A Glossary Of Basic Finishing Technology
As with any technical field, understanding the terminology of finishing is critical. It’s also critical that we all mean the same thing with the terms we use.
With that in mind, here are some of the most common finishing terms, in alphabetical order, and their definitions.
- Bleach is a chemical that removes stains and, sometimes, the natural color from wood. The three types of bleach are chlorine (sodium hypochlorite), which removes dye color without changing the color of the wood; oxalic acid, which removes rust marks and lye stains without changing the color of the wood; and two-part bleach (sodium hydroxide and hydrogen peroxide), which removes stains, and also removes the wood’s natural color.
- Dye is a chemical that dissolves in a liquid solvent. Typical solvents include water, alcohol and toluene. Dye can be applied to wood dissolved in the solvent alone, or dye can be added to a finish and thinner (with or without pigment included) to make a wiping stain or toner. Dye is transparent.
- Film Finish is any finish that can be built up on the wood (by repeated applications) to a noticeable thickness. The key requirement for a film finish is that it cures hard. Varnish (including polyurethane varnish), shellac, lacquer, water-based finish and a number of two-part finishes are all film finishes.
- Finish is a substance that changes from a liquid to a solid after it’s applied to the wood. The purpose of a finish is to protect the wood from moisture and to enhance the wood’s appearance. The term “finish” can also refer to all the steps in a built-up finish. These steps could include a stain, several coats of transparent finish, and coloring steps (glazing and toning) in between the finish coats. The context usually makes it clear which definition of “finish” is being referred to.
- Gel stain is a stain that has been made thick and resistant to flow, similar to glaze. Because of the resistance to flow, gel stains don’t penetrate well, so they are used primarily to reduce blotching on blotch-prone woods such as pine and cherry.
- Glaze is a stain that has been thickened enough so it stays where you put it. It doesn’t run on vertical surfaces or flatten out on horizontal surfaces. Glaze is used to decorate (graining, antiquing, highlighting, etc.) and to add the appearance of age and depth.
- Glazing is the application and manipulation of a stain or glaze between coats of finish with the purpose of adding decoration. Typical effects include graining, antiquing, distressing and highlighting recesses.
- Paste wood filler (or “pore filler”) is a finish (usually varnish or water-based finish) with a pigment colorant and a solid filling substance added. The filling material is usually silica (resembles fine sand) in varnish-based fillers and wood dust in water-based fillers. Paste wood filler is used to fill the pores of porous woods such as oak, mahogany and walnut before the application of topcoats. The filler can be applied directly to raw wood to fill and stain in one operation, or it can be applied over a washcoat or sealer coat to add color only to the pores (the best procedure). All excess paste wood filler has to be wiped or sanded off the surface to avoid muddying the wood.
- Patina is the mellowing and color change that occurs in wood over time due to oxidation (from exposure to air) and bleaching (from exposure to light). Patina is also the dings, scratches and wear in the wood or finish that give old furniture and woodwork character.
- Penetrating finish is any finish that cures too soft to be functional if built up on the wood. The excess finish has to be wiped off after each application. All oil and oil/varnish blend finishes are penetrating finishes. By this definition, wax is also a penetrating finish, but it isn’t usually referred to as such because it doesn’t penetrate much. A better terminology would be “film-building” for all finishes that harden and can be built up, and “non-film-building” for all finishes that don’t harden and have to be wiped off.
- Pigment is finely ground earth or colored synthetic particles made to resemble earth. Pigment is used to add the color to stain, glaze, toner, paste wood filler and, of course, paint. Pigment is opaque and hides the wood if built up thick. But if the excess is wiped off, the pigment remaining in the wood’s pores highlights them, which adds depth and grain definition to the wood. The pigment settles in cans and must be stirred into suspension before each use.
- Rubbing and polishing is the procedure used to level the surface of the final coat of finish and raise or lower the sheen. Various types of abrasives are used to do this, including sandpaper, steel wool, abrasive pads and rubbing compounds in liquid, paste or powder form.
- Sealer is the first coat of finish applied to the wood. This first coat seals the pores of the wood so that any liquid applied on top, including the next coat of finish, will remain on top; it won’t penetrate into the wood. A sealer can be the first coat of the same finish you’re using for the topcoats, a special varnish or lacquer “sanding” sealer made to sand easily, a water resistant vinyl sealer commonly used under catalyzed finishes, or shellac, which forms an effective barrier over oil, resin and odors (from smoke or animal urine) in the wood.
- Shading stain is a toner used to change or adjust the color of part of the wood without affecting other parts. Examples of situations where shading stain (toner) is used include highlighting (darkening the areas around the area to be highlighted) and matching sapwood to heartwood. Shading stains are always sprayed.
- Sheen is the degree of gloss in a cured finish. All film finishes cure to a gloss sheen unless they have flatting paste (gloss-reducing solid particles) added. Semi-gloss, satin, matte and flat finishes have flatting paste added. These finishes must be stirred before use to put the flatting particles into suspension.
- Solids content is the ratio of finish (the part that hardens) to thinner (the part that evaporates) in a finish. Most finishes are supplied with a solids content ranging from 20 to 40 percent.
- Solvent is any evaporating liquid that will dissolve a dried finish. The solvent for a finish can also be used to thin the finish. Examples of common solvents are lacquer thinner for lacquer, denatured alcohol for shellac and mineral spirits for wax.
- Stain is any liquid that colors wood. Two colorants are used in stains: pigment and dye. Pigment requires a binder (finish) to glue it to the wood. Dye can be used with or without a binder.
- Thinner is any evaporating liquid that can be added to a finish, stain, glaze, toner or paste wood filler to thin it and make it easier to apply with a brush, cloth or spray gun. Examples include mineral spirits for oil-based products, water for water-based products, lacquer thinner for lacquer products and denatured alcohol for shellac products.
- Toner is finish, usually lacquer, thinned with up to six parts thinner and a pigment and/or dye colorant added. Toner is always sprayed and can be used to add color in very thin layers between coats of finish. Toner is most often used to adjust the wood’s color after it has been stained and sealed, or used as a shading stain.
- Topcoat refers to all coats applied over a sealer coat.
- Washcoat is any finish thinned to 10 percent, or less, solids content. A washcoat can be applied directly to the wood to partially block the penetration of a stain and prevent blotching, or to harden end grain so it becomes easier to sand smooth. A washcoat can also be applied between coloring, filling and glazing steps to keep them separated and make them stand out better.
- Wood Conditioner (also called stain controller and pre-stain) is a commercial washcoat product made by thinning varnish with two parts mineral spirits, or thinning water-based finish an equivalent amount. Wood conditioner is used to prevent blotching in pine, cherry and other blotch-prone woods. To be effective, the product should be allowed to cure thoroughly before applying a stain.