Michigan
State University Extension
Home Maintenance And Repair - 01500572
06/24/03
Paint Problems
Moisture and paint
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,
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 melts and 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 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, 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
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 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 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 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.
Blistering
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 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.
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 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 pain film from the old
paint coat, indicating a weak bond between the two. Intercoat peeling
usually results from inadequate cleaning of the weathered paint and
usually occurs within one year of repainting. This type 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 coat and top coat in a new paint job. If you wait longer
than two weeks before applying a top coat 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 two 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 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 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 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. 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 they
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
adhesion of the new paint.
Repainting Procedures
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-base
primer or a stain- blocking latex primer, followed by at least one top
coat 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-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 and openings after treating and
priming. If intercoat peeling is a problem, clean and prepare the
entire surface. Then apply the desired top coat. 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 penetration 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 paint The oil-base primer will prevent penetration of the
wood by rain or dew. To insure good adhesion, 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 will result with 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 top coats) thickness of 4-6 mils, or about the
thickness of a single sheet of newspaper, will result in the maximum
service life.
References
Materials here are from North Central Regional bulletin Wood Finishing
Paint Failure Problems and Their Cure, with Purdue Extension and USDA
Forest Service.
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