sign in sign up
<<
Home , Varnish

Tips and Techniques

Paint, Stain, Varnish, or Preservative? It’s Your Choice Although you cannot completely tex are suspensions of inor- arrest or reverse the weathering of ganic and various The north side of a building does exterior , you can slow the in water. Acrylic latex res- not need to be repainted as often process dramatically by using the ins are flexible and durable. A as other sides because it is ex- right type of finish. Two basic good acrylic latex outdoor house posed to less direct sunlight. Ap- types of finishes or treatments are will generally outlast a good ply one topcoat to this side instead used to protect wood surfaces oil-based house paint. Latex of two, or paint this side less of- from weathering: those that form paints are very porous; they can ten. The optimum thickness for a a film or coating on the wood “breathe” while they shed water. and two topcoats is 4 to 5 (film-forming finishes) and those If water enters the wall from an mils that penetrate the wood surface. interior source, it’s more likely to (1 mil = 1/1000 in.), about the Film-forming materials include get trapped in the wood beneath thickness of a sheet of newspaper. paints, solid-color stains, and an oil-based finish, and it can varnishes. Penetrating finishes cause the paint to blister. Moreo- If the paint has peeled, the easiest include preservatives, water ver, oil-based paints tend to be- way to prepare the wood for repellents, and pigmented come repainting is by power washing. semitransparent stains. brittle. Power washing or sanding (with 50–80 grit ) also Film-Forming Finishes Acrylic are more resis- improves the ability of new tant to sunlight than oil-based smooth siding to hold paint and Paint—Latex is Best paints. However, like red- removes weathered fibers of Paints form a thin layer over the wood and cedar have water- unfinished wood that have been surface to which they are applied. soluble extractives that can bleed exposed to sunlight for more than The pigments in the paint protect through latex paints. The best way a few weeks. the surface completely from the to prevent this is to seal the wood damaging (UV) rays of with an oil-based or stain- Before applying paint, treat wood the sun and erosion. The finish blocking latex primer paint, then siding and trim with a paintable retards the movement of moisture topcoat with acrylic latex paint. water-repellent preservative. This into and out of the wood and seals in the natural resins. However, Wood is easier to repaint before Life Span of Various Finishes paint will not prevent decay if the first coat fails. Paints weather ______Water repellents 6–12 months conditions are favorable for fun- away; self-cleaning paints weather at a faster rate than other Clear water-repellent gal growth. preservatives 1-2 years paints. Ideally, wood should be Pigmented water-repellent Oil-based or paints are es- repainted when the topcoats have preservatives 2-3 years sentially suspensions of inorganic weathered to the point that the Varnish 2–3 years pigments in an oil or modified oil primer is just starting to show. Solid-color stains 3–7 years vehicle that binds the Painting too often can build up Semitransparent stains 3–8 years particles to the wood surface. La- too many layers of paint. Paints 7–10 years

May 2000 ote: The life span of water repellents, Solid-color stains are preferable to Semitransparent Stain— water-repellent preservatives, and paints when the wood is refinished Oil is Best semitransparent stains can be extended frequently. Semitransparent stains are pene- by if discoloration caused trating finishes with inorganic by mildew or other factors is a Varnish pigments. High quality stains problem. Because varnishes are clear, they contain wood preservatives and require frequent maintenance to be water repellents. Semitransparent can be done by brush after the attractive. Sunlight degrades both stains penetrate the wood without siding or trim is in place or by the varnish and the wood fibers radiation that would otherwise dipping the wood before directly beneath it. Some varnishes degrade the wood. The properties installation. All lap and butt joints have chemicals that partially block of the pigment, , preservative, of solid wood and all panel edges UV light, but all varnishes will and water repellent determine the should be well saturated. eventually crack, peel, and flake, durability of exterior stains. taking along the fibers of photo- Allow the treatment to dry for at chemically degraded wood. The Penetrating stains perform best on least 2 warm, sunny days before wood needs to be cleaned and weathered or coarse-textured wood. applying the primer. If the wood revarnished as soon as this break- If you are finishing smooth wood, has been dip treated, let it dry for down occurs. Varnish lasts only 1 powerwash, sand, or wet the about a week. Two coats of a good to 2 years on wood exposed to full surface to relieve stresses and open quality acrylic latex house paint sun. The life span of varnish can be the surface pores. Unlike paints, should be applied over the primer. extended by first staining the wood, stains can be applied to weathered In general, paint quality is directly then applying many coats of surfaces without preparing the related to price. varnish. surface. If the surface is dirty or has mildew, clean it with bleach Brush application is always super- Penetrating Finishes and detergent before applying the ior to roller or spray application, Penetrating finishes absorb into finish. Penetrating stains are especially with the first coat. Each the wood, saturating the surface suitable for siding, trim, decking, coat should be applied within 2 fibers and partially or completely fences, and roof shingles. weeks of the previous one, to avoid filling the surface pores. Many formation of a soap-like or penetrating finishes contain water Stains may be applied by brush, chalky substance on the surface. If repellents, usually in the form of spray, or roller. To prevent lap more than 2 weeks have elapsed, paraffin dissolved in mineral marks, finish the board or panel in scrub the paint with water and a spirits. one working session. stiff-bristled brush before applying the next coat. Water repellents are usually clear. Working in the shade will give the

They can be used alone as a natural best results because longer drying Temperature is important. Do not penetrating finish or as a treatment time means greater penetration of paint on a cool surface that will be prior to painting. Check the label the stain. heated by the sun within a few of the finish to make sure that it is hours or at the end of a cool day paintable. For best results, rough sawn or when heavy dew will form at night. weathered should be treated The temperature should stay >40°F Many clear, penetrating finishes with two coats of penetrating stain. (>4°C) for 24 hours for oil paints contain wood preservatives in Apply the second coat before the and >50°F (>10°C) for latex. addition to water repellents; these first is dry. If the first coat has

penetrating finishes are much completely dried, it may seal the Solid-Color Stain better for outdoor use. The wood surface so that the second Solid-color stains also form a film. preservatives control the growth of coat scarcely penetrates. To prevent They look and act like thin paints. mildew and other fungi. Some formation of a film, an hour after Use a primer and two coats of preservatives also discourage insect applying the second coat, use a stain. Refinishing may require only infestation. This is especially cloth, sponge, or dry brush lightly one coat. Like paints, latex solid- important in moist, shady wetted with stain to wipe off excess color stains are usually more locations. stain that did not penetrate the durable than oil-based stains. wood. Remember that sponges or cloths soaked with oil-or alkyd- Wood Properties based stains are particularly Wood properties vary; therefore, susceptible to spontaneous the type of wood is an important combustion. They should be consideration in choosing a finish. immersed in water or sealed in an Wood that has minimum tendency a airtight metal container. to shrink and swell is the best for painting because there is less Natural Finish chance that the paint will crack as To some, a natural look means the wood shrinks and swells. b rough, gray, and weathered. To Vertical-grained (edge-grained) others, a successful natural exterior wood is subject to about half the wood finish is one that retains the shrinking and swelling of flat- a original, attractive appearance of grained lumber the wood with the least change in (Fig. 1). Wood density also affects color and the least masking of the swelling. Low-density woods are b grain and surface texture. In this better for painting than denser case, the finish should inhibit the species. Low-density like growth of mildew, protect against western redcedar and redwood boards may check or crack as part moisture and sunlight, and not have excellent properties for of the natural weathering process. change the surface appearance or finishing; and also retain Weathering can be reduced by us- color of the wood. paint well, particularly if the wood ing a penetrating finish that con- is free of knots. As more wood is tains a water repellent early in the The “weathered look” can be being cut from younger trees, more finishing process. Allowing a deck obtained in several ways. Do boards are containing juvenile to weather for a year opens the sur- nothing until the wood turns gray, wood— wood formed during the face pores and permits the wood to then treat the wood regularly with first few years of growth. Juvenile accept more finish, but this weath- a water-repellent preservative or a wood shrinks and swells at a very ering damages the wood. Research gray semitransparent stain. This high rate and requires a indicates that it is better to finish a will extend the life of the wood. If penetrating finish. Like wood with deck as soon as it is dry (a few you want to obtain a weathered knots, juvenile wood easily releases weeks after construction) and to look quickly, apply a gray stain. As extractives, which bleed through refinish it in 2 years. the stain weathers, the wood will paint. turn gray naturally. Maintain the Figure 1—The position of wood finish with a stain or water- In general, it is best to paint over a in the log determines the grain of the sawn lumber: (a) vertical- repellent preservative. fairly smooth and stable surface. or edge-grained lumber, (b) flat- Flatsawn and dense woods can be You can preserve the original color grained lumber. stabilized with a resin-treated of the wood in several ways. From paper overlay. This type of the start, treat the wood with a lamination is most frequently To paint treated wood, make sure water-repellent preservative at least applied over exterior or once a year, or apply a semi- the wood is clean and dry. Porches stabilized fiberboard, and it makes and decks can be painted, but be transparent oil-based stain an excellent surface for painting. (containing a water repellent and prepared for problems like decay, a Rough sawn exterior plywood slippery surface, cracking, check- preservative) of the same color as siding is best finished with a the wood. The pigment in the stain ing, and flaking. Use a very good semitransparent penetrating stain porch and deck enamel for the two protects the wood and extends the or solid-color latex stain. life of the preservative. If the wood topcoats. Water-repellent preserva- tives and semitransparent penetrat- turns gray, the preservative has Preservatives failed. Most gray discoloration is ing stains must be reapplied more Preservative-treated wood, such as caused by mildew. Power-washing, frequently than paint, but re- wood pressure treated with chro- bleach, and some commercial application is easy. Simply brush a mated copper arsenate (CCA), is cleaners remove gray discoloration. generous amount of finish onto the frequently used for outdoor porches wood; after 20 to 40 minutes, wipe and decks. After a few years, some