Moisture and paint
Outside moisture such as rain and dew can penetrate a paint coat and result incracking, peeling, discoloration and premature paint failure.
These problems may be observed in both heated and unheated buildings, are morepronounced on edges and ends of boards and are also observed where water is held on thesurface. Porous paints are particularly vulnerable to moisture penetration.
Proper construction and maintenance will eliminate most exterior water problems. Icedams occur in cold northern climates when snow on the upper warmer parts of a roof meltsand 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 anice dam begins to build up. The melted water may penetrate the roof and drain into theexterior walls, thus causing moisture and associated paint problems.
Ice dams can be reduced or eliminated by adequate insulation in attics and by properattic ventilation. Inside moisture (water vapor) can destroy paint on the outside of abuilding by diffusing through the walls. Water vapor from cooking, dishwashing, clothesdryers, bathing and normal respiration by an average family of four can contribute threegallons of water per day to the humidity.
If the inside of all exterior walls does not have a vapor barrier or if the vaporbarrier is improperly installed, water vapor passes into the walls during cold winterweather and condenses to a liquid. The water eventually soaks into the siding and wets thepaint and is a common cause of blistering and peeling. The problem may be particularlypronounced 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 deckingand eventually work its way down the side walls, causing paint peeling near the tops ofthese 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 squarefeet of ceiling area. Hip roofs should have continuous slotted vents in the eaves to allowair to enter the attic and ridge vents to allow its exit.
Condensation problems in cold climates are best prevented by installation of acontinuous 6 mil polyethylene vapor barrier on the warm side of all exterior walls andceilings. The vapor barrier should fit tightly around electrical outlets, doors, windowsand other openings. A vapor barrier, sometimes called a soil cover, should be installeddirectly over the soil in all houses with crawl spaces. This will keep moisture frommoving out of the soil and up into the living space and then through the walls andceilings. 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 bathareas to the outside. Clothes dryers should be vented directly to the outside and not tothe attic, basement or crawl space. Mechanical humidifiers add large quantities ofmoisture 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 paintfilm as early as a few hours or as long as one to two days after painting. They occur onlyin the last coat of paint. They are caused when a thin dry skin has formed on the outersurface of the fresh paint and the liquid thinner in the wet paint under the dry skinchanges to vapor and cannot escape.
A rapid rise in temperature, as when the direct rays of the sun fall directly onfreshly painted wood, will cause the vapors to expand and produce blisters. Usually onlyoil- based paint blisters in this way. Dark colors which absorb heat and thick paint coatsare more likely to blister than white paints or thin coats.
To prevent temperature blisters, avoid painting surfaces that will soon beheated."Follow the sun around the house" for the best procedure. Thus, the northside of the building should be painted early in the morning, the east side late in themorning, 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 pointwhere condensation will occur.
If blistering does occur, allow the paint to dry for a few days. Scrape off theblisters, smooth the edges with sandpaper and spot paint the area. Moisture blisters arealso bubble-like swellings on the surface of the paint film. As the name implies, theyusually contain moisture when they are formed. They may occur where outside moisture suchas 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 endgrain of wood. Paint failure is most severe on the sides of buildings facing theprevailing 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 wallsare sources of inside water. Moisture blisters usually include all paint coats down to thewood surface. After the blisters appear, they dry out and collapse. Small blisters maydisappear completely, fairly large ones may leave a rough spot and in severe cases thepaint will peel.
Thin coatings of new, oil-based paint are the most likely to blister. Old, thick coatsare usually too rigid to swell and form blisters. Therefore, cracking and peeling willusually result.
Elimination of the moisture problem is the only practical way to prevent moistureblisters in paint. The moisture source should be identified and eliminated to avoid moreserious problems such as wood decay or rot and loss of insulating value.
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 inadequatecleaning of the weathered paint and usually occurs within one year of repainting. Thistype of intercoat paint peeling can be prevented by following good painting practices.
Intercoat peeling can also result from allowing too much time between the primer coatand top coat 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 withthe bonding of the next coat of paint. When the period between applications exceeds twoweeks, scrub the surface before applying the second coat. Do not apply a primer coat inthe 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, repaint a small area with the new paint andallow 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 thetape is free of paint,the new paint is well bonded to the old surface. If the new paintadheres to the tape, the old surface is too chalky and needs more cleaning or the use ofan oil-based primer.
Cross-grain cracking occurs when paint coatings become too thick. This problem oftenoccurs on older homes that have been painted several times. Paint usually cracks in thedirection it was brushed onto the wood. Once cross-grain cracking has occurred the onlysolution is to completely remove the old paint and apply a new finishing system.
To prevent cross-grain cracking, follow the paint manufacturer's recommendations forspreading rates. Do not repaint unweathered, protected areas such as porch ceilings androof overhangs as often as the rest of the house. If possible, repaint these areas only asthey weather and require new paint. However, if repainting is required, be sure to scrubthe areas with a sponge or bristle brush and detergent in water to remove anywater-soluble materials that will interfere with adhesion of the new paint.
If a new paint coat is to be successful over a problem area, any moisture problems mustbe eliminated. Good surface preparation is also essential.
If the wood has been completely stripped or is being painted for the first time, itshould 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-repellentpreservative or water repellent has dried for at least two warm days, apply an oil-baseprimer or a stain- blocking latex primer, followed by at least one top coat of highquality acrylic latex paint. One top coat of paint should last four to five years, but twotop coats can last up to 10 years.
If only isolated areas of the paint coat have failed by peeling to the bare wood, spotpainting 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 cleanthe old painted surface by scrubbing with a sponge or bristle brush. Rinse the scrubbedsurface with clean water. Wipe the surface with your hand. If the surface is still dirtyor chalky, scrub it again using a detergent, and rinse with clean water.
After the surface has been thoroughly dried, apply one coat of a paintablewater-repellent preservative or water repellent to the bare wood, being careful toliberally treat end and lap joints. Any water- repellent preservative or water repellenton the painted surface should be wiped dry with rags. At least two warm days should beallowed for the water-repellent preservative or water repellent to dry before painting.
An oil-base primer or stain-blocking latex primer should be applied to the bare wood.At least one top coat to match the rest of the house should follow. Caulk large cracks andopenings after treating and priming. If intercoat peeling is a problem, clean and preparethe entire surface. Then apply the desired top coat. Where intercoat peeling has been aproblem, it is particularly important to clean areas protected from sun and rain such asporches, eaves and side walls protected by overhangs. It is probably adequate to repaintthese protected areas every other time the house is painted. If paint fails because ofpenetration of rain and dew through porous paint, clean and prepare the paint surface.
Then apply one coat of an oil-base primer and top coat with acrylic latex house paintThe oil-base primer will prevent penetration of the wood by rain or dew. To insure goodadhesion, apply the top coat 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 willresult with excessively thick paint coats. Therefore, it is important that the paintweathers 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, willresult in the maximum service life.
Source of material: Michigan State University Extension Service
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