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Wood Finishing |
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Wood finishing
is the process of applying some type of solvent-based polymer that
penetrates the surface of a wood item in order to highlight its natural
beauty and aid in its resistance to degeneration. Usually this means applying a coating, but can
also mean rubbing in an oil or wax that leaves the surface of the wood
accessible to the touch.
The solvent could be anything - turpentine, naptha, mineral spirits,
amyl acetate, xylol, water, alcohol, to name a few. It's important to
know which uses what because they're usually incompatible and one
coating when put over another different one will often react
unfavorably with it. Staining is the process of altering the color of the wood by applying either pigmented solutions that distribute ground up pigments evenly over the surface of the wood or dyes that penetrate the structure of the wood to alter its color. Both of these are done prior to finishing and must be chemically compatible with the chemistry of the final finish. Another part of wood finishing is "rubbing out" the finish. This is a secondary operation of sanding down the completed coating with ever increasingly finer abrasives until a desired reflective luster is achieved, from satin finish to high-gloss finish. This requires a physical technical ability distinct from the chemistry of finishes. Oils - These are "drying" oils that have been dissolved in a solvent so they're easy to apply and soak into wood. They have added chemicals that speed up the reactivity of the oil with oxygen to form a hard polymer (called polymerization) that doesn't come off on your hands. It "dries". But actually it is more accurate to say that it "cures" - changes from a liquid to a hard substance. The advantage to oils is that they are a "penetrating" finish that penetrates into the cellular structure of the wood, encouraging the play of light in that structure that brings out a natural luster of the wood, but they do not form a coating so you can feel the texture of the wood itself. Because they've penetrated the cells and hardened into a polymer within them, the wood is resistant to staining. But, they're not very resistant to abrasion, and should be re-applied every year or two. Waxes - There are many kinds of waxes. They all get rubbed into the wood, whereupon they "harden" through their reaction with oxygen. They are very similar to oils in that they penetrate and are not really a "coating", but they allow the wood to be buffed to a higher luster than oils. They are not very "protective" of abrasion, heat, or spillage, but they are beautifully visually "soft" finishes. Varnishes - These are similar to oils and waxes, but suspended in a solvent that allows them to be brushed or sprayed onto the wood surface and remain as a thin coating. The solvent evaporates, leaving an even thinner coating of thick liquid material that gradually reacts with the oxygen in the air to polymerize into a hard coating. They are very durable, very beautiful, and very good finishes. Often they are applied in several coats, each coat adhering to the one below without dissolving the one below because the curing process of polymerization has rendered the coating impervious to the very solvent in which it was originally dissolved. Shellac - This is a unique finish. It is a suspension of natural resins in alcohol. It is a very old recipe but retains its place today because unlike nearly every other finish it is compatible as an undercoat with all the others, takes stain, penetrates wood, is easy to apply, relatively non-toxic, and it's beautiful in and of itself. It has two drawbacks - one is that the material in the can has a limited shelf life, the other is that it is easily re-dissolved with alcohol and alot of furniture has to endure its owners and their guests spilling alcohol on it. French Polish is a shellac application process. Lacquers - This is a special group of finishes that, like shellac, the finished coatings are forever dissolvable in their solvent. This may seem disadvantageous, but unlike shellac's solvent (alcohol), very seldom does the owner of the furniture have occasion to spill the solvent (lacquer thinner, amyl acetate, toluene, xylol, etc) on their furniture. The advantage to lacquers is that they are put on in very thin coats, each coat chemically melding with the one before, making a single-layer coating. This process allows a thorough flat-sanding between each coat of ever-increasingly fine abrasives during the application process, resulting in an exceedingly high-quality single-layer coating that is very flexible, has excellent adhesion to the wood, penetrates the wood deeply bringing out its natural luster more readily than any other coating, is the flattest smoothest and the least "coated" look of all the coatings. Lacquers are time-consuming, but are used on some of the highest quality works. Polyesters - This group of finishes can be categorized as "catalyzed" finishes, meaning that a small amount of catalyst is added to the liquid coating prior to application. This catalyst greatly speeds the curing time of the finished coat, as well as adding durability and hardness. These coatings are often applied so thickly as to require only one coat - so they speed up the finishing process for the shop doing the work. They're not particularly penetrating, so the natural lusters of wood are not accentuated by these finishes. However, for durability and abrasion resistance, they far exceed the finishes I've mentioned prior. Urethanes - There are Polyurethanes, which are varnishes and covered in the varnish description above, but there are also an entire group of urethane finishes that have been developed by the automobile industry to coat vehicles. If ever there were a test for the durability of a coating, it would be a car. They retain their gloss, their corrosion protection, their all around integrity far longer under far more duress than nearly any other situation. They are usually catalyzed, and require a professional setup to spray on. They can air-dry or bake dry. Either way, they are enduring, though like polyesters they somewhat mask the natural beauty of wood. Epoxies - These are high in abrasion resistance, high in chemical resistance, high in UV resistance, and can be applied in very thick coatings. They're not particularly good looking, but that is not their primary function. Powder Coated Finishes - These are all finished by baking in a big oven at really high tempuratures, so they're not applicable to wood. However, they're great for metal. They are initially applied to the completed work by statically charging the work and spraying a cloud of powdered coating material near the charged work. The work attracts the powder like a balloon attracts dust. When the powder has deposited itself thickly enough over the work, the entire piece is put into an oven and baked for a half hour or so. This melts the powder grains together to form a coating. The advantage to this that an even-thickness coating can controllably be applied to complex pieces without drips or dry spots. The chemistry, color, and luster of the powder coating material can be adjusted by the manufacturer for optimizing whatever characteristic the fabricator desires. These finishes are the most durable finishes regularly available. They are not excessively expensive, but they very seldom can be applied in one's own shop. |
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