What you need to know about
exterior finishes for Log Homes
characteristics of wood make it suitable for a variety of
applications in the building industry: log or timber homes, decks,
rough-sawn siding, fences, and shingles. Without protection from
sun and moisture, wood "weathers" and deteriorates.
Finishes are used to enhance appearance and dimensional stability,
and to prevent deterioration.
Weathering of wood is a combination of chemical,
mechanical, biological, and sunlight-induced processes that change
the appearance and structure of wood. After two months of
exposure, all woods will turn yellowish or brownish, and then
gray. Dark woods will become lighter, while light woods eventually
darken. Surface checks, raised grain, cupping, and warping develop
as wood continues to weather.
Recent research conducted by the Forest Product
Laboratory indicates that failure to properly treat new lumber can
reduce the average life of wood by 20 percent.
Understanding the differences between finishes
makes it easier to select the right product. In the past, finishes
were made from alkyd or natural oil resins such as linseed, tung,
soya and paraffin. The resins were often blended with waxes to
provide additional water repellency, and then diluted with a
mineral spirits solvent.
Technological advances and environmental
regulations on emission level of volatile organic compounds (VOC's)
have spurred the development of new products. Water-based
products, particularly those formulated with certain
water-reducible synthetic oils and resins, have excellent
penetration and perform as well as, or better than, oil-based
The performance of commercially available wood
finishes is often listed on a product label or in literature
supplied by the manufacturer. The American Society for Testing and
Materials (ASTM) has standardized test methods to measure the
water repellency and color retention of wood finishes.
In ASTM test D5401-93, a finish is applied to a
2" by 4" section of wood, allowed to cure for seven days
under controlled conditions, and then tested for water absorbency.
Standard ASTM G53-88 evaluates the water repellency of coatings
exposed to ultraviolet light and condensation in a weather
exposure chamber for 1000 hours. Manufacturers also use outdoor
tests to measure weathering in various climates, and they might
provide test results if you request them.
Finishes are generally classified into two basic
categories: those that form a film or coating on wood and those
These products form a coating, or film, that is
a barrier between wood and the elements. Film-formers include many
alkyds, latex/acrylics, and varnish resins in solvent or
water-based finishes. Products without pigments are considered to
be a clear or transparent finish, and have little or not
protection from ultraviolet (UV) radiation. Pigments are added to
paints, solid color stains, and semi-transparent finishes to
change the appearance of the wood and to provide protection from
Some of the newer water-based coatings are
semi-transparent acrylic blends that have excellent flexibility.
Unfortunately, due to their higher molecular weight, acrylics
still form a film on the surface of wood, and are subject to the
cracking that is characteristic of all film-forming finishes. A
film finish cracks as wood expands and contracts during normal
moisture cycling and water gets underneath the finish and
deteriorates the wood. Removing film-forming wood finishes can be
difficult, but is often necessary before re-application. If the
failing coat is not removed, then the new coat may blister and
Penetrating wood finishes are oil or water-based
products that saturate wood pores to prevent water penetration.
They typically contain a drying oil or resin in a transparent or
semi-transparent stain. Advantages of penetrating finishes over
films are that they provide long-term water repellency, they do
not trap moisture in the wood, and they do not peel or blister.
Natural oils (linseed and tung, for example) are
initially very effective in stopping the absorption of water into
wood, but tend to darken over time because they are a food for
fungi. Buildings treated with natural oils and resins generally
need extensive cleaning before reapplying the finish.
Some of the newer water-based systems have
synthetic oils and resins and they provide excellent water
repellency and color retention. One of the main advantages of
synthetic resins is that unlike natural oils, they do not serve as
a food for most biological growth, making future coats easier to
APPLYING THE FINISH
Correct application is critical to performance.
Follow the manufacturers' instructions, particularly with the
newer water-based formulations. All finishes should be applied to
a clean surface, but penetrating finishes must be applied to
surfaces that are porous and free from previous coatings.
Although chlorine beach will effectively remove
many stains like mold and mildew, it can damage wood and is toxic
to people and plant life. Newer, chlorine-free cleaners are
environmentally safe and can actually increase product penetration
up to 25%. Wood that is pre-treated with a cleaner or pressure
washer will probably have some raised grain, but will also have a
better finish penetration.
Water-based finishes tend to dry faster than
oil-based products. To avoid lap marks, particularly on hot sunny
days, apply these only in the shade: the cooler surface will
absorb better and allow for easier application of a second coat.
Log structures can pose special application
problems. Moisture contents higher than 20% can cause a finish to
creak and peel as the log dries. Test logs with a moisture meter
before applying or re-applying, especially when using film-forming
or water-based products.
Another problem is that the up-facing curves of
logs are subjected to intense UV rays and moisture when when and
snow accumulate in cracks and crevices causing the finish to crack
and peel. Log homes at higher elevations are especially subject to
temperature extremes that cause wood to continually expand and
contract. This affects adhesion, water repellency and color
retention of finishes.
Exposed end grain at corners can encourage water
penetration. Make sure that end grain is adequately treated and
that large checks are sealed properly. Apply finish liberally to
the courses of logs near the foundation where moisture and dirt
are likely to be a problem.
Protecting logs from rain prolongs a finish and
greatly reduces maintenance. Wide roof overhangs, gutters and down
spouts, and good drainage around a foundation can help you avoid
many problems. The combination of a high performance finish and
good design and construction will help ensure that moisture does
not deteriorate logs.
Routine maintenance is necessary, but the
life-span of a finish depends on a variety of factors.
Construction details, exposure to the elements, product choice,
surface preparation and application techniques are all essential
to success. Some finishes may even require chemical stripping or
sandblasting to restore wood to the proper condition before
re-treatment. Finishes that weather unevenly and are re-coated
without removing the old finish will have an unsightly, patchy
Although the wood finish is only a small percent
of the cost of a log home, it is one of the more critical elements
in construction. To most consumers, aesthetic appeal is just as
important as performance when selecting a wood finish.
Understanding the properties and expected performance of various
products makes the decision process much easier for you.
Greg Herbert is a research chemist for
SaverSystems, a division of Meredith Inc., a manufacturer of
finishes and water repellents for concrete, masonry and wood.
SaverSystems is the manufacturer of DEFY and DEFY Epoxy Fortified,
water based penetrating finish.
You can find additional information, color
charts and estimating graphs, as well as on-line ordering at Log