The finish line

A Forest Products Laboratory finishing factsheet

Forest

Service

Forest

Products

Laboratory

United States

Department of

Agriculture

House paint can fail prematurely—

the following identifies some

 reasons and remedies.

•Wood was wet when it was

painted.

If only the surface of the wood is

wet, then only 1 sunny day is

usually needed for drying prior to

painting. If the wood is saturated,

several sunny or windy days are

necessary for drying prior to

painting.

•Unfinished siding was exposed

to several weeks of sunlight

before painting.

Sunlight degrades the unfinished

wood surface, thus it will never

hold paint as well as fresh wood.

If the unfinished wood was exposed more than 3 to 4 weeks,

lightly sand or power wash the surface to remove the thin layer of degraded wood before applying

paint.

•Temperature was too cold

when the wood was painted.

Oil-based paints should be applied

when the temperature is at least

40 ̊F; for latex paints, the

temperature should be at least

50 ̊F. Conditions should remain

above these temperatures for

24 hours after painting. When pretreating the wood with a paintable

water-repellent preservative (a

recommended practice), best results are achieved if it is applied

when temperatures are greater

than 70 ̊F.

•Humidity in the house was

too high during the heating

season.

A high level of humidity inside the

house is probably the cause if

paint failure occurs on the outside

walls of the bathroom or kitchen,

and it can be even more pronounced on the outside of an upper floor. In multistory buildings,

there is a chimney effect. Warm

moist air is trying to vent upstairs,

and eventually this moisture travels out through the siding. Paint

failure may be more noticeable

near electrical outlets or other

breaks in the vapor barrier. Drier

air enters the house through

cracks on the main level; therefore, paint failure caused by high

humidity is usually not a problem

on the main level. Condensation

on the windows also indicates excessive humidity in the house.

Turning down your humidifier or

turning on a bathroom exhaust

fan will help lower the humidity

level inside the house. An energy

efficient but somewhat expensive

solution to high levels of humidity

is to install an air-to-air heat exchanger. Here, warm moist air

gives its heat to the incoming

fresh, dry air.

•Wood was installed directly

over foam or foil-faced insulation board.

Water can travel in behind the siding of the house through various

routes but has to travel out

through the wood, pushing the

paint off. Even if the paint remains

Why House Paint Fails

•Wood was too hot when it was

painted or was heated soon

after painting.

Do not paint when the temperature is greater than 90 ̊F. To

prevent temperature blisters,

avoid painting surfaces that will

soon be heated. The best procedure is to “follow the sun around

the house.” The east side of the

building should be painted late in

the morning, the south side in the

middle of the afternoon, the west

side late in the afternoon. The

north side can be painted at any

time during the day. However, at

least 2 hours are needed for the

fresh paint to dry before weather

conditions cool to the point where

dew forms. If blistering on the

wood surface does occur, allow

the paint to dry for a few days,

scrape off the blisters, smooth the

edges with sandpaper, and paint

the area.

•Weather was too humid when

the surface was painted.

When water-based paints cure,

the water should evaporate as

fast or faster than the solvents. After the water has evaporated, the

paint will shrink to nearly its final

shape. As the solvents evaporate,

the paint chemically reacts to form

a hard material. When it is too humid, water cannot evaporate and

the solvents may evaporate first,

causing the paint to cure while still

in a water-filled state. You cannot

recover from this type of disaster.

Oil-based paints will also fail if

conditions are too humid.
on the surface, this moisture can

cause other problems. Large

overhangs, proper caulking, and a

12-inch-minimum ground clearance may decrease the chance of

water getting in behind the siding.

Additional suggestions to prevent

paint failure in this situation include the following:

– Driving small wedges (1/16

inch) under every sixth row of

siding may permit water to escape and reduce the moisture

problem. However, wind-driven

rain may also use this as an access and aggravate the situation.

– Back priming (painting the back

of the siding before installation)

may help reduce or prevent

paint failure.

– Install roofing paper (15- to 30-

lb felt) beneath the siding. Note:

The best solution is to attach

furring strips to the studs

through the insulation board,

making air spaces behind the

siding. Furring strips also make

a nice home for bugs, if you do

not screen the bottom. A new

spacer-type webbing called “cedar breather” is sometimes

used under wood shingles and

may have merit for use under

siding.

•House has no interior vapor

barrier.

The absence of an interior vapor

barrier is related to the problems

of high levels of humidity inside

the house during the heating season and wood that was installed

directly over foam or foil-faced insulation board. Driving wedges

(previously noted) under the siding may be the easiest solution.

Applying certain interior vaporretarding paints and installing

electrical outlet gaskets may also

be effective, especially on the upper floor of a multistory house.

•Wood siding is dirty.

If the siding is dirty, the surface of

the siding should be power

washed or cleaned with detergent

and a stiff bristle or brass brush

and rinsed well. Never use steel

or iron, which causes iron stain

and may glaze the surface.

•Wood has mill glaze.

Mill glaze can be caused by several factors. For example, if during

planing to make smooth wood,

the planer blades were dull, running too fast, or pressing too hard

on the wood surface, the surface

of the wood can become hardened or resins may be drawn to

the surface causing a glaze.

Whatever the cause, the surface

appears to be case hardened. If a

drop of water beads up on the

wood surface but does not on a

lightly sanded surface, you may

have what is commonly called mill

glaze. If you have mill glaze, the

smooth surface must be lightly

sanded or power washed to remove the hardened surface.

Oddly enough, controlled wetting

of new siding with a garden hose

may promote better adhesion to

the redried wood. The water releases the stresses in the wood.

You can also create a type of mill

glaze by sand blasting or using a

wire wheel on the wood surface.

Mill glaze is not a problem on

rough-sawn siding.

•Brown stains appear on the

surface of the paint.

Paint does not have to fall off to

fail. Moisture traveling through

wood pulls water-based extractives through the paint, leaving

brown stains on the surface of the

paint. If the wood is kept dry, the

water-based extractives in the

wood will not bleed through paint.

Keeping all moisture out may be

difficult. Oil-based primers usually

block extractive stains better than

latex primers and may be a better

choice on redwood and cedar;

however, oil paints can increase

mildew. Compared with oil-based

primers, latex primers produce a

more flexible paint film with better

durability and can be used when

extractive staining is not a problem. Improved stain-blocking latex

primers will probably be available

in the future.

•Wood has decayed (rotted).

Decayed wood can result if the

wood has been wet for extended

periods. If the wood is soft and

spongy, it is degraded to the point

that it will never hold paint and

should be replaced.

In summary, note the following

to prevent house paint failure

(not applicable to semitransparent or solid-color stains):

– Install siding properly.

– Sand or power wash the surface of the wood if it is smooth.

– Apply paint during recommended weather conditions and

temperatures.

– Treat the surface of the wood

with a paintable water-repellent

preservative (especially the end

grain).

– Prime the surface of the wood

with a stain-blocking primer.

– Properly apply caulking material.

– Apply two latex topcoats over

the primer.

Mark Knaebe, Chemist

Wood Finishing Research

USDA Forest Service

Forest Products Laboratory

One Gifford Pinchot Drive

Madison, Wisconsin 53705-2398

October 1995