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Guide to Exterior Wood Finishes, Paints, Stains InspectAPedia®  -     Life expectancy of various exterior paints & stains Paint & stain preparation advice Which wood species & grains hold paint best? Comparing latex vs. oil-based paints (alkyds) Using solid color stains on wood exteriors Instructions for application of paints & solid stains Discoloration, mildew, mold on painted surfaces

Here we discuss the selection and proper application of paints and stains on exterior wood surfaces.This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION for details of paint failure cause, diagnosis, cure and prevention. Our page top photo shows a 1940's wood and wood shingle sided house in Poughkeepsie, NY, painted by the website author and Arthur Cady in 1988.

© Copyright 2009 Daniel Friedman, Steve Bliss, Wiley & Sons, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

A Best Practices Guide to Choosing & Applying Exterior Wood Finishes (Paints & Stains)

Our photo (left) shows a failing paint coating on wood clapboards. Notice that the separation is down to bare wood - a diagnostic clue about this early paint failure. According to Best Practices Guide to Residential Construction: 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.

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 this PAINT & STAIN LIFE CHART courtesy of Best Practices Guide to Residential Construction. Whether painting, staining, or finishing in any manner, the FPL makes the following recommendations:

Moisture Content Impact on Paint Job Success

Never paint wood with a moisture content over 20%. Ideally, the wood should be painted at its average moisture content for that climate—about 12% for most of the United States, 9% for dry southwestern states.   Our moisture meter is shown during a paint failure investigation. Not only is it critical to measure moisture before painting wood, but where the measurements are made is equally critical. In this case even when the upper wall portions were quite dry and ready for painting, the lower wall, soaked by recent rain splash-up, was still very wet and should not have been painted until later.

The table below gives the recommended moisture content for wood surfaces at the time of painting.

Continuing with building painting advice from Best Practices Guide to Residential Construction:

Wood Surface Paint Holding Characteristics

A rough-sawn wood surface will hold paint and stain much longer than a smooth, planed surface, which is why many contractors prefer to install siding rough side out. Also most lumber and siding today is flat-grained, which holds paint less well than vertical (or edge) grained.

The combination of flat grain and planing can create a burnished surface called “mill glaze,” which can cause problems with paint adhesion. To avoid problems, it is best to lightly sand with 50 to 80 grit sandpaper before painting smooth siding. The optimal approach is to first wet the lumber to raise its grain and then let it dry for two days before sanding.

Weathering Surfaces Before Finishing to Improve Paint Life

Some painters recommend letting smooth siding weather for a few weeks to open up the grain. However, research at FPL has shown that after two weeks of exposure, the wood surface begins to degrade and to loosen the wood fibers on the surface, which weakens the paint adhesion.

The FPL therefore strongly recommends painting within two weeks of installation, whether the rough or smooth side is facing out. If you need to paint wood that is badly weathered, the wood should be sanded, power rinsed, and allowed to dry before priming. Once the primer is dry, the top coat should be applied as soon as possible.

Species and Grain and Paint Durability

In general, less dense woods hold paint better than more dense woods. Also, within a single species, vertical-grain (also called edge-grain) wood holds paint much better than the more common flat-sawn lumber, primarily because flat- sawn wood shrinks and swells more from changes in relative humidity.

Also vertical-grain wood has narrower bands of latewood, the denser and harder portion of each annual ring in a tree. When paint, particularly oil-based, becomes brittle with age, it tends to peel from the latewood. See the table just below.

Dense woods with wide, flat grain will present the greatest problems in holding paint. This is true for most hardwoods as well as dense softwoods with wide, flat grain, such as southern yellow pine and Douglas fir, especially if planed smooth.

Paints for Building Exterior Surfaces

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 Paints

In addition to its easy cleanup, latex paint has always held certain advantages over oil. 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, 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.

Oil-Based Primers

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.

Solid-Color Stains

Solid-color, or “opaque,” stains are not true penetrating stains, since they form a film on the surface of the wood as paint does. 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.

Application of Paints and Solid Stains

The best paint in the world can fail within the first year if applied over a wet, dirty, or degraded substrate. So the first priority is to make sure that the material being painted is sufficiently dry and clean.

Sealers

For the best protection of the underlying wood and the longest lasting finishes, bare wood should be sealed with a water-repellent preservative (WRP) before priming and painting or staining. WRPs contain a small amount of wax or other water repellent and a mildewcide, fungicide, or both, usually in a solvent base. The preservatives help prevent mildew and decay in above-ground applications but are not meant for ground contact. Some WRPs contain UV blockers as well, which slow down the degradation of the outer wood fibers.

While sometimes formulated as a finish treatment for siding, some WRPs can be used as a pretreatment for painting and are recommended for that use by the USDA Forest Products Laboratory (FPL) and Western Wood Products Association (WWPA). Research shows that WRPs resist water entry better than acrylic primers. On bevel siding, they also reduce warping, splitting, and mildew growth. They can also improve paint performance on hard-to-paint woods, such as southern yellow pine and Douglas fir.

In new construction, the FPL recommends that siding and trim be coated on all sides with a paintable WRP such as DAP Woodlife® or Cuprinol’s Clear Wood Preservative, preferably by dipping or with a brush, roller, or pad. If the siding or trim is already installed, they suggest treating all places vulnerable to water entry, including door bottoms, window sills, lap and butt joints, edges and ends of trim, and any end grain on panel products such as plywood sidings.

If used as a pretreatment for paint, apply to bare, dry wood when it is above 50°F, and use only a single coat or excess wax buildup on the surface could affect the paint adhesion. Allow two days of warm weather to dry, or up to a week if the material was dipped. If painted before the solvent has evaporated and the wax absorbed, the paint can be discolored and not bond well.

Priming Surfaces is Needed Before Painting a Building

All paints and most solid stains require priming on new wood. Primers are formulated with a higher ratio of binder to pigment than paints. This forms a durable film that bonds well to the surface and blocks water. However, without much pigment, it offers limited UV protection.

For woods with water-soluble extractives, such as cedar and redwood, use an oil-based primer or a stain- blocking acrylic primer formulated to seal in the extractives. Also use a stain-blocking primer on any knots. Otherwise the extractives can bleed through the finish and stain the siding. For wood species relatively free of extractives, use a 100% acrylic latex primer. If sprayed or rolled on, back brushing is recommended for a good bond.

Many manufacturers now sell siding and trim preprimed. In addition to the convenience for the contractor, the factory-applied coating is applied uniformly without the risk of bad weather or other job-site variables. The only concern is the thickness of the primer. While most major manufacturers of preprimed siding do a good job, some third-party prefinishers may ship material with too thin a coating. In general, the primer should be 1.5 to 2 mils thick—thick enough that it hides the wood grain.

Back Priming Wood Surfaces Before Painting or Staining

Most paint failures are related to moisture moving through the wood either from wind-driven rain that reaches the back of the siding or moisture escaping from the house. In some cases exposed end grain picks up moisture and causes localized peeling. Use of a water- repellent preservative or primer on the back of the siding and on all edges and cut ends, in addition to the visible face, will minimize these problems. Sealing the wood properly also helps prevent moisture from being driven through the siding by solar radiation.

Advice for Painting Top Coats

For paints and solid stains, apply the top coat as soon as the primer is dry but not more than two weeks later. For best performance, apply two top coats. Latex paints can typically be recoated within a few hours. Oil must cure for one or two days between coats. Apply paint at the coverage recommended on the can. Too thin a coat will wear quickly and too thick a coat may crack.

While brushing provides the best adhesion, a properly done spray job can yield good results. When spraying or rolling, the best results are achieved by back-brushing the paint to help work it evenly into the wood, particularly on rough-sawn surfaces.

Temperature and Time of Day Impact on Paint Job Success

Oil-based paints should be applied when it is over 40°F; for latex coatings the temperature should be at least 50°F during application and for 24 hours after. Also it is best not to apply paint too early or too late in the day. If the dew has not evaporated in the morning, both oil and latex may have adhesion problems. If applied within two hours of sunset and a heavy dew forms before the paint dries, latex paints may streak and oil-based paints may not cure properly.

Painting stucco exteriors in windy or hot sun conditions can lead to early paint failures. See PAINT on STUCCCO, FAILURES for details.

Advice When Painting or Staining Pressure-Treated Wood

Wood that is pressure-treated with waterborne preservatives, such as chromated copper arsenate (CCA), ammoniacal copper zinc arsenate (AZCA), and ammoniacal copper quaternary (ACQ), present special problems for painted finishes.

First, pressure-treated lumber is often shipped to lumberyards with very high moisture contents. If painted while wet, the moisture may get trapped by the paint film and cause peeling. Also the species most commonly pressure- treated—flat-sawn southern yellow pine in the eastern United States and Douglas fir and Ponderosa pine in the West—do not hold paint well to begin with.

Whether or not you intend to paint the wood, pressure- treated exterior trim should be sealed with a water- repellent preservative as soon as the surface is sufficiently dry. This will protect cut ends and help keep the wood from checking, cupping, and warping as the wood dries out. If this is the only treatment, it will need recoating every one to two years. Factory-sealed treated lumber is now available that only requires treatment of cut ends when installed.

The most common treatment for pressure-treated wood is an oil-based, semitransparent stain. Since this type of finish is relatively permeable to moisture, for best results apply it over a sealer or over factory-sealed lumber. While the sealer can be applied to wood that is still wet inside, it is best to air dry the wood before staining. This will take from a few days to a few weeks, depending on conditions, with two weeks on average. Two coats of an oil- based, semitransparent stain over a sealer should last several years. The second coat should be applied before the first coat dries completely, or the second coat cannot penetrate the wood.

If a painted finish is desired, you will need to seal the wood first and allow it to dry for two to three weeks before applying a compatible primer and two coats of 100% acrylic top coat. The longer the wood dries, however, the greater the risk that UV radiation will damage the wood surface, interfering with the paint’s adhesion. To avoid these problems and the long delays, consider using kiln- dried treated lumber that can be finished immediately.

Paint or Stain Discoloration Problems on Building Exteriors

Extractive bleeding and mildew 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.

Extractive Bleeding on Painted or Stained Wood Surfaces

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, 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.

See STAIN DIAGNOSIS on Building Exteriors for details on the diagnosis, cure, or prevention of stains on buildings.

"Mildewed" or Moldy Building Exteriors

Speaking accurately, there is no mildew found on building surfaces. Mildew, which is comprised of two groups of fungi within the larger class of molds, grows only on living plants: mildew is an obligate parasite that is found, for example, on grapes. So if you see what you think is mildew on a building exterior, unless the exterior is made of living plants like grapes, it's mold, but it's not not mildew. However because the terms mildew and mildewcide are used very widely among the building trades we have permitted that topic name in this article.

Various species of outdoor mold will grow on just about any surface with sufficient moisture and heat. In new construction, it 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. Continuing from Best Practices Guide to Residential Construction:

To remove mildew, 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.

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.

White Blotches or Blooms in Paint on Exterior Stucco

Painting over high pH surfaces (over pH of 11), or over moisture, both conditions found when new stucco is insufficiently cured can lead to white blooms or effloresence on the painted surface as well as a short life of the paint job as our photo shows (left). See PAINT on STUCCCO, FAILURES for an explanation of white blotches or white bloom that appears on painted stucco building surfaces. Also see see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION for a complete inventory of causes and signs of types of paint failure on buildings.

Guide for Using Semitransparent Penetrating Stains on Buildings

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).

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.

Guide to Clear and Lightly Tinted Finishes for Building Surfaces

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 Total Wood 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.

Guide to Using Bleaching Oils on Cedar Shingle Exteriors

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.

Decay Resistance of Unfinished Siding and Trim

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 of Best Practices Guide to Residential Construction).

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 mildew (see Figure 1-19, page 19 of Best Practices Guide to Residential Construction).

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.

Exterior Building Finish Product Suppliers

Suppliers of building paper, flashing tapes, prefab cedar panels, fiber cement siding and trim, exterior insulation and finish systems (EIFS), hardboard trim, polyurehtane trim, cellular polyvinyl chloride (PVC) trim, exterior adhesives, water repellent preservatives, and clear wood finishes as well as trade associations are listed at the end of Chapter 1 of Best Practices Guide to Residential Construction, by Steven Bliss.

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

PAINT & STAIN GUIDE, EXTERIOR
  Moisture Impact on Paint
  Exterior Surface Paints
  Solid-Color Stains
  Application of Paints & Stains
  Discoloration Problems
  Extractive Bleeding
  Mildewed Exteriors?
  Semitransparent Penetrating Stains
  Clear & Lightly Tinted Finishes
  Bleaching Oils
  Unfinished Siding & Trim
  Product Suppliers
  PAINT & STAIN LIFE CHART
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION
PAINT FAILURE CHECKLIST
  EXTERIOR CLUES
  INTERIOR CLUES
  SITE HISTORY
  PAINT FAILURE INDICATORS
COMMON PAINTING MISTAKES
  BAD SURFACE PREP
  PAINTING SHORTCUT ERRORS
  PAINT OVER MOISTURE
  INCOMPATIBLE PAINTS
  PAINT in SUN or WIND
  PAINT on STUCCCO, FAILURES
PAINT FAILURE DICTIONARY
PAINT FAILURE ANALYSIS LAB PHOTOS
Paint Failure Case Photographs-SITE
Paint Failure Case Photographs-LAB
PAINT LAB SAMPLE PREPARATION

PAINT REFERENCES

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Wiley.com and also at Amazon.com. See our book review of this publication.
• Paint Failure Diagnostic Procedures - see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION
• Paint faiures on stucco walls - see PAINT on STUCCCO, FAILURES

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