topcoat, that determines the final sheen. The sheen doesn’t get flatter when you apply several satin coats, as is often said.
It should also be obvious that you aren’t limited to the sheens of finishes available. You can make any sheen you want from just one can of satin finish.
First, let the flatting agent settle to the bottom of the can. The easy way to do this is to tell the store clerk not to shake the can. Otherwise, let the can sit undisturbed on a shelf for a week or two.
Then pour off or decant some of the gloss finish at the top of the can. Leave the finish with the flatting agent at the bottom of the can.
Now you have two parts: one gloss and one very flat. You can mix these to get any sheen you want. You’ll have to experiment a little, because you don’t know the sheen of the mix until the finish has dried.
Remembering that it’s the last coat of finish you apply that is responsible for determining the sheen means that it’s possible to change the sheen with the next coat if you don’t get it right.
There’s one caveat, however. Creating your own sheen works easily with solvent-based finishes, but not with water-based finishes because the flatting agent doesn’t settle nearly so quickly.