This article provides details about wood surface preparation, selection of an exterior paint or stain product and type, and proper paint or stain application for a successful job. A successful paint or stain job means that the result looks good and is durable.
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Best Practices for Choosing & Applying Paints & Stains on Wood Building Exteriors: Wood Preparation, Product Choices, Installation Details
This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. The series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
For a detailed guide to selecting and using exterior paints and stains, readers should see PAINT & STAIN GUIDE, EXTERIOR. Also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION for details of paint failure cause, diagnosis, cure and prevention.
The USDA Forest Service Forest Products Laboratory (FPL) has done extensive research on how to keep paints and stains on wood sidings and trim. In general, they recommend paint for the longest lasting finish and best protection of the underlying wood, followed by solid or semi-transparent stains. Clear finishes need the most frequent recoating and offer the least protection from water damage and UV radiation (see Table 1-15).
How long a finish will last depends on many variables, including the quality of the finish, type and texture of wood, application conditions, and exposure. South- and west-facing walls get the most sun and are, therefore, often the first to need recoating. See PAINT SURFACE PREPARATION where we provide and illustrate some paint surface preparation suggestions from the U.S. Forest Products Laboratory.
Guide to Choosing Paints for Building Exteriors: Latex Paints vs. Oil / Alkyd-based Paints & Primers
Paints offer wood the greatest protection from the elements and can last from 7 to 10 years if properly applied with one prime coat and two top coats of quality paint. The longevity of a particular job will depend on a number of variables, including paint quality, surface preparation, climate and exposure, and the type of wood.
Latex vs. Oil-Based Exterior Paints
In addition to its easy cleanup, latex paint has always held certain advantages over oil-based paint. [Note that "oil-based paint" today is typically an alkyd product.
Older traditional oil- based paints have been modified to reduce environmental and health concerns with traditional oil-based paint solvent and paint-vehicle VOCs. Also see VOCs VOLATILE ORGANIC COMPOUNDS.
Perhaps most important, latex paints stay flexible over time while oil-based paints get brittle as they age. This is particularly true of 100% acrylics, which makes them less likely to crack due to seasonal movement of the substrate.
Also, while oil is more resistant to liquid water, latex is more permeable to water vapor, making it less likely to blister in situations where moisture must pass through. Latex also fades less over time, is not prone to chalking, and is less likely to support mildew growth than oil-based paint. The best quality latex paints use 100% acrylic binders, offering increased flexibility and durability over latex-vinyl blends.
Oil-based paints, however, are still favored by many professional painters for their better appearance and better adhesion due to the oil penetrating the surface of the wood. Oil paint’s flow characteristics help hide brush strokes and provide better coverage, particularly in high-gloss paints. Also, window sash and doors painted with oil paint dry to a harder finish that is less likely to stick to other painted surfaces.
In the past two decades [to 2010-ed.], however, manufacturers have greatly improved the quality of latex paint, overcoming many of the problems associated with it in the past, while oil-based paints have suffered somewhat as manufacturers have had to adjust their formulas to comply with air-quality regulations that restrict the use of VOCs (volatile organic compounds) found in paint solvents. Since latex now dominates the market in residential paint sales, most development efforts now and in the future will focus on improving latex rather than oil-based paints.
Many painters still prefer oil as a primer for woods with water-soluble extractives, such as redwood and red cedar, although specially formulated stain-blocking latex primers can also work for this application. Many painters also favor oil primer when repainting over chalky or degraded surfaces because of its penetrating oils and strong adhesion. Painting over high gloss surfaces also may be easier with oil-based paints.
Finally, oils offer greater temperature flexibility in both hot and cold weather. In hot weather, latex may dry too fast; while below about 50°F, latex should not be used without special additives. Oil-based paints can be safely used to about 40°F. Newer formulations of latex paints, however, promise to extend their temperature range.
Use a Paint or Stain Primer Compatible with the Paint or Stain Top Coat
Watch out: be sure that the primer you select for your paint job is compatible with the type of coating you intend to use for the top-coat. Read the manufacturer's label-instructions and check with your paint supplier. Painting some top coats atop the wrong primer is a guarantee of a failed paint job that shows up as wrinkling, cracking, or loss of adhesion of the building coating.
Our paint failure photo (left) shows a top coat (gray) with poor adhesion over a previous paint coating that appeared sound. While further inspection and tests may be required to accurately diagnose this paint failure, it looks a lot like a paint adhesion problem, perhaps due to an incompatible top coat, or due to painting over an inadequately prepared older surface.
For example, if a solvent in the top coat is incompatible with the primer coat, it may cause the primer to dissolve, lose adhesion, or wrinkle. And if the chemistry of a primer is not compatible with a top coat, the top coat may simply fail to properly adhere to the primer.
We like alkyd paints because of their compatability: alkyd-based paints can generally be applied over either older oil-based painted surfaces or over latex based paints, reducing the need to strip otherwise sound paints from an older building surface. If you don't know the chemistry of the paint on a building exterior, using an alkyd primer will help assure a good bond to the old surface, even if your final top coat is going to be latex.
Finally, don't use an indoor-rated paint on a building exterior. Interior paints not intended for outdoor use may lack adequate UV protection, weather resistance, or a binder and adhesion necessary for success on an outdoor surface exposed to the weather.
Details about incompatible paints are at INCOMPATIBLE PAINTS. - Ed.
In fact, they are formulated the same as paints, only with fewer solids, leaving a thinner, less protective film.
They may also contain water-repellents and preservatives.
Like paints, they help protect wood from UV degradation; but also like paints, they can peel and blister if applied incorrectly. Most require a primer for best results.
The thinner coating of these products tends to hide the wood grain but allows the wood texture to show through, particularly on rough-sawn siding (see Figure 1-39).
Most solid-color stains sold today are latex-based, which makes them fast-drying and likely to show lap marks if not applied carefully.
The most durable latex solid-color stains are 100% acrylic. Oil-based solid stains are sometimes used on redwood and cedar.
Two coats of top-quality latex solid stain over a primer on a solid-wood siding should provide 3 to 7 years of service versus as many as 10 years for an acrylic latex paint of equal quality.
More information about using solid color stains on building exteriors is at Solid-Color Stains.
Guide to Choosing Semitransparent Penetrating Stains for Building Exteriors: Selection, Preparation, Application
Most semitransparent stains are oil-based, and they penetrate the surface of the wood. They have a moderate level of pigment that offers some UV protection and provides some color without hiding the wood grain. Because these stains do not form a film on the surface, they are not subject to blistering and peeling like paints and solid-based stains.
Penetrating stains last longer on rough than on smooth siding materials. One coat of oil-based penetrating stain on rough-sawn siding or plywood will last two to five years, depending on exposure and other variables; two coats may last as long as seven or eight years. In general, subsequent coats last longer than the first coat because the weathered wood will accept more stain. For decks, steps, or other wood subject to foot traffic, use a special deck stain formulated with better abrasion resistance (see “Finishes for Decking,” page 154 in Best Practices Guide to Residential Construction).
Like paints, penetrating stains can be applied by brush, spray, or roller. If sprayed or rolled on, back-brushing will improve the penetration and performance. Also spraying without back-brushing can cause a splotchy appearance. If two coats are desired, apply the second coat before the first has fully dried or the second coat will not be able to penetrate the surface.
Because oil-based stains are thin and dry quickly, lap marks may form if the applicator is not careful to maintain a wet edge. It is best to work on a small area at a time and, if possible, to work in the shade to extend the drying time.
Clear and Lightly Tinted Semi-Transparent Stain Finishes on Wood Siding or Trim
Some customers want to retain the look of “natural” wood siding, particularly with the warm-toned hues of premium red cedar or redwood. Unfortunately, there is no finish that will magically preserve the look of new wood.
Wood turns gray as UV radiation degrades the outer surface and as mildew spores develop. Clear water-repellent preservatives (described under “Sealers,” previous page) with UV blockers can slow down this natural aging process, but will need to be reapplied every year or two to keep the wood from turning a weathered gray.
To retain the tone of new wood, the best approach is to use a WRP or penetrating oil with UV blockers and a tint added to match the redwood or cedar.
Amteco’s TotalWood Protectant (TWP®), Flood’s Clear Wood Finish (CWF®), and Penofin® (Performance Coatings Inc.) are proprietary formulations designed to maintain a natural wood appearance.
A similar product called Sikkens Translucent Cetol® (Akzo Coatings) darkens the wood somewhat and creates a thin film, but it does not peel like paint or varnish. Apply one to three coats, according to the manufacturer’s recommendations.
Even with “one-coat” finishes, a second coat may be worthwhile on south or southwest sides of the building due to increased UV exposure.
If applied correctly, a high-quality tinted finish can keep redwood or cedar siding looking close to new for three to five years. Before recoating, you may need to clean the siding with a bleach solution to remove any mildew and dirt that has started to discolor the siding. After cleaning, another coat of the original finish should restore the new wood look for another three to five years.
Bleaching Oils for Exterior Wood Siding or Trim Finish
In some regions, homeowners like the silver-gray, weathered look of unfinished cedar shingles, but they do not want the splotchy, uneven coloring that sometimes results from uneven wetting and sun exposure. Bleaching oils solve this problem by combining a lightly pigmented semitransparent stain with a bleaching agent. Initially, the pigment colors the wood a silver-gray color, and over time, the bleach lightens the underlying wood to a uniform color.
The uniform weathered look can last for a number of years, but the oil and pigments in the original finish protect the wood for only two or three years. Beyond that, a clear water-repellent preservative can be used periodically to protect the wood from UV degradation and decay. If, after several years, the siding begins to darken or lose its uniform appearance, another coat of bleaching oil should restore the original look.
Guide to Selecting Wood Siding or Trim to be Left Unfinished
Due to their high level of extractives, the heartwood of some species is naturally resistant to decay and insects and can be used on the exterior unfinished. The woods most commonly used this way are western red cedar, northern white cedar, redwood, and bald cypress (see Table 1-16 below).
In salty coastal air with good exposure to sunshine, untreated wood tends to weather to an attractive silver gray. In other regions, uneven staining from mildew is likely.
Even in coastal regions, areas of the house that get frequent wetting from splashback, snow, or other types of weather exposure may become darkened from mold (see the weathering at the bottom of wood shingle siding at Figure 1-19 below) and see details at RAIN SPLASH-UP SIDING DAMAGE.
Also, the wood extractives do nothing to prevent cupping, warping, or cracking from uneven absorption of moisture. For a uniform appearance without leaving the results to chance, it is best treat the wood with a WRP or bleaching agent.
Additional help in diagnosing stains and paint job discoloration are at STAIN DIAGNOSIS on BUILDING EXTERIORS.
Extractive bleeding and mildew [mold] can discolor either bare wood or finished surfaces. They should be removed before finishing or refinishing. After washing, it is important to allow the surface to dry before applying the new finish.
Excess moisture in wood species such as cedar, redwood, Douglas fir, and mahogany can dissolve the natural tannins in the wood and cause them to migrate to the surface, leaving a reddish-brown stain on the finish. Sealers and stain-blocking primers help to minimize this problem but do not always eliminate it.
If staining occurs on your building exterior walls or trim, the first step is to eliminate the moisture problem.
Then, if the extractive bleeding is mild, remove the stains with a mild detergent and water. More severe cases will require cleaning with an oxalic acid solution.
Carefully follow the manufacturer’s instructions when using oxalic acid, as the bleaching solution will harm plants and may bleach existing finishes on siding, trim, and other woodwork.
After washing, the oxalic acid must be thoroughly rinsed with clean water and the wood dried before finishing or refinishing. If the extractive bleeding has been allowed to bake in the sun, it may have hardened and be difficult to remove. In this case, you will need to apply a stain-blocking primer before refinishing. Details are at are at STAIN DIAGNOSIS on BUILDING EXTERIORS. .
[A wide variety of common fungi (molds) grows on just about any surface with sufficient moisture and heat and organic "food", but actually not mildew. Mildew is a a sub-class of molds that are obligate parasites that only grow on living plants. So while people commonly call mold on painted surfaces or wood "mildew", it's not. It is, however, mold -ed.]
In new construction, mold growth on wood surfaces can be minimized by storing wood off the ground and providing adequate ventilation. Although sealers and stains contain a mildewcide, any mildew should be removed before finishing or refinishing, or it will continue to grow through the new finish.
To remove mold from wood surfaces ["mildew"], one option is to use a sodium hypochlorite solution, which can be made with household chlorine bleach. Depending on the severity of the problem, the solution should range from 1 to 8 parts bleach to 1 part water. Spray the solution onto the siding (avoid sprayers with aluminum parts), starting at the top and working down. If two applications do not remove the stains, you may need to scrub in the solution with a brush. Thoroughly rinse everything with water.
Watch out: Bleach can harm plants, discolor the finishes on trim, and corrode aluminum, brass, and copper. It is best to cover plants with tarps and protect any stained or painted surfaces.
More about paint or stain discoloration problems is at Discoloration Problems.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Water-Repellent Preservatives (WRPs)
Cuprinol www.cuprinol.com Cuprinol Clear Wood Preservative
Dap www.dap.com DAP Woodlife
Wolman www.wolman.com Premium Water-Repellent Sealer
Clear Wood Finishes
Amteco www.amteco.com Total Wood Protectant (TWP)
The Flood Company www.floodco.com Clear Wood Finish (CWF)
Performance Coatings Inc. www.penofin.com Penofin wood finishes
Sikkens/Akzo Nobel www.nam.sikkens.com Sikkens Cetol finishes
For More Information on Building Practices for Exterior Wall Products
California Redwood Association www.calredwood.org
Cedar Shake and Shingle Bureau www.cedarbureau.org
USDA Forest Products Laboratory (FPL) www.fpl.fs.fed.us
Vinyl Siding Institute www.vinylsiding.org
Western Wood Products Association (WWPA) www.wwpa.org
-- Adapted with permission from Best Practices Guide to Residential Construction.
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