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.