Despite the growing use of wood-plastic composites and all-plastic
or metal decking, most new residential decks are still made from
wood. And while pressure-treated lumber and naturally
decay-resistant species such as cedar and redwood can last a very
long time outside, they aren't immune from the effects of weather.
Thus, most homeowners and deck professionals will turn to a deck
finish to slow the aging process and prolong the life of their
The available finishes run the gamut from semisolids that hide wood
grain to clear preservatives designed to keep wood looking
lumberyard fresh — and the options continue to multiply. In
addition to traditional finishes made from natural oils, there are
finishes made from modified oils called "alkyds"; water-based
acrylics; and coatings that combine acrylics and alkyds.
No finish does everything perfectly, especially in a grueling
environment where heavy foot traffic, unrelenting sun, and exposure
to rain and snow are the norm. Also, there's no such thing as a
permanent, maintenance-free finish — all will need periodic
cleaning and recoating.
That said, there are fundamental differences between families of
finishes that affect performance, appearance, longevity, and (of
course) price. And just as your painter will tell you, surface
preparation is everything (see "Before You Finish, Get the Surface
The Basics: What Pigments Do
There are three basic flavors of deck finishes: clear,
semitransparent, and semisolid. Formulations differ widely, but a
key difference among them is the amount of pigmentation they
contain. Whether the finish is water-based or oil-based, finely
ground pigments are the ingredients that prevent surface damage and
natural graying due to ultraviolet radiation. Increasingly, coating
manufacturers are exploring the use of extremely fine pigment
particles to limit UV damage (see "Big Promise From Very Small
Clear sealers last only about a year, or possibly two, before they
need recoating, but they are relatively easy to apply and require
the least amount of prep work before a reapplication (Figure 1).
They may be either oil- or water-based, and may contain ingredients
to absorb UV radiation to slow natural graying and a fungicide to
retard the growth of mold. What makes clear finishes appealing is
they allow the natural figure of the wood to show through —
and if you've just spent a boatload of money on a deck of all-heart
redwood, that argument can be persuasive.
Figure 1. Clear sealers are the easiest to
apply and they show the wood's figure. However, they're the least
durable type of finish — with an expected life span of only a
year or two — and many clear finishes offer little protection
against UV degradation of the underlying wood.
For these reasons, a clear water repellent containing an agent to
combat mold is the first choice of Sam Williams, a research chemist
at the Forest Products Laboratory in Madison, Wis., when he
finishes his own deck. Although he can count on reapplying the
finish every couple of years, Williams can prep the deck and
reapply finish in an elapsed time of only about 60 minutes.
After clear finishes, next on this gradient of increasing opacity
is a class of tinted sealers and stains, sometimes called toners or
semitransparents, that contain some light-blocking pigments (Figure
2). Their advantage is that they offer some protection from UV
damage while allowing some wood grain to show. Depending on
weather, sun exposure, and foot traffic, you can count on two or
possibly three years of service on a horizontal surface before
Figure 2. Tinted sealers offer a couple of
years' durability and show off the grain of the wood. Also called
"semitransparent," these finishes are fairly easy to apply. Their
pigments change the wood's color while providing some defense
The most opaque finishes are the semisolid stains, which contain a
lot of pigmentation — enough to offer the best long-term
protection against sun and weather damage (Figure 3). They might
not be the first choice for a deck of clear redwood or cedar, but
semisolid stains come in a wide range of colors that can spruce up
the uniformity of pressure-treated softwood. These stains last
longer than the others — up to five years, according to one
Figure 3. Semisolid stains show only a hint of
the underlying wood grain. Their heavy pigments offer the best UV
protection, and they can be expected to last as long as five
There are a couple of downsides, however, to semisolid finishes.
They are harder to apply evenly and may show brush marks where the
surface doesn't dry evenly. The best advice is to apply the finish
on just a couple of deck boards at a time, following them from one
end to the other, and then move on to the next section of
Semisolid stains will make wear patterns on the deck more obvious,
and they require the most prep work before they can be recoated.
The manufacturer may, in fact, recommend the surface be completely
stripped and cleaned before any new finish is applied. Bottom line:
Semisolid stains offer more protection for a longer period of time,
but they're harder to apply evenly and require more work to prepare
Avoid Film-Forming Finishes
Paint forms a tough film designed to block sun and water for long
stretches of time. These attributes make paint ideal for exterior
trim and siding, but they're not advantages for a deck. Penetrating
deck stains and sealers are designed to protect without forming a
As explained by lumber-industry veteran Ed Burke, who's a member of
the coatings advisory committee at the Forest Products Laboratory,
film-forming finishes are very good at protecting wood, but they
inevitably develop hairline cracks — particularly on a deck.
When they do, rainwater will get beneath the film. "And then these
coatings are going to do an equally good job of keeping it there,"
Burke says. "The problem with that is that any time wood does not
dry out between rainstorms, you have the potential for decay and
Opaque deck stains can be paintlike in this respect when too much
is applied. Homeowners sometimes take a look at the result and
think the deck needs multiple coats. Yet when they try to make the
surface of the deck look as uniform as a freshly painted ceiling by
applying more finish, the stain morphs into a film-forming
"The problem is," explains Burke, "there's so much pigment in this
stuff that if you put it on too heavily or you put on two coats,
bingo — you've got an oil-based paint. It doesn't matter what
you put on the can. It's paint because it forms a paint film."
Avoid the problem by following the recommendations of the
Oil- or Water-Based?
For a variety of reasons, acrylic latex paint is gradually
replacing traditional oil-based formulations, and manufacturers are
showing equal resolve in developing water-based deck finishes.
There have been steady improvements in their durability: Some now
perform as well as their oil-based counterparts (Figure 4). "Three
years ago I would have said don't ever use them," Williams says.
"But some of the acrylics are starting to get a little better. It's
a brand-by-brand situation. Some of the waterbornes seem to work as
well as some of the old [oil] systems."
Figure 4. Water-based finishes can be as
durable as oil-based. This clear finish looks milky during
application but dries to a satin finish. Water-based finishes wear
differently than oil-based, tending to eventually flake or peel
away, rather than abrading with time. Refinishing may be more
likely to require stripping the deck to bare wood than with
Water-based finishes, however, typically don't penetrate very
deeply into wood decking (if they penetrate at all), so they wear
differently — they're more likely to peel than gradually
erode like oil-based products, according to the makers of
Thompson's deck products. But they are lower in volatile organic
compounds (VOCs), making them a natural choice where air-quality
regulations are toughest. They also hold color well and can be
cleaned up with soap and water — all decided
Everett Abrams, owner of Deck Restoration Plus in Shamong, N.J.,
calls water-based finishes the "enigma of the industry." He
explains that while water-based finishes are durable, they also are
harder to strip back when it's time to recoat. And if the surface
isn't consistent when a new coat is applied, he notes, the finish
can look blotchy. Abrams says strippers that effectively remove
water-based finishes are becoming more widely available, but when
faced with a deck that's been finished with a water-based product,
he's still likely to sand before recoating.
Among oil-based finishes, linseed and soy oils have been
traditional choices. They are easy to apply and they penetrate more
deeply than water-based coatings. Another plus: They wear gradually
On the other hand, natural oils have two big disadvantages: First,
these finishes contain more solvents containing VOCs than
water-based coatings do — and VOCs are falling under
increasingly stringent government regulations, though rules vary
around the country. In California, the South Coast Air Quality
Management District regulates products sold in several counties in
the Los Angeles area. In the Northeast, the Ozone Transport
Commission has jurisdiction over a dozen states plus the District
of Columbia. Manufacturers may sometimes adjust their formulations
to meet these local or regional requirements.
Second, natural oil can be what Burke calls "mold fertilizer."
Given the right moisture and temperature, mold colonies thrive as
they feed on the organic compounds in oil as well as on the
extractives in the wood decking itself (Figure 5). "When you load
up a piece of redwood or western red cedar with linseed oil, the
mildew just goes to town on it," Williams says. "In fact, you can
turn a piece of redwood black in about a week under the right
weather conditions. It's really a disaster."
Figure 5. Natural oil-based finishes soak in
to seal out water but may bring other problems. Linseed oil is
itself gourmet mold food. If you're planning to use such a product,
be sure it's got a factory-blended fungicide.
Fungicides added to the mix by the manufacturer can control the
problem. Beware of the anti-mildew additives sold at paint stores,
however: They probably have limited usefulness.
Alkyds, which are modified oils, don't have the same potential for
mold. Greg Portincasa, a technical and customer support supervisor
for Sikkens Wood Finishes, says alkyds are more expensive than
natural oils, but they cure faster and last longer.
Increasingly, manufacturers are finding ways of combining alkyds
with acrylics for a finish that forms a shell of acrylic resin
around a core of alkyd resin — what Thompson's describes as
something like a Tootsie Pop. These finishes can be produced with
lower VOCs, and they wear by erosion like other oil-based
Ipe and Wood-Plastic Composites
While most woods last longer and look better with a finish, caring
for super-hard tropical hardwoods, such as ipe, and for the growing
number of wood-plastic composites can be more complicated.
Ipe is a naturally decay-resistant wood that will last a long time
in the weather, even if it turns gray in sunlight and the board
ends show some minor cracking or checking. These characteristics
might seem to make this family of South American hardwoods a good
candidate for a clear UV protector, except that finishes have a
tough time penetrating the surface. Some manufacturers have
developed finishes that are specifically marketed for this purpose.
But in the end, ipe, like other naturally durable woods, can be
allowed to weather naturally if the homeowner doesn't find fading
objectionable. A water-repellent preservative applied to the end
grain can reduce checking.
Composites pose a different problem (Figure 6). There are now many
brands on the market, each with its own formulation. But because
these man-made planks contain significant amounts of wood fiber,
they are susceptible to fading and mold growth — just like
wood. The material itself is challenging, too. A mix of wood flour
and recycled or virgin plastics, composite decking combines two
materials with dramatically different characteristics. The
finishing industry has been stymied at times as decking producers
continue to modify their formulations.
Figure 6. Yes, you can finish composite
decking. No manufacturer requires that composite decking be
finished, but colors can be changed, and older decks can be made to
Should some kind of finish be applied? It depends. Trex says its
decking needs no stains or sealants, just regular cleaning.
CorrectDeck, on the other hand, says clear or solid-color stains
may be used. The Forest Products Lab has found that composites with
high wood content can be maintained with a clear penetrating finish
after the boards weather for several years. Given the difference in
composite content, however, it would be a good idea to check with
the manufacturer to see which, if any, type of finish or stain is
recommended. Regular cleaning will take care of any mold and
No Magic Answer
If you're looking for the single best deck finish, one that works
on every decking material all the time, no matter where you are,
you're going to be disappointed. There are just too many
Keeping the pros and cons of various families of finishes in mind
will help determine what finish to use, as will the particular
requirements of the homeowner. Should, for example, the finish
enhance the natural color and grain of the wood (clear
preservative) or mask it (pigmented stain)? How much maintenance is
the homeowner willing to accept — a minor cleaning before
recoating or more extensive chemical stripping and sanding? How
long does the finish have to last? What is the decking
Long-term third-party testing of finishes would be useful, but
there's not much of that around. Consumer Reports magazine conducts
multiyear weather tests for a variety of finishes, periodically
reporting the results. Those surveys may provide clues about
performance, as will asking around for what brands and types of
finishes seem to work best locally.
Williams suggests trying a couple of brands to see which look best
and then contacting the manufacturer's customer service department
to ask specific questions about the formulation (Figure 7). Does
the finish contain alkyds? Or is it made with a natural oil or with
a polymer such as acrylic or vinyl acrylic? If it's a polymer base,
don't expect much penetration. If it's made from a natural oil,
expect a higher risk for mold.
Figure 7. Sample boards help customers decide
between finish options. There's no substitute for seeing the real
Finally, take a close look at the surface of the wood after the
finish has been applied. If it's shiny, it means the finish hasn't
really penetrated the surface. What you're looking for is a dull
surface, meaning the finish has soaked in.
Ultimately, Mother Nature is going to prevail. Finishes can help
preserve color, at least temporarily, and a preservative applied
every couple of years won't hurt.
"Does the wood need it?" asks Williams. "To a certain extent, yes,
but not to the extent that you're out there every year for three or
four hours finishing your deck. Most people finish their decks
because they want a certain look. They want to maintain the natural
look of the wood or match the color scheme of their home. If you
have a redwood or cedar deck, it's really important to put a
penetrating preservative on it every two or three years. Other than
that, decks don't require much maintenance."
Scott Gibson is a writer in East Waterboro,