Flat, Semi-gloss or Low-sheen Coatings
How they work, how to achieve them, and their consequences
The difference between a gloss coating and a semi-gloss, satin, “low sheen” or flat coating is simply the surface roughness. A gloss finish has a surface roughness comparable to window glass, a mirror, or a waxed car finish. But why does it shine?
Light behaves in certain circumstances as a wave, and will reflect an image from a surface which is smooth compared to the wavelength of light. That is about twenty millionths of an inch. A surface which is much more smooth than that (say, a surface roughness of only a few millionths of an inch, much less than one wavelength) will reflect an image without much scattering of the light and consequent blurring of the image, and so the surface appears smooth, or high-gloss.
A surface which has a roughness of many wavelengths of light, or about a few ten-thousandths of an inch, will appear "flat.", “eggshell”, “satin” or other-such terms, even though it may be “smooth-to-the-touch”.
The typical thickness of a single coat of varnish or clear urethane finish is about one to two thousandths of an inch (dry film thickness). The surface roughness necessary for a flat coating is less than a tenth of that, and satin finish, semi-gloss, etc., surface roughness is even smaller. Incidentally, slight variations in that surface roughness will produce variations in the sheen of the surface, and these will be visible as blotches or patterns. Sprayed low-sheen finishes therefore can appear more uniform than brushed or rolled applications.
In order to understand how to make a high-quality flat, semi-gloss, satin finish, or low sheen surface of consistent sheen and professional appearance it is necessary to understand a little more about exactly how a coating is made to dry rough instead of glossy-smooth, as liquid coatings usually do.
Gloss coatings consist of the resins or polymers that will become the final coating, dissolved in some compatible solvent blend. The resins may be very viscous liquids or even solids by themselves. The solvents reduce the viscosity and aid the surface leveling of the coating.
A gloss finish, as it dries, leaves a surface that is smooth within a small fraction of a wavelength of light. That's a few millionths of an inch.
Gloss coatings for wood normally have a flexibility and elongation capability that is compatible with the natural seasonal and weather-related movements of wood.
Low-sheen finishes are made by the paint manufacturer starting with a gloss finish and adding little particles of transparent or neutral-colored minerals, so that there is somewhat more of a volume of the minerals (think of them as very small rocks) than of the resin. As the solvents evaporate, the remaining resin shrinks down in-between the rocks (which are now piled up at random, almost touching), creating a surface that is microscopically rough on the order of the size of the rocks, which need only be a few millionths of an inch in order to scatter the light and give a low-sheen, flat or "satin" finish. A little less resin, a few more rocks and the surface is rougher and so appears more "flat."
The inevitable consequence of this is that the coating, which of course would need to have a flexibility comparable to that of wood in order to stay stuck to wood as the wood naturally expanded and contracted, now has the flexibility of that resin mixed with a layer of rocks.
Coatings intended to provide superior long-term protection to wood must have exceptional elongation (many tens of percent) and retention of that elongation with age.
In order to obtain both the low-sheen finish and the flexible finish, one may apply several coats of the gloss finish, and then one final coat of the thinned, flat or semi-gloss finish.
It is extremely difficult to remove the artifacts of the application process if brushing or rolling a low-sheen coating. Even if sprayed, one may have artifacts. One way to reduce or eliminate blemishes and other artifacts of the application process is to add an excess of solvent, so that the leveling effect of the solvent disperses ridges and the thinning effect of the solvent deposits a thinner film, blemishes and all. Such over-thinned coatings must not be allowed to puddle, as when doing a floor. When the puddles dry the sheen will not be uniform. Two or more such applications, brushed or rolled perpendicular to each other, should produce a finish of uniform sheen
There is another consequence of having a low-sheen coating on, for example, the outside front door of a house: When a clear high-gloss finish loses its gloss, it’s time to recoat, as that means the UV absorbers on the surface have worn out, and the coating has reached its life expectancy. If not recoated, the ultraviolet from the sun will continue to break down the varnish protection and begin to bleach the underlying wood, and then it won’t look pretty any more. When the underlying color of the wood goes, the only possible “maintenance” is to strip the surface entirely and start again from scratch. You want to avoid this if at all possible. Clear gloss finishes give you that advance warning as loss-of-gloss. With a low-sheen finish, even if everything is done as correctly as possible, there is no advance warning. The only solution is to have a written recoat maintenance-schedule and not “forget”.
So, there are a few reasons why satin or any low-sheen finishes outdoors fail sooner...much sooner...than gloss finishes, whether clear or pigmented. The paint manufacturers don't care, as they will sell you (they think) more product more often, and that's good for their business. You and I want your finish to last as long as it possibly can, and that’s why I make Clear Penetrating Epoxy Sealer™, also known as MultiWoodPrime™ (The World’s Greatest Primer™), and why I write these Application Notes.
The way you can get a better value for your labors is to apply a high-gloss finish. To start, use the Clear Penetrating Epoxy Sealer on the wood first, per the application note on priming at www.smithandcompany.org/technical.html ; that's in the third application-note down, but you should read all of the first group. The reason to use the high-gloss finish, is that, not having all those rocks for a filler, it will be the most flexible and give the best mechanical match to the wood. I would recommend the Epifanes Gloss varnish in this regard.
In summary, there are two ways to get the low-sheen finish you may still desire, and get a better life-expectancy than using only a low-sheen store-bought coating. One is to apply the clear gloss varnish per my recommendations, allow a couple weeks for a *full* chemical cure, and then wet-sand with a little soapy water and a green Scotchbrite pad to get a uniform low-sheen. The second way is to apply one last topcoat of a low-sheen varnish; there will be enough of a mechanical buffer-layer underneath that you should not get the premature-failures that are certain with just a low-sheen finish on the wood. *However* you will get "artifacts of the application process" as variations in sheen due to brush-marks. These are virtually impossible to eliminate unless the flatted finish is sprayed, and that’s not always practical. I therefore recommend the wet-sanding method.
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