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Painting Problems from Moisture
by Michigan State University on May 11, 2012
Outside moisture such as rain and dew can penetrate a paint coat and result in cracking, peeling, discoloration and premature paint failure.
These problems may be observed in both heated and unheated buildings, which are more pronounced on edges and ends of boards and are also observed where water is held on the surface. Porous paints are particularly vulnerable to moisture penetration.
Proper construction and maintenance will eliminate most exterior water problems. Ice dams occur in cold northern climates when snow on the upper warmer parts of a roof melt sand runs downward.
As the melted snow reaches the roof overhang which is not heated, it freezes once more. With repeated thawing and freezing on the roof, more water moves to the roof edge and a nice dam begins to build up. The melted water may penetrate the roof and drain into the exterior walls, thus causing moisture and associated paint problems.
Ice dams can be reduced or eliminated by adequate insulation in attics and by proper attic ventilation. Inside moisture (water vapor) can destroy paint on the outside of a building by diffusing through the walls. Water vapor from cooking, dishwashing, clothes dryers, bathing and normal respiration by an average family of four can contribute three gallons of water per day to the humidity.
If the inside of all exterior walls does not have a vapor barrier or if the vapor barrier is improperly installed, water vapor passes into the walls during cold winter weather and condenses to a liquid. The water eventually soaks into the siding and wets the paint and is a common cause of blistering and peeling. The problem may be particularly pronounced around bathrooms, laundry rooms, kitchens and other areas of high humidity.
Interior water vapor can also move into the attic space and condense on the gable ends, causing paint peeling. Moisture may also condense on the attic side of the roof decking and eventually work its way down the sidewalls, causing paint peeling near the tops of these walls. To prevent condensation problems in the attic, it should be well ventilated. Gable roofs should have screened vent areas of at least one square foot per 300 square feet of ceiling area. Hip roofs should have continuous slotted vents in the eaves to allow air to enter the attic and ridge vents to allow its exit.
Condensation problems in cold climates are best prevented by the installation of a continuous 6 mil polyethylene vapor barrier on the warm side of all exterior walls and ceilings. The vapor barrier should fit tightly around electrical outlets, doors, windows, and other openings. A vapor barrier, sometimes called a soil cover, should be installed directly over the soil in all houses with crawl spaces. This will keep moisture from moving out of the soil and up into the living space and then through the walls and ceilings. If a vapor barrier is absent, it can be installed under new paneling or drywall.
To reduce water vapor in the house, vent high humidity areas such as kitchens and bath areas to the outside. Clothes dryers should be vented directly to the outside and not to the attic, basement or crawl space. Mechanical humidifiers add large quantities of moisture to the air and should not be used if paint peeling is a problem.
Temperature blisters are bubble-like swellings that occur on the surface of the paint film as early as a few hours or as long as one to two days after painting. They occur only in the last coat of paint. They are caused when a thin dry skin has formed on the outer surface of the fresh paint and the liquid thinner in the wet paint under the dry skin changes to vapor and cannot escape.
A rapid rise in temperature, as when the direct rays of the sun fall directly on freshly painted wood, will cause the vapors to expand and produce blisters. Usually only oil-based paint blisters in this way. Dark colors that absorb heat and thick paint coats are more likely to blister than white paints or thin coats.
To prevent temperature blisters, avoid painting surfaces that will soon be heated."Follow the sun around the house" for the best procedure. Thus, the north side of the building should be painted early in the morning, the east side late in the morning, the south side well into the afternoon, and the west side late in the afternoon. However, at least two hours should elapse before the fresh paint film cools to the point where condensation will occur.
If blistering does occur, allow the paint to dry for a few days. Scrape off the blisters, smooth the edges with sandpaper and spot paint the area. Moisture blisters are also bubble-like swellings on the surface of the paint film. As the name implies, they usually contain moisture when they are formed. They may occur where outside moisture such as rain enters the wood through joints and other end grain areas of boards and siding. Paint blisters caused by outside water are usually concentrated around joints and the end grain of the wood. Paint failure is most severe on the sides of buildings facing the prevailing winds and rain. Blisters may occur in both heated and unheated buildings.
Moisture blisters may also result from inside liquid water moving to the outside. Plumbing leaks, overflow of sinks, bathtubs or shower spray and improperly sealed walls are sources of inside water. Moisture blisters usually include all paint coats down to the wood surface. After the blisters appear, they dry out and collapse. Small blisters may disappear completely, fairly large ones may leave a rough spot and in severe cases, the paint will peel.
Thin coatings of new, oil-based paint are the most likely to blister. Old, thick coats are usually too rigid to swell and form blisters. Therefore, cracking and peeling will usually result.
Elimination of the moisture problem is the only practical way to prevent moisture blisters in paint. The moisture source should be identified and eliminated to avoid more serious problems such as wood decay or rot and loss of insulating value.
Peeling and Cracking
Intercoat peeling is the separation of the new paint film from the old paint coat, indicating a weak bond between the two. Intercoat peeling usually results from adequate cleaning of the weathered paint and usually occurs within one year of repainting. This type of inter coat paint peeling can be prevented by following good painting practices.
Intercoat peeling can also result from allowing too much time between the primer coat and topcoat in a new paint job. If you wait longer than two weeks before applying a topcoat to an oil-base primer, soap-like materials may form on the surface and interfere with the bonding of the next coat of paint. When the period between applications exceeds 2 weeks, scrub the surface before applying the second coat. Do not apply a primer coat in the fall and wait until spring to finish with the topcoat.
A simple test can be conducted to determine if the new paint coat is likely to peel. First, clean the old paint surface. Then, repaint a small area with the new paint and allow it to dry for at least two days. Then, firmly press one end of a"band-aid" type adhesive bandage onto the painted area. Jerk it off with a snapping action. If the tape is free of paint, the new paint is well bonded to the old surface. If the new paint adheres to the tape, the old surface is too chalky and needs more cleaning or the use of an oil-based primer.
Cross-grain cracking occurs when paint coatings become too thick. This problem often occurs in older homes that have been painted several times. Paint usually cracks in the direction it was brushed onto the wood. Once cross-grain cracking has occurred the only solution is to completely remove the old paint and apply a new finishing system.
To prevent cross-grain cracking, follow the paint manufacturer's recommendations for spreading rates. Do not repaint unweathered, protected areas such as porch ceilings and roof overhangs as often as the rest of the house. If possible, repaint these areas only as of the weather and require new paint. However, if repainting is required, be sure to scrub the areas with a sponge or bristle brush and detergent in water to remove any water-soluble materials that will interfere with the adhesion of the new paint.
If a new paint coat is to be successful over a problem area, any moisture problems must be eliminated. Good surface preparation is also essential.
If the wood has been completely stripped or is being painted for the first time, it should be brush treated with a paintable water-repellent preservative or water repellent. This treatment will reduce the uptake of moisture by the wood. After the water-repellent preservative or water repellent has dried for at least two warm days, apply an oil-based primer or a stain-blocking latex primer, followed by at least one topcoat of high quality acrylic latex paint. One top coat of paint should last four to five years, but two top coats can last up to 10 years.
If only isolated areas of the paint coat have failed by peeling to the bare wood, spot painting may be the best alternative. First, scrape away all loose paint. Sandpaper or"feather" the edges of any remaining paint smooth with the bare wood. Then clean the old painted surface by scrubbing with a sponge or bristle brush. Rinse the scrubbed surface with clean water. Wipe the surface with your hand. If the surface is still dirty or chalky, scrub it again using a detergent, and rinse with clean water.
After the surface has been thoroughly dried, apply one coat of a paintable water-repellent preservative or water repellent to the bare wood, being careful to liberally treat end and lap joints. Any water- repellent preservative or water repellent on the painted surface should be wiped dry with rags. At least two warm days should be allowed for the water-repellent preservative or water repellent to dry before painting.
An oil-based primer or stain-blocking latex primer should be applied to the bare wood. At least one topcoat to match the rest of the house should follow. Caulk large cracks and openings after treating and priming. If intercoat peeling is a problem, cleans and prepares the entire surface. Then apply the desired topcoat. Where intercoat peeling has been a problem, it is particularly important to clean areas protected from sun and rain such as porches, eaves and side walls protected by overhangs. It is probably adequate to repaint these protected areas every other time the house is painted. If paint fails because of the penetration of rain and dew through porous paint, clean and prepare the paint surface.
Then apply one coat of an oil-base primer and topcoat with acrylic latex house paint The oil-base primer will prevent penetration of the wood by rain or dew. To ensure good adhesion, apply the topcoat within two weeks of priming. Always use a high quality paint, and apply it as recommended by the manufacturer.
Do not skimp on the amount of paint. On the other hand, cross-grain cracking will result in excessively thick paint coats. Therefore, it is important that the paint weathers normally before repainting. Protected areas may be painted every other time. Research has shown that an entire paint film (consisting of a primer and one or two topcoats) thickness of 4-6 mils, or about the thickness of a single sheet of newspaper, will result in the maximum service life.
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