Take this for what it is worth, because I am not a physicist, chemist, or even a college graduate, but I have worked with coatings for over 35 years and done a lot of research and conversed with many of the above chemists, physicists, and PhDs over those years:
Coatings provide protection from the environment and esoteric appeal. For wood, the deteriorating factors are erosion, organics (mold, etc), chemicals (i.e acid rain, exhaust pollution), oxidation, and ultraviolet light.
Components of coatings consist of three main categories: Resin (the binder that holds the coating together when dry, and give it flexibility or brittleness, hardness, toughness, or softness, moisture resistance, and some gloss characteristics); Pigment (the particles that give it color, abrasion resistance, UV resistance, heat resistance and gloss characteristics depending on its leafing qualities); and Solvents (which keep it fluid enough to apply, but leave through evaporation before the coating cures). Sometimes additives such as insecticides are combined with the resin to enhance a particular property.
Curing of coatings is by three main methods by which the resin becomes inert (not necessarily hard): Evaporation (lacquer, shellac, silicon sprays); Oxidation (varnishes, enamels); Catalytic or chemical reaction (epoxies, 2-3 component polyurethanes); or a combination or two or three.
Resin provides resistance from oxidation and moisture as it seals the wood. It is also the compnent that is destroyed most readily by UV.
The pigments provide the UV protection by reflecting the suns rays. If they are fully leaved like flat leaves laying flat on top of each other they give the most reflection (gloss coating). Interleaved means the pigment flakes are somewhat flat and allow light (UV) penetration at different angles (semigloss paint). Open leaved has the pigment flakes standing on end, offering the least reflection and most penetration of UV (Flat paint). Most commercial coatings have mineral pigments as they are the least photochemically reactive to light in their own right (rocks stand up for a while). The only clear/translucent pigment is silica (glass) and it still allows some UV penetration.
Deterioration of a coating comes from the UV deteriorating the resin, allowing the pigments to fall out or be washed away. Gloss, which reflects most light, last longest and flat lasts the least.
Most varnishes (and that includes the "Polyurethane" varnishes) cure by oxidation, and will deteriorate from both oxidation from air exposure that is expedited by UV deterioration. The longer the molecules in the resin (i.e tall-oil or long-oil varnishes), the more resistant to the UV. Spar varnishes are the only long-oil varnishes in the average commercial store. The varnish is the resin.
Bottom line: No 2 coat clear coating will provide UV protection for more than 3-5 years. The 5 year ones are those that contain silica, and the protection factor degrades per year, so some UV damage will be seen.
Pigmented (i.e color type which masks the wood) coatings can provide 10 year+ protection, depending on the pigment. More light reflection, especially in the UV spectrum, and the higher the intial gloss, the longer the coating will last. (i.e white high gloss paint lasts longer than purple or black). There are some silicon based aluminum pigmented coatings that will last longer, but need high temperature to cure enough to be moisture resistant.
This is submitted with the expectation that Howard will correct any misconceptions I have.
I do not know if the color change in cedar is photochemical or oxidation. If oxidation, keeping the moisture away will keep its hue. If photochemical, light will change it regardless. I suspect it is photochemical as is cherry.
Go
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grey cedar looks rustic!:rotflm:
