Popular Woodworking 2006-10 № 157, страница 74

All the sheen is created in the last coat of finish applied. Previous coats have no impact. Here's the proof. On the left half of this board, I brushed on three coats of gloss water-based finish. On the right half, I brush on three coats of satin water-based finish. Then I brushed satin water-based finish onto the right half of the gloss side and gloss water-based finish onto the left half of the satin side. The resulting glosses and satins are the same irrespective of what's underneath. The same would be the case no matter what type of finish I used.

To easily create a sheen not available at a store, pour off some of the gloss from a satin or flat finish after letting all the silica settle to the bottom of the can. Then mix the two (gloss and very flat) until you get the sheen you want.

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.

Manipulating Sheen

As mentioned, manufacturers aren't very informative about the sheen of the products they sell us. First, their terminology is vague. (Among themselves, they use a more exact numbering system from 1 to 100, with 100 being perfect gloss; but they rarely share this information with us.) Second, they often give us only two choices.

What if you want a sheen that is flatter than anything offered in the store, or you want a sheen that is in between the two choices offered?

To create a flatter sheen, let the flatting agent settle to the bottom of the can (tell the paint clerk not to shake the can). Then pour off some of the gloss finish at the top into a second container. You will then have some gloss in one container and some flat in another. Blend these two until you get the sheen you want.

Of course, you'll need to experiment by applying some of your blend to wood to see the sheen you are creating. Be sure to apply two coats because it's only the second coat that will produce an accurate sheen. It doesn't matter, of course, what you use for the first coat.

It's even easier if you want a sheen in between those of two products offered. Simply stir the flatting agent to put it into suspension and then mix the two.

Sheen Problems

Once you have determined the sheen you want and have a product that will produce it, problems are rare. The most common is not keeping the finish stirred. More rare, but also possible, is white specks appearing in the dried finish film.

It's not necessary to continually stir a flatted finish while brushing or spraying. But it is necessary to stir between coats.

If you are brushing, some of the flatting agent will have settled and the finish you are pulling off the upper level of the container will be glossier than you intend. If you are spraying, some of the flatting agent will have settled and the finish entering from the lower part of the spray-gun cup will be flatter than you intend.

Though it is rare, you may find tiny white specks in your dried finish and wonder where they came from. Unless you are using a dirty finish, it's most likely you have broken some clumps of dried silica away from the lip of the container. These clumps don't break up in the finish so they show up as white specks in the film.

There's no way to effectively strain these clumps because they are very small. It's best to toss the container and all the finish in it and begin again from a fresh can. You'll have to sand out the white specks from the dried film to remove them. PW

Coming next issue: Making sense of dyes: Don't let the packaging confuse you!

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