Defining Finishing Terms
All technical fields have their own vocabulary. You’ll have difficulty mastering any technical field without understanding its vocabulary. Here are some of the most commonly used finishing terms and their meanings.
Sealer (sanding sealer) is the first coat of any finish. The first coat penetrates, dries, and seals the pores so the next coat of finish (or any other liquid) has difficulty penetrating into the wood. The sealer coat also locks raised wood fibers in an upright position causing the surface to feel rough. To achieve a smooth final finish, you should sand the sealer coat smooth before applying additional coats. Special sanding sealers are made for lacquer and alkyd varnish that are difficult to sand. Sanding sealers don’t clog sandpaper as easily as these finishes.
Finish is a substance that changes from a liquid to a solid after it has been applied to the wood. The purpose of a finish is to protect the wood and enhance its appearance. By adding colorant to a finish (usually called a binder in this situation), you can make a stain, glaze, toner or paint.
Film finish is any finish that can be built (by repeated applications) to a hard, thick layer, or layers, on top of the wood. The key requirement for a film finish is that it must dry hard. This is in contrast to so-called “penetrating” finishes, such as oil, which don’t dry hard. Alkyd and polyurethane varnish, shellac, lacquer, water-based finish and catalyzed (two-part) finishes are all film finishes.
Washcoat is any finish thinned with two or more parts thinner to partially seal the wood and prevent uneven stain penetration (blotching) on soft woods such as pine and tight-grained hardwoods such as cherry and birch. So-called wood conditioners are oil or varnish washcoats. To be effective these (and all) washcoats have to be allowed to dry thoroughly before a stain is applied. A washcoat can also be used between coloring steps with minimum build to prevent the colors from running together.
Thinner (mineral spirits, naphtha, lacquer thinner, alcohol, water) is any evaporating liquid that can be used to thin a finish, stain, glaze or pore filler to make application easier.
Solvent (mineral spirits, naphtha, lacquer thinner, alcohol, water) is any evaporating liquid that dissolves a dried finish, stain, glaze or pore filler. Often a solvent for a solid substance is also the thinner for that substance in liquid form.
Sheen is the degree of gloss in a dried finish. Most film finishes dry to a gloss sheen unless flatting agents (gloss- reducing solid particles) are added. Semi-gloss, satin, matte and flat varnishes, lacquers, and water-based finishes have had flatting agents added. These finishes must be stirred before use to put the flatting agents into suspension.
Stain changes the color of wood. There are two types of colorant used in stains: pigment and dye. Pigment particles are opaque and resemble colored earth. They settle to the bottom of the can and must be stirred into suspension before using. Applied to wood, pigment lodges in recesses, such as pores and sanding scratches, large enough to hold it and remains there after the excess stain is wiped off. Built up on wood, pigment obscures the wood like paint. To glue the pigment particles to the wood, a binder (oil, varnish, lacquer or water-based finish) must be included in the stain. Dye is transparent. It dissolves rather than suspends in the liquid. Once dissolved, dye remains in solution. It can be combined with a binder, or simply dissolved in a liquid and applied.
Glaze is a stain that has been made thick and resistant to flow so it stays where you put it, even on vertical surfaces. Gel stain, for example, makes a good glaze. You can use a glaze to darken or change the color tone of wood after the wood has been sealed. You can leave a glaze in the recesses of carvings, turnings and moldings to give the appearance of age and three- dimensional depth. You can feather out a glaze to highlight certain areas, such as the centers of cabinet doors.
Or, using special glazing tools, you can make patterns in a glaze that resemble wood grain or marble. When the glaze is dry, protect it from being scratched off by applying one or more coats of a clear, film finish.
Pore filler is essentially a glaze with silica (fine sand) added to provide bulk. Pore filler is used to produce a mirror-flat effect by filling the pores of porous woods such as quarter-sawn oak, mahogany and walnut before the application of a film finish. Pore filler doesn’t take stain well, so pigment should be added before application. You can apply pore filler directly to raw wood to fill and stain in one operation, or you can apply a different colored filler to a sealed (and stained) surface to highlight the pores, as I’ve done on the right half of the panel below.
Toner is finish, usually lacquer, with dye or pigment added and sprayed. To provide better control, thin with up to six parts thinner. Toner adds color in very thin layers without penetrating into the wood. Toner can be used to change or adjust a color after the wood has been sealed.
Shading stain is a toner used to change or adjust the color of certain parts of wood without affecting other parts. Shading stains can be used to blend sapwood to heartwood and to highlight certain parts, such as the centers of cabinet doors, by darkening the surrounding areas.
Rubbing and polishing is the procedure used to level the surface of the final coat of finish and raise or lower the sheen. Various abrasives, including fine sandpaper, steel wool and rubbing compounds, are used. Sandpaper removes dust nibs, orange peel, brush marks and other imperfections in the surface. Steel wool and rubbing compounds (fine abrasive powders in a liquid or paste) raise or lower the sheen.
Bob Flexner is the author of “Flexner on Finishing,” “Wood Finishing 101” and “Understanding Wood Finishing.”
Understanding Wood Finishing is, quite simply, the most practical, authoritative and comprehensive book ever published on this important subject. If you’ve ever struggled with finish application (in other words, if you’re a woodworker), this book is a must-have. Get your copy today on shopwoodworking.com