Without the right preparation, you can run into a lot of problems with your exterior paint job. Sometimes, this can happen even if you do, just thanks to unexpected weather complications. Study up before you try to paint your home so you know what to avoid and how to make your project as painless as possible.
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, 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.
While proper construction and maintenance will eliminate most exterior water problems, some may still occur in cases when construction has been shoddy. For example, ice dams happen in northern climates when snow on the upper, warmer parts of a roof melts and runs downward. As the melted snow reaches the roof overhang which is not heated, it freezes once more. Repeated thawing and freezing on the roof moves more water to the edge and an ice 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, dish washing, 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, this water passes into the walls during cold winter weather and condenses to a liquid. It eventually soaks into the siding and wets the paint. This is a common cause of blistering and peeling. The problem is particularly pronounced around bathrooms, laundry rooms, kitchens, and other areas of high humidity.
Interior water vapor can move into the attic space and condense on the gable ends, causing paint peeling there, or it may also condense on the attic side of the roof decking and eventually work its way down the side walls, 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 installation of a continuous six 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. Another vapor barrier, sometimes called a soil cover, should also 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 dry wall. To reduce water vapor in the house, vent high humidity areas such as kitchens and bath areas to the outside. Clothes dryers should also 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 and 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, not latex. Dark colors, which 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; 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 wood, and they may occur in both heated and unheated buildings. This issue will most severe on the sides of buildings facing the prevailing winds and rain.
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, and after the blisters appear, they will eventually dry out and collapse. Small blisters may disappear completely, but fairly large ones may leave a rough spot. 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 instead. 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.
Inter-coat peeling is the separation of the new paint film from the old paint coat, indicating a weak bond between the two. Inter-coat peeling usually results from inadequate cleaning of the old, 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.
Inter-coat peeling can also result from allowing too much time between the primer coat and top coat in a new paint job. If you wait longer than two weeks before applying a top coat to an oil-based primer, soap-like materials may form on the surface and interfere with the bonding of the next coat of paint. If the period between applications exceeds two weeks, scrub the surface before applying the second coat. Never apply a primer coat in the fall and wait until spring to finish with the top coat!
A simple test can be conducted to determine if the new paint coat is likely to peel. First, clean the old paint surface. Then, apply the new paint to a small area 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. If the new paint adheres to the tape, the old surface is too chalky and needs more cleaning or a new layer of oil-based primer.
Cross-grain cracking occurs when paint coatings become too thick. This problem often occurs on 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 altogether.
To prevent cross-grain cracking, follow the paint manufacturer's recommendations for spreading rates. Also, do not repaint un-weathered, protected areas such as porch ceilings and roof overhangs as often as the rest of the house. If possible, repaint these areas only as they weather and require new paint. 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 adhesion.
This article has been contributed in part by Michigan State University Extension