wood use Finishing Exterior Wood Surfaces

Wood properties and distributed more evenly on the finish durability surface and because vertical-grain The durability of an exterior finish is lumber shrinks and swells less in affected by the characteristics of the width than flat-grain material. The Vertical grain wood. Hardwoods, obtained from side nearest the bark in flat-grain trees with broad leaves, generally lumber holds finishes better and is require more care in finishing than less subject to surface roughening softwoods, which are derived from than is the pith side. trees with needlelike leaves. Satis- factory finish life is usually more Wood that is free of knots, pitch difficult to achieve on woods of pockets, and other defects is the higher density. preferred base for exterior finishes. Planed surfaces are best for most A tree adds a layer of new wood paint finishes, while rougher or each year. In softwoods, softer, sawed surfaces may be preferred for lighter springwood forms early in the stains or water-repellent treatments. season and heavier, harder summer- wood forms later in the year. Fin- Always use corrosion-resistant fasteners when installing any wood ishes, particularly paints, usually last Flat grain longer on surfaces with a low propor- product outside. Use only aluminum, tion of summerwood. stainless steel, or hot-dipped galva- nized fasteners. Pith face Bark All wood shrinks as it loses moisture face and swells as it absorbs moisture, Follow recommended procedures for but some species are more stable nailing wood siding to allow for Figure 1. Vertical and flat grain lumber and than others. Species that shrink and natural expansion and contraction how they are cut from a log. swell the most cause more stress on with changes in moisture content (figure 2). paint films than woods that are more during transition from the original to stable. Checking and warping are Types of exterior finishes the final gray color. The time required more likely to be critical on woods to produce a fully weathered appear- Each type of finish that is suitable for that are sensitive to changes in ance depends on the severity of the exterior wood surfaces has advan- moisture content. exposure to sun, wind, and water and tages and disadvantages. Carefully will not be the same on all sides of a examine the different types before Vertical-grain or edge-grain lumber is building. Untreated wood may exhibit selecting a particular finish. cut from the log with the wide face at blotchy discolorations during the right angles to the annual growth weathering process due to the action Natural weathering rings; flat-grain boards are sawed so of microorganisms. Stable woods The simplest and most economical the wide face is almost parallel to the such as baldcypress, western way of finishing an exterior wood growth rings (figure 1). Finishes redcedar, and redwood, are best surface is to allow it to weather usually perform best on vertical-grain suited for natural weathering. lumber because the summerwood is naturally. Wood exposed to the weather will undergo color changes

Pm-362 Revised February 1991 Electronic version November 1999 Board An inclination to warp increases with Plain and the width of the lumber. The width of bevel batten boards should not exceed eight times the thickness. The orientation of the annual rings also influences the Overlap tendency to warp. Flat-grain boards 1" usually warp more than vertical-grain lumber.

1 Overlap Space ⁄2" The color and appearance of weath- 1" ered wood can be adversely affected Figure 3. The action of water on treated (left) by mildew and other microorganisms and untreated (right) wood following Nail in warm, humid climates. The growth application of a water-repellent. clears of fungi on the wood surface can tip make unfinished wood exposed to the weather appear blotchy and also is effective. When wood is unsightly. treated in place, liberal amounts of the solution should be applied to all Extractives in woods such as red- lap and butt joints, edges and ends wood and western redcedar may of boards, and panels. One gallon gradually diffuse to the surface and will cover about 250 square feet of Figure 2. Recommended nailing methods for produce a dark brown stain that may smooth surface or 150 square feet of horizontal bevel siding and for vertical board- persist. and-batten siding. rough surface. The life expectancy is only 1 to 2 years. Treatments on Clear film finishes rough surfaces are generally longer- The color of wood is affected soon Clear film finishes such as varnishes lived than those on smooth surfaces. after exposure to outdoor conditions. should not be used on wood that is Dark-colored woods tend to become fully exposed to the weather. These Water-repellent preservatives can be lighter and light-colored woods finishes are relatively expensive to renewed by a simple cleaning of the become a little darker. All woods apply and often begin to deteriorate old surface with a bristle brush and eventually exhibit a gray color. within 1 year. The surface film must an application of a new coat of finish. be removed before refinishing. To determine if a water-repellent Exposure to light and moisture Refinishing is a frequent, expensive, preservative has lost its effective- changes the anatomy of the surface and time-consuming chore. ness, splash a small quantity of cells and induces minute checks. The water against the wood surface. If checks or cracks enlarge and are Water-repellent preservatives the water beads up and runs off the easily visible as wood surfaces are A water-repellent preservative may surface, the treatment is still effec- repeatedly exposed to alternate be used as a natural finish. It con- tive. If the water soaks in, the wood wetting and drying. The degree of tains a fungicide, a small amount of needs to be refinished. Refinishing weather checking varies with spe- wax as a water repellent, a resin or also is required when the wood cies. Aspen, baldcypress, cedar, drying oil, and a solvent such as surface becomes gray. redwood, and yellow poplar are turpentine or mineral spirits (figure woods on which weather checking 3). Water-repellent preservatives do Steel wool and wire brushes should tends to be inconspicuous. Actual not contain any coloring pigments. not be used to clean surfaces to be erosion of wood from the surface The preservative slows graying and finished with water-repellent preser- proceeds very slowly. Unfinished reduces mildew infection. vatives since small iron deposits may wood will wear away at the rate of be left behind. Chemicals used may only about 1/4 inch in 100 years on Water repellents also are available. cause iron remaining on the surface exposed vertical surfaces. These are simply water-repellent to corrode. The corrosion products preservatives with the preservative may then react with certain wood Unfinished lumber exposed to the left out. Water repellents are not extractives to form a dark-blue, weather warps and pulls at the good natural finishes but can be unsightly discoloration. fastenings. Warping varies with spe- used as a stabilizing treatment cies, density, width, and thickness of before priming. Preservatives the lumber, and the orientation of the Wood preservatives are not consid- annual rings. Baldcypress, western The most effective method of apply- ered to be finishes. Common wood redcedar, and redwood are species ing a water-repellent preservative is preservatives fall into three general that have only a slight tendency to to dip the entire board into the categories: creosote, pentachlo- warp and loosen fastenings. solution. However, brush treatment rophenol in oil, and water-borne salt treatments. Wood treated with These stains are made with a much Exterior paints creosote or pentachlorophenol in oil higher concentration of pigment than Of all the finishes, paints provide the is not recommended for use around the semitransparent penetrating most protection against weathering the home where people will come in stains and will obscure the natural and offer the widest selection of contact with it. However, wood wood color and grain. Oil-base solid colors. An adequate paint film will treated with water-borne salts is color stains tend to form a film much retard penetration of moisture and suggested for use as patio decks, like paint and, as a result, can also reduce checking and warping. Paint, outside steps, privacy fences, and peel loose from the substrate. however, is not a preservative and other home uses. This material is will not prevent decay if conditions generally light to bright green or Semitransparent penetrating stains are suitable for fungi attack. brown in color. Make sure wood may be brushed or rolled on. Brush- treated with water-borne salts is ing will give better penetration and The durability of paint coatings on completely dry before finishing. performance. These stains are exterior wood is affected by several generally thin and runny, so applica- factors. The species, density, texture, Bleaching oil stains tion can be messy. Lap marks will moisture content, orientation of Bleaching oil stains may be used to form if stains are improperly applied. annual rings, type and amount of produce a permanent, uniform gray Lap marks can be prevented by extractive, and number of defects are color on wood in a much shorter time staining only a small number of characteristics important in determin- than through natural weathering. boards or a panel at one time. This ing paint life. method prevents the front edge of Apply one or two coats of bleaching the stained area from drying out Almost all native softwoods can be oil stain with a brush or roller accord- before a logical stopping place is painted satisfactorily, but species ing to the manufacturer’s directions. reached. One gallon will usually differ in regard to ease of finishing Since the chemical action is aided by cover about 300 to 400 square feet and expected durability. Common sunlight and water, spraying the of smooth surface and from 150 to species are classified according to bleached surface with water is 200 square feet of rough surface. their finishing performance in Table 1. helpful. The bleaching oil stain Woods classified as fair or poor for should contain a chemical to retard For long life with penetrating oil-base exterior painting require extra care in mildew growth. Reapply a bleach stain on rough-sawn or weathered the selection of primers and top- only if the wood begins to darken. lumber, use two coats and apply the coats, while higher rated woods can second coat before the first is dry. be expected to perform well when a Pigmented stains About an hour after applying the broad range of paints are used. Exterior stains may be oil-base or second coat, use a cloth or sponge to latex-base. Oil-base stains may be wipe off the excess stain that has not Table 1. Grouping of woods for semitransparent or solid color; latex penetrated into the wood. exterior painting based upon stains are typically solid color. A finishing ease and durability. variety of colors are available. Solid color stains may be applied to a smooth surface by brush or roller Excellent: Baldcypress, cedars, Semitransparent, penetrating, oil- application, but brush application is redwood base stains are suitable for any wood best. One coat of solid color stain is surface exposed to the outdoors. adequate, but two coats will provide Good: White and sugar pines They are especially appropriate for better protection and longer service. rough-sawn and weathered wood, Fair: Hemlocks, spruces, ponderosa textured surface plywood, and woods Lap marks may also form with a solid pine, lodgepole pine that do not hold paint very well. color stain. Latex-base stains are Poor: Douglas fir, red pine, southern more likely to show lap marks than pine, western larch Pigmented stains obscure the grain the oil-base ones. and texture of wood but do not completely conceal the surface Semitransparent penetrating stains characteristics. The finish penetrates are relatively easy to refinish. Exces- into the wood without forming a sive scraping and sanding are not continuous film on the surface. The required. Simply use a stiff bristle treated surface appears flat or dull. brush to remove all surface dirt, dust, Darker stains containing more and loose wood fibers, and then pigment are usually more durable apply a new coat of stain. The than lighter colored stains. second coat of penetrating stain often lasts longer since it penetrates Solid color stains, also called heavy into small surface checks that open bodied stains, are opaque finishes. up as wood weathers. Hardwoods generally are more use over these woods. The primer To avoid wrinkling, fading, or loss of difficult to finish than softwoods. The seals in the extractives so they will gloss of oil-base paints and streaking best hardwoods for painting are not bleed through the top coat. A of latex paints, the paint should not those of lower density with small primer should be used, whether the be applied in the evenings of cool pores such as aspen, basswood, top coat is an oil-base or latex-base spring and fall days when heavy cottonwood, or yellow poplar. How- paint. For species that are predomi- dews form before the surface of the ever, these species require care in nantly sapwood and free of extrac- paint has thoroughly dried. the selection of primer and finish tives, such as pine, a high-quality paints for good finish life. acrylic latex paint may be used as Refinishing both a primer and top coat. Enough If you are refinishing an old paint Vertical-grain boards of any species primer should be applied to obscure coat, proper surface preparation is are best for painting. If flat-grain the wood grain. Follow the spreading essential if the new coat is to give the lumber is used, paint durability will be rates recommended by the manufac- expected performance. First, scrap improved by exposing the bark side turer. away all loose paint. Use sandpaper rather than the pith side. on any remaining paint to “feather” 3. Two coats of a high-quality acrylic the edges smooth with the bare Only well-dried wood should be latex house paint should be applied wood. Then scrub any remaining old painted. Properly seasoned material over the primer. One coat of a good paint with a brush or sponge and should be kept clean and dry. Place it house paint over a properly applied water. Rinse the scrubbed surface under cover or store it indoors until it primer should last 4 to 5 years, but with clean water. Wipe the surface is installed. two topcoats over a prime coat can with your hand. If the surface is still last 10 years or more. dirty or chalky, scrub it again using a High-grade, clear lumber is best for detergent. Mildew should be re- painting. Knots, pitch pockets, cross- 4. One gallon of paint will usually moved with a dilute household grain, and other defects increase the cover about 400 square feet of bleach solution. Rinse the cleaned risk of paint failure. Apply a knot surface area. Optimum thickness for surface thoroughly with fresh water sealer over resinous knots in species the total paint coat (primer and two and allow it to dry before repainting. such as pine before priming to avoid top coats) is 4 to 6 mils or about the Areas of exposed wood should be discoloration and early failure of the thickness of a sheet of newspaper. treated with a water-repellent preser- paint film. The quality of paint is usually, but not vative, or water repellent, and always, related to the price. Brush allowed to dry for at least 2 days, Paint wood surfaces within 2 weeks application is always superior to any and then primed. High quality top after installation if weather permits. other application method. coats can then be applied. Weathering for longer periods of time results in significant reduction of To avoid future separation between Prepared by Dean R. Prestemon, former adhesion. To achieve maximum paint paint coats, the first top coat should extension forester. life, follow these steps. be applied within 2 weeks after the primer and the second coat within 2 File: Forestry 3 1. Wood siding and trim should be weeks of the first. If more than 2 treated with a paintable water- weeks elapse before applying Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with repellent preservative or water another paint coat, scrub the old the U.S. Department of Agriculture. Stanley R. repellent. Water repellents can be surface with water using a bristle Johnson, director, Cooperative Extension Service, applied by brushing or dipping. Lap brush or sponge. Then rinse well with Iowa State University of Science and Technology, Ames, Iowa. and butt joints and the ends of panel water and allow the surface to dry products should be especially well- before painting. . . . and justice for all The U.S. Department of Agriculture (USDA) prohibits treated. Allow at least 2 warm, sunny discrimination in all its programs and activities on the days for adequate drying before To avoid temperature blistering, oil- basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and painting the treated surface. If the base paints should not be applied on marital or family status. (Not all prohibited bases wood has been dip-treated, allow at a cool surface that will be heated by apply to all programs.) Many materials can be made available in alternative formats for ADA clients. To file least 1 week of favorable weather. the sun within a few hours. a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten Building, 14th and 2. After the water-repellent preserva- Oil-base paint may be applied when Independence Avenue, SW, Washington, DC 20250- 9410 or call 202-720-5964. tive or water repellent has dried, the the temperature is 40°F or above. A bare wood must be primed. For minimum of 50°F is desired for woods with water-soluble extractives applying latex-based paints. For such as redwood and cedar, the best proper curing, the temperature primers are high-quality oil-base and should not drop below 50°F for at alkyd-base paints. Some latex-base least 2 hours after painting. primer paints also are designed for