Exterior paint is subject to wetting from rain, dew, and frost.
Equally serious is “unseen moisture” (water vapor) that
moves from inside to outside structures in cold climates and
from outside to inside of air-conditioned buildings in hot
climates. Effective air and vapor barriers can minimize wa-
ter vapor movement (see Chapter 13).
Paint Cracking (parallel to grain)
Cracking parallel to grain occurs on smooth flat-grain lum-
ber, particularly with wood species having abrupt transition
between EW and LW bands (such as southern yellow pine,
Douglas-fir, and oak). LW bands are compressed into EW
during planing. Normal rebound of LW bands after wood is
in service causes films to crack along the EW–LW bound-
ary. Other contributing factors are coatings having insuf-
ficient thickness and lacking flexibility. If the cracking is not
too severe, scuff sand and apply one or two top-coats to give
additional film-build.
Peeling and Flaking
Peeling and flaking (adhesion failure between wood and the
primer) can have several causes: water, wood weathering,
and dimensional change of thick LW bands on flat grain of
high-density wood species. Flaking often follows cracking;
small cracks in paint caused by raised grain allow water to
enter. Flaking is similar to peeling; small pieces of finish
peel from the surface usually along an EW–LW boundary.
Flaking often occurs with cracking parallel to grain and is
attributed to thin films. It can occur with thinly applied film-
forming finishes and with oil-based semitransparent stains
if they do not absorb properly. Water is the main cause, but
other factors can also cause it.
Water speeds the failure by other causes. One cause is
weathering of wood prior to primer application. Protect
wood from the weather prior to installation and paint it as
soon as possible after installing it. Leaving smooth-planed
lumber exposed to the weather for as little as 2 weeks de-
creases its paint-holding properties. If wood was exposed
more than 2 weeks, scuff sand it prior to painting. In fact,
scuff sanding is always a good idea on planed lumber. The
wide bands of LW on flat-grain surfaces hold paint poorly.
If possible, flat-grain boards should be installed “bark-side”
out to minimize raised grain particularly with wood species
having abrupt EW/LW transition. Paint applied to weathered
wood often fails over large areas and can be easily diag-
nosed by inspecting the back side of the peeled paint. Wood
fibers are attached to the film clearly showing the grain of
the wood.
Priming end grain with oil-alkyd paint eliminates peeling
at the ends of boards. Saw-texture greatly improves finish
adhesion to all species and grain angles. Paint and solid-
color stains adhere quite well to difficult-to-paint wood
species such as flat-grain southern yellow pine, Douglas-
fir, and radiata pine, if applied to saw-textured surfaces.Cross-Grain Cracking
Modern waterborne latex finishes seldom fail by cross-grain
cracking. If latex finishes crack across the grain, dimen-
sional instability of wood under the finish causes it. For ex-
ample, cross-grain checking of juvenile wood causes paint
to crack. In this case, replace the board and repaint.
If juvenile wood is not to blame, cross-grain cracking usu-
ally occurs on structures having thick layers of oil-alkyd
paint. If the wood is not the cause of paint failure, remove
the old paint and apply new finish to the bare wood. Old
paint probably contains lead (see Lead-Based Paint).Water Blisters
Water Blisters (also called moisture blisters) are bubble-like
deformation of paint films (Fig. 16–21). As the name im-
plies, these blisters usually contain water when they form.
Water blisters form between the wood substrate and the first
coat of paint. After the blisters appear, they may dry out and
collapse. Small blisters may disappear completely and large
ones may leave rough spots; in severe cases, the paint peels.
Oil-alkyd paint recently applied to wet wood is most likely
to blister. Old paint films are too rigid to swell and form
blisters; they usually crack and peel. Water blisters are not
common on latex paint systems.
Minimizing water absorption into wood is the only way
to prevent water blisters. Water blisters may occur on sid-
ing and trim where rain enters through improperly flashed
doors, windows, and vents; they are common near unsealed
end grain of siding and trim. Water from ice dams and
overflow from blocked gutters can also cause water blisters.
Movement of water vapor from the inside of a structure to
siding and trim may also cause water blisters. Plumbing
leaks, humidifiers, and shower spray are sources of inside
water. Minimizing water absorption also prevents decay
(rot), warping, and checking of wood.Mill Glaze
Since the mid-1980s, a condition known as “mill glaze”
(also called planer’s glaze) has been reported to cause paintGeneral Technical Report FPL–GTR– 190Figure 16–21. Water blisters (also called moisture blis-
ters) caused bubble-like deformation of paint film.