Making Paint Stick to Wood Siding & Trim
Here’s a plausible scenario: You’ve just
completed a new custom home — the clients are ecstatic,
the project looks great. With fresh paint on the outside, the
house’s curb appeal promises to bring new client
referrals. Then, within six months or a year, the phone call
comes: "The paint is peeling; can you do something?"
Premature paint failure on wood siding has become
commonplace in recent years. The reasons cited are varied. Some
blame paint quality, claiming that paints are not as durable
since the lead content was banned. Others point to the
declining quality of the wood siding itself. Still others argue
that the high interior moisture levels in today’s tight
houses cause water vapor to move through the exterior walls
into the back of the siding, which eventually causes the paint
to peel and blister.
All of these arguments have some merit. There is often a
complex combination of causes behind a particular paint
failure. But paint failure is not inevitable: In most cases, if
the job had been designed and executed properly in the first
place, the failure could have been prevented.
Success with painted siding starts with an understanding of
how the siding you choose will perform under local weather
conditions. Next, it’s necessary to match the finish to
the siding — a given finish performs differently on
different types of siding.
In this article, I’ll look mainly at two areas: 1) the
properties of wood that most affect paint durability; and 2)
varieties of paints, stains, and other finishes, and how they
perform on various types of siding.
Although it’s not covered here, ventilation details
are critical to good paint performance. This includes proper
roof venting as well as using adequate indoor exhaust
ventilation to remove excess moisture from the building. When
the budget allows, vented siding, where an air space is created
between the sheathing and siding, is unquestionably one of the
best details for ensuring long-term paint performance (see
"Rain Screen Siding Retrofit," 4/98).
The Wood Makes a
How wood siding performs varies not
only from one wood species to another but within the same
species. These natural variables, and the variables created
during the manufacturing processes, have important influences
on wood’s finishing properties and its durability.
Density. The density of
wood, or its "weight," is one of the most important factors
affecting paint life, for a simple reason: "Heavy" woods shrink
and swell more than "light" woods (see Table 1).
Paint-Holding Ability of Selected
Weight per cubic foot
at 8% moisture content
(I best, IV worst)
Western red cedar
Eastern white pine
Southern yellow pine
Note: Lighter-weight woods shrink and swell less than
denser woods, so tend to hold paint better. Besides
being lower density, redwood and cedar have narrow
bands of latewood compared with Southern yellow pine
and Douglas fir, which are higher in density and have
wide bands of latewood.
Excessive dimensional change constantly stresses a
film-forming finish, such as paint or a solid-color stain, and
may result in early failure. Finishes that don’t form a
film, such as penetrating stains, are not affected by these
Flat-grain vs. edge-grain. Softwood lumber is referred to as
either flat-grained or edge-grained (plainsawn or quartersawn
in hardwoods). Most standard lumber grades contain a high
percentage of flat grain. Flat-grained lumber shrinks and
swells more than edge-grained lumber (see Figure 1), so
edge-grained lumber will usually hold paint better than
1. Flat-grained lumber shrinks and swells more
than edge-grained (quartersawn) lumber and also has
wider bands of dark latewood. Therefore, edge-grained
siding will usually hold paint better than flat-grained
Some bevel sidings are produced in both a flat-grained
standard grade and an edge-grained premium grade (sometimes
called vertical grain).
Earlywood and latewood.
Another reason that edge-grained siding holds paint better is
that edge-grained wood has narrower bands of latewood.
Earlywood and latewood form in two distinct bands within an
annual growth ring. Latewood is denser, harder, smoother, and
darker than earlywood. Although new paint or solid-color stain
will adhere well to both earlywood and latewood, old alkyd
paints and solid-color stains that have become brittle with age
and weathering will peel first from the smooth, hard surface of
sapwood. Mature trees have a darker central column
of wood called heartwood, surrounded by a lighter cylinder of
wood called sapwood. Heartwood is formed as the individual
cells die and are impregnated with extractives, pitch, oil, and
other extraneous materials. These natural materials give the
heartwood of some species, such as redwood, cedar, and cypress,
a natural resistance to decay and insects as well as an
attractive color. Extractives, however, can sometimes cause
discoloration problems when the heartwood is finished with
paints or solid-color stains. Because the extractives are water
soluble, they can dissolve when water is present in the wood
and be transported to the wood surface. When the solution of
extractives reaches the painted surface, the water evaporates,
and the extractives remain, showing through as a reddish-brown
Sapwood is not decay resistant, but also does not normally
cause discoloration problems through paints or solid-color
Knots and other irregularities, such as bark, splits, pitch
pockets, and insect damage, also affect paint adhesion. Knots
are mainly exposed end grain. End-grain wood absorbs more
finish than flat- and edge-grained lumber, which will affect
the appearance of paint. In pine, knots often contain a high
percentage of resin, which may cause the paint over the knot to
discolor. Furthermore, large knots usually check and crack, and
can leave a noticeable split in the wood surface.
Good painting practices can eliminate or control brown stain
over knots. First apply a primer recommended for blocking the
extractives in the knot, then follow with two top coats. Some
manufacturers recommend orange shellac for controlling knot
Wood Properties You
Can Control So far, we’ve been
talking about properties of wood that vary from species to
species and grade to grade. But once you’ve selected and
bought wood siding or trim, there are a few variables under
your control that will affect the life of the finish.
Moisture content is critical in determining the service life
of paint. The best time to paint wood is when its average
moisture content is about the same as that expected to prevail
once the wood is put in service (Table 2).
Optimal Moisture Content for Wood Siding &
Most areas of United States
Dry southwestern areas
Warm, humid coastal areas
Note: It’s best to paint exterior wood
when its moisture content is within the prevailing
range for the region. Source: Williams, Knaebe
& Feist, Finishes for Exterior Wood.
Wood above 20% moisture content should never be painted, as
the paint will most likely peel.
Paint lasts longer on smooth, edge-grained surfaces than on
smooth, flat-grained ones. However, paint will last longest on
roughsawn or rough-sanded wood, whether the wood is
edge-grained or flat-grained. Sand smooth siding with 60-grit
paper before painting.
Avoid weathering before painting.
Much research has been done that indicates that whenever wood
is to be painted, stained, or finished in any manner,
weathering of the unprotected wood before finishing may be
detrimental to the service life of the finish. The USDA Forest
Service’s Forest Products Laboratory in Madison, Wisc.,
recommends that any dry, unprotected wood should not be allowed
to weather for more than two weeks before it is protected with
some finish that will prevent photodegradation and water
Choosing a Finish
finishes range from opaque, film-forming paints and solid
stains to penetrating semi-transparent stains, which impart a
rustic appearance and allow some wood grain to show through, to
clear finishes, which accentuate the grain and natural beauty
of the wood. The choice of finish should be made at the same
time that the siding treatment is chosen. For example, while an
expensive all-acrylic latex paint may perform well on a clear
grade of siding, it may not be the best choice for a roughsawn,
Here’s a rundown of the available types of
products, with comments on their suitability for various types
of sidings ().
form a film, and provide the most protection against weathering
and wetting. It’s possible to get up to ten years from a
top-quality paint applied as two top coats over a primer
Oil-based vs. latex.
The most durable house paints are the all-acrylic latex paints.
Although oil-based paint films usually provide the best
protection from liquid water and water vapor, they are not
necessarily the most durable, because they become brittle over
time. No matter how well sealed, wood still moves with seasonal
humidity changes, thus stressing and eventually cracking the
brittle paint film. On the other hand, latex paints,
particularly the acrylic paints, remain more flexible with age.
Even though latex paints allow more water vapor to pass
through, they hold up better by swelling and shrinking with the
Gloss. Paints are
available in different degrees of gloss, including flat, matte,
semigloss, and gloss. Generally, high-gloss paints contain more
paint resin and less pigment, and will perform better and last
longer than the low-gloss or flat paints. Flat paints tend to
pick up dirt and absorb water more readily than do the
high-gloss paints. Because of this, mildew growth is often
greater on the flat paints.
Back-prime. Paints do
not penetrate the wood deeply, but form a film on the surface.
This film can blister or peel if the wood is wetted or if water
vapor from inside the house moves through the exterior wall
into the siding because of the absence of a vapor barrier.
It’s important to back-prime the siding with one coat
of primer or a paintable water-repellent preservative before
installation. This helps reduce wetting up the back of the
siding. Carefully coating butt ends and cut ends is also
Solid-color stains (also called hiding, heavy-bodied, or opaque
stains) are essentially thin paints, not true stains.
Solid-color stains have a higher concentration of pigment than
semitransparent penetrating stains, but a somewhat lower
concentration of pigment than standard paints. As a result,
solid-color stains obscure the natural wood color and grain,
while the wood’s surface texture is retained. They are
often the finish of choice on textured or roughsawn siding
products. They can also be applied over existing paints and
solid-color stains, and normally leave a flat finish
Like paints, solid-color stains protect wood against UV
degradation. Lifetimes of three to six years can be expected
for two-coat applications.
A penetrating water-repellent
preservative may be used as a natural wood finish. This type of
finish contains a preservative (a fungicide), a small amount of
wax (or similar water repellent), a resin or drying oil, and a
solvent such as turpentine, mineral spirits, or paraffinic oil.
Some may be lightly pigmented, and waterborne formulations are
also available. The unpigmented finishes provide minimal
protection for wood and may last only one to three years
depending on exposure. Water-repellent finishes reduce warping
and checking, prevent water staining at the edges and ends of
wood siding, and help control mildew growth. Wood treated with
preservative is easily refinished and usually requires minimal
Paintable water-repellent preservatives (such as DAP’s
Woodlife II) are also an excellent treatment for bare wood
before priming and painting or in areas where old paint has
peeled, exposing bare wood. This pretreatment keeps rain or dew
from penetrating the wood, especially at joints and on end
grain, thus decreasing the shrinking and swelling of the wood.
As a result, less stress is placed on the paint film, and its
service life is extended. While these treatments protect
against liquid water, they do not protect wood from water
There are many
penetrating oil-based and alkyd-based finishes available, most
using linseed or tung oil. However, these oils may serve as a
food source for mildew if applied to wood in the absence of a
mildewcide. The oils will also perform better if a water
repellent is included in the formulation. All these oil systems
will protect wood, but their average lifetime may only be one
to three years.
Semi-transparent penetrating stains are pigmented
water-repellent preservatives with higher resin or binder
content. Lifetimes may vary from three to six years, depending
on wood surface texture and quantity of stain applied. The
solvent-borne stains (oil or alkyd-based) penetrate the wood
surface to a degree, are porous, and do not form a surface film
like paint. Thus, they do not totally hide the wood grain and
will not trap moisture that may encourage decay. As a result,
the stains will not blister or peel even if moisture penetrates
The better solvent-borne penetrating stains contain a
fungicide (preservative or mildewcide), ultraviolet radiation
stabilizer or absorber, and a water repellent. Latex-based
(waterborne) stains are also available, but do not penetrate
the wood surface as do their oil-based counterparts. Newer
latex formulations are being developed that may provide some
You Get What You Pay For
When buying paint and other exterior wood finishes, it is best
to use the top-of-the-line product of a supplier you know and
trust. For example, the top-of-the-line and most expensive
latex house paint of most manufacturers is usually the
all-acrylic latex type.
Consider a factory
Many wood siding products are available
either factory-primed or factory-finished (primed and
top-coated). Factory finishing offers many advantages. Because
of the controlled environment, it’s easier to apply the
correct amount of finish and to have it dry and cure under
Bill Feist is a former wood finishes researcher with the
Forest Products Laboratory in Madison, Wis., and co-author of
Finishes for Exterior Wood. This article was adapted with
permission from Wood Design & Building magazine. For
subscription information, call 888/438-7771 or visit