The A-to-Z guide to finishing vocabulary - Woodshop News

The A-to-Z guide to finishing vocabulary

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Most professions and trades have their own vocabulary. Here are some of the more obscure terms we use in wood finishing:

Aliphatic hydrocarbons — This is simply the more technical name for petroleum-derived mineral spirits and Varnish Makers and Painters Naphtha, which are more often called petroleum distillates. It contrasts with aromatic hydrocarbons, which include toluene and xylene. The aromatics are generally stronger, evaporate faster and are more toxic. Some manufacturers of oil-based products such as stains and varnishes list the more complex “aliphatic hydrocarbons” term as an ingredient rather than the more common “petroleum distillates.”

Amino resins — These are the resins, usually melamine formaldehyde and urea formaldehyde, used in combination with alkyd resin to formulate conversion varnish and catalyzed lacquers. The difference between the two is that catalyzed lacquers also contain a little nitrocellulose lacquer to make them easier to use (and also a little less durable).

ASTM — The acronym for the American Society for Testing Materials, which establishes the standards for testing used by finish manufacturers and manufacturers in many other industries.

Barrier coat — A coat of finish that isn’t affected by contaminants (for example, silicone) in the wood; or a finish based on a solvent that doesn’t dissolve or damage an existing stain, glaze or finish. For example, if you were using a water-soluble dye to be followed by a water-based finish, you might choose to apply a barrier coat of shellac, especially if you were brushing the finish.

Bridging — A finish spanning the gap of a crack or void rather than sinking in and conforming to the sides. A typical case is at the joint of a poorly constructed leg and rail or style and rail. In time, the bridge will crack and peel because of wood movement.

Chatoyancy — Areas in finished wood with a darker color when looked at in one direction and a lighter color when looked at in the opposite direction. Consequently, the wood takes on an iridescent or shimmering quality as you move around it. Mottled cherry and ribbon-stripe mahogany are examples.

Coalescence — Refers to the way water-based finishes and paints dry. As the water and solvent evaporate, the emulsified resin particles come together and stick to form a hard and continuous film. The water has to evaporate first so the solvent can make the particles sticky. Contrast this method of film forming with the way shellac and lacquer dry, which is solely by solvent evaporation and the way varnishes and catalyzed finishes dry, which is by crosslinking.

Cosolvent — This is the slow-evaporating and water-compatible solvent in water-based finishes and paints that remains after the water evaporates. It makes the resin particles sticky so they form a film. Then the solvent evaporates. The solvents used are generally glycol ethers, one of which is the common solvent butyl cellosolve, which is also used to retard the drying of lacquer.

Diluent — An uncommon term that’s easier to think of as “diluting.” A diluent is a solvent that doesn’t dissolve the finish, but is compatible with the solvents that do and can be added at less expense to reduce viscosity. The most common examples in finishing are lacquers and lacquer thinners, which can contain half or more diluent. But if the thinner contains too high a percentage of diluent and is added to lacquer, the lacquer will come out of solution. This is what generally happens if you use a lacquer thinner meant for cleanup rather than thinning.

Holiday — A missed or thin area in an applied finish film. A refinish shop down the street from my shop in the 1970s called themselves “Holiday Refinishing.” We all thought it was funny, of course.

Lightfast — Resistance to fading. Pigments are much more lightfast than dyes. This is a consideration when staining an object that will be exposed to strong UV light, such as sunlight or fluorescent light.

Makeup air — A system that heats outside air and blows it into a finishing area to make up the air that is being exhausted through the spray booth. Without makeup air, the spray booth loses its exhaust efficiency.

Metamerism — The phenomenon where two colors will match under one type of light (daylight, incandescent, fluorescent) but not under another. You need to be aware of this when matching colors. Try to do the matching in a light as close as possible to that where the object will be placed.

Mil — One mil is one thousandth of an inch (.001). Typically, a sprayed finish will be 3 to 5 wet mils thick and will dry to 1 to 2 mils thick. We use a mil gauge, available from many finish suppliers, to measure the wet thickness of a film, and then figure the dry thickness by factoring in the solids content of the finish.

MFFT — The acronym for Minimum Film Forming Temperature. This is the temperature below which a water-based finish or paint won’t coalesce properly to form a continuous film. The result can be poor water resistance or color or gloss variations.

PEL — The acronym for Permissible Exposure Limit, a standard set by OSHA that establishes a maximum concentration or time-weighted average exposure limit for evaporating liquids, primarily solvents. You can find the PEL on the Material Data Safety Sheet for the product.

Print resistance — The ability of a coating to resist pressmarks when parts are stacked or placed in contact with each other. Print resistance will vary with temperature and, especially for water-based finishes, humidity conditions. Print resistance can often be improved by choosing faster-evaporating solvents.

Resin — A vague term that originally referred only to natural substances such as shellac, kauri and copal, but now is also often used for the synthetic film formers that make up most of the finishes today. It refers to that part of the coating that remains after all the solvent has evaporated. The resin provides the hardness, gloss, durability, adhesion, drying and handling characteristics to the film.

Surface tension — The tendency of a liquid to contract or spread due to molecular forces. Water, with a high surface tension, contracts and beads on gloss surfaces. Silicone, an oil with a very low surface tension, spreads so easily that it repels liquids with higher surface tension, such as finishes, so they pull away and form craters (“fish eyes”).

Surfactant — An additive, most common in water-based finishes, that reduces the surface tension of the water so the finish flows out level.

Thixotropic — The characteristic in a liquid product that causes reduced flow until the product is spread, stirred or shaken. Then it flows easily but returns to its gel-like state when the sheering stops. Catsup is a good example of a thixotropic substance. It doesn’t flow out of the container easily until you shake it, and it doesn’t flow over the food until you spread it. Latex wall paint, gel stain and most glazes are thixotropic. 

Bob Flexner is the author of “Understanding Wood Finishing” and “Flexner on Finishing.”

This article originally appeared in the November 2013 issue.

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