Finishes is a general term that includes single or multiple coatings or processes applied to a surface using either spray, brush, rag, sponge, electrolysis, mechanical, or abrasive methods (Figure 7.35). Historically, societies have placed a high value on the finish of furniture. Wood, stone, and fabrics are porous materials that need sealers to protect them and enhance their visual appeal. Wood is finished primarily to protect and enhance its surface. Glass can be finished to minimize the oily finger marks left by touching. Metal can be finished to minimize the effects of oxidation (though the protection is temporary).
The first industrial use of an aniline dye finish was in the manufacture of mauverine, a purple dye discovered in 1856 by William Henry Perkin. During the mid-nineteenth century, aniline was an expensive laboratory compound, but within 10 years of its first manufacture, it was produced in significant volumes, utilizing a manufacturing process that was previously invented by Antoine Bechampe.3
Aniline dyes are used on wood because of their intense color. Aniline dyes are available in water-based, alcohol-based, and oil-based formulas. Water-based formulas require preparation of the wood’s surface with a damp cloth prior to application in order to create a
smooth and seamless finish. Aniline dyes penetrate the surface of the wood rather than remaining on the surface (as stains tend to do) and are available in a wide range of bright and primary colors. When using water-based aniline dyes, one must carefully sand down any raised grain that results from the damp pretreatment. All dyes require a sealer and topcoats, whether oil, lacquer, or water-based.
Natural lacquer is extracted from insects that secrete a resinous substance called scale. Scale was used in lacquer and in some varnishes, and has been known for 1,000 years as the best finish for furniture. Once dried, the natural lacquer protects the furniture from humidity, water, heat, and chemicals. The process takes a long time to cure and requires repeated applications, so it is not suitable for mass-produced furniture. Modern lacquer was developed to mimic natural lacquer and is the industry standard for mass-produced furniture (Figure 7.36).
Nitrocellulose and Precatalyzed Lacquer
Wood furniture is commonly finished using nitrocellulose and catalyzed lacquers in a matte, satin, or gloss sheen. Lacquer finishes protect wood surfaces and, when applied properly, greatly increase the perceived value of wood furnishings. When combined with aniline and pigmented dye stains, a fine lacquer finish can enhance the color, texture, and grain of wood. Boiled linseed and Danish oil finishes applied with a cloth or brush are beautiful but offer less protection from use.
Catalyzed lacquer finishes are harder and more durable finishes than standard nitrocellulose lacquers but require more skill in their application. They are also relatively difficult to repair outside of a factory environment because of an ingredient that cures the lacquer in 21 days to a nearly nonreactive finish.
Lacquered finishes are particularly susceptible to water stains, especially within the first 30 days of their application. Generally, vertical and horizontal wood surfaces receive a sprayed application. Horizontal surfaces generally receive one additional coat more than vertical surfaces. Care must be given not to overapply a nitrocellulose lacquer finish because each application will fuse with the previously applied lacquer application, whereas catalyzed finishes will not.
Polyurethane finishes are less attractive and slower to dry than lacquer finishes, but they are more durable and resistant to moisture and water when fully cured. When polyurethane is
applied, cold joints result between each application, because the applied coat does not dissolve the previous coats. For this reason, each coat must be applied to a properly prepared surface using a brush, cloth, or spray. Between applications, care must be given to lightly sanding or cleaning the wood’s surface. Polyurethane-finished wood does not have the same luster that some lacquers have. Its finish has the look and feel of "plastic" and turns increasingly amber in color with age.
Spar varnishes make an excellent protective finish. When moisture is an issue, a polyurethane or spar urethane wood finish is typically used, because lacquer finishes are subject to moisture damage. Spar varnishes were first used to finish the long, extended poles that supported the sails on boats, because the varnish finish remained flexible and weathered UV radiation well. Today, spar varnishes often utilize modified tung oil and resin for their finish and provide a durable and beautiful wood finish.
Powder-coating processes utilize colored powder that has been given a positive electric charge. After the furniture’s surface is given a negative electrical charge, the powder is sprayed onto the surface of the component receiving the finish and the electrostatic charge enables the powder to adhere to the component’s surface. Once the powder is attached to the surface, the component is then baked to fuse the powder to the surface.
One-part waterborne finishes are essentially waterborne acrylic finishes that produce a hard, clear, low-odor protective finish film adhered to the wood. Waterborne polyacrylic finishes have many advantages. They apply easily and dry quickly. They are environmentally safer than petroleum-based finishes. Of all available finishes, gloss polyacrylic finishes are water white and retain the natural appearance of wood without causing it to amber or darken. The problem with polyacrylic finishes is that they appear to generate less depth and luster than oil-based or lacquer finishes. Polyacrylic finishes generally look better on light-colored woods, such as maple, as opposed to dark woods, such as walnut.
Waterborne finishes are an excellent finish for cabinet interiors, drawer boxes, case goods, and work surfaces. In waterborne finishes, the acrylic resin results from the process of the water evaporating from the finish. It does not cure through a chemical reaction, as would be the case in a true catalyzed finish.
In Denmark, a common method of finishing wooden furniture is to apply a mixture of soap flakes and water on unfinished wood. The number of coats varies from 1 to 10, depending on the desired sheen and depth of the finish. Soap flakes are mixed with water, applied in coats, and then rubbed to a finish. They offer an excellent satin sheen finish, retain the true color of the wood without causing it to turn amber, are environmentally safe, and can be easily cleaned and reapplied as necessary or desired.
Wax is generally used over an oil-based stained finish to seal the surface and add luster to the finish. Rubbing carnauba wax with a fine grade of steel wool over light scratches is an easy way to repair lacquer or polyurethane finishes. Using wax as a lubricant between the steel wool and the wood, one can rub the finish to the desired sheen. Once the wax has dried, it can be polished using a cloth diaper or cheesecloth. Wax, however, is not a good finish for furniture, because it offers little or no protection for the wood. Periodically, wax finishes must be refinished.
Purposes of finishing wood furniture include:
■ To protect the wood from dirt, moisture, oils, odors, or minor scratches.
■ To minimize the expansion and contraction of furniture due to changes in moisture content. It is important to finish all sides, not just the exposed surfaces.
■ To provide a smooth, nonporous surface that is easy to clean.
■ To improve the appearance of the wood by enhancing the color and quality of the grain.
Painting furniture has a long tradition as an inexpensive way to protect and embellish the piece and should not necessarily be undervalued. The Shakers painted their furniture with milk-based paint, especially large armoires made in the 1700s (Figure 7.37). With the technical advances made in powder-coat finishing techniques, painted furnishings have seen an enormous renaissance in popularity.