Clear Exterior Finishes
Protecting the beauty and warmth of exterior natural wood surfaces

One of nature's treats is the appearance, color and beauty of "bare" wood used in exterior applications.  Many people prefer the appearance of natural wood used on houses in the form of rough sawn siding, clear decking, on porches and patios, park benches, fences and countless other applications.  They also prefer protective coatings that do not mask this beauty.  Transparent stains are one way of protecting the "natural appearance".  Clear finishes are another way of protecting wood without masking the natural beauty.

Unfortunately, the coatings industry has been unable to provide clear coatings that perform as well as the average consumer expects.  An average exterior "paint job" lasts seven to ten years.  An average "penetrating clear" vertical (e.g. siding/fences) application with a quality product will last three to five years.  The average horizontal (e.g. flat deck) application of a penetrating clear will last about one year.

What causes the short life span of exterior clears?
Sunlight and rain cause all exterior coating to degrade.  The ultraviolet rays in sunlight cause destruction of wood and particularly the coatings applied to wood.  However, moisture is the major cause of wood degradation.  As a direct point of reference, think of what happens to a newspaper left out in the weather.  Moisture and sunlight cause the paper to yellow and decompose.  Newspapers are made from wood (more precisely from the cellulose in wood); therefore, the same degradation takes place with "natural" wood . . . just slower.  Another way to describe the effects of sunlight is in terms of sun screen or sun blocking lotion.  These lotions will shield or block the sun's skin burning rays until they wash or wear off.  Then, the skin will darken and crack due to lost moisture in the outer layers.
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No. 2


All wood shrinks as it dries, but parts of the wood that run parallel to the growth rings shrink about twice as much as portions running perpendicular to the rings.  Black areas around the cuts above indicate where shrinkage occurs.

 How does the wood decompose?
Micro-checks develop through pits concentrated in the area between the spring and summer growth and raising begins to occur.  As the cracks grow in size, face-checking, warping and cupping often result.  Cell wall bonds near the wood surface lose strength, and as the sun and water continue to impact the softened surface, it becomes increasingly uneven and slowly erodes.  The process becomes visible to the naked eye when the wood begins to warp and/or when it begins to turn gray.

See the three drawings depicting:
(1)  a close-up of wood cells and fibers.
(2)  the different cuts of lumber and their respective warping tendencies, and
(3)  the time-line for a typical softwood during the weathering process.
How can the decomposition be minimized?
Use of a quality pigmented or clear coating will control decomposition, until the coating, itself, has weathered away.  Pigmented "clears" and pigmented stains will last longer than clear coatings.  The added pigments absorb and/or block the damaging U.V. rays from attacking the resin, as well as the underlying surface.  The more pigment, the better the performance.

What can be done to extend the life of penetrating clear coatings?
As is true in most things in life, the higher the quality, the higher the price and the longer the life.  The lower cost, most frequently advertised, water sealer products may last only a year or two on vertical surfaces.  Whereas, the higher quality penetrating clear products will last three to five years.  The reasons for these differences are: 1) the choice of resin or binder, the amount of resin used and incorporating ultraviolet absorbers in the formulation - all of which help extend the life of the clear.  The better the resin, the longer the life of the clear.  The more resin (or higher solids) results in a thicker film which takes longer to weather.  UV absorbers sacrifice themselves until they are gone, then the resin begins to be affected.  Adding anti-mildew agents also helps slow the degradation process.

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Artist's rendition of color and surface wood changes for a typical softwood during ourdoor weathering process.  Photo courtesy U.S. Dept. of Agriculture's Forest Products Laboratory, Madison, WI.

How important is "Water Repellency" in extending the life of clear coatings?
Anything that can be done to keep water from penetrating the clear will help to extend the life of the clear . . . just a little bit longer.  However, much of the advertised effect of water sealers is overplayed.  "Surface active" waxes are incorporated into clear formulations, but weather off in the first year or so.  Thus, the water beading feature is truly short-lived.  Don't confuse the "water sealers" with quality "water repellent" coatings and "wood preservatives". 
How long should wood be allowed to weather before it is coated?
The decomposition process starts as soon as the wood is installed.  If the surface is to be primed or painted with a pigmented coating, the wood should be coated as soon as possible after installation.  If a penetrating clear coating is going to be applied, it's better to let the wood "season" one winter.  The wood needs a chance to release unstable sap, sugars and other extractive materials.  These "residues" take up valuable space that is better filled by a good clear finish.
What is the biggest mistake people make when using penetrating clear finishes?
They fail to clean the wood prior to applying any clear finish or penetrating stain.

If the wood is gray and weathered, how should it be prepared for painting?
There are many products on the market from ARMOR ALL, The Flood Company, OKON, Masterchem and Roman Adhesives that are great cleaners for weathered wood.  Properly applied, they will clean mildew, algae, dirt and stains with minimal scrubbing.  The bare wood is restored to near natural color.  Thoroughly rinse the surface with water and allow it to dry before finishing with a clear coating.

What is the best method of applying penetrating clear coatings or stains?
Brushing is best.  Most coatings can also be sprayed or rolled, including penetrating products.  However, spray and roller application techniques don't really "work the products into the wood".

What clear product is best for stained garage doors and wood park benches?
Spar Varnish - without question.  The film build and pliable nature of the coating make spar varnishes ideal for exterior stained doors.  The name spar is derived from the central mast or spar on sailing ships, as the product was originally used in marine applications.  It is still used today on the "Teakwood" on pleasure boats.  Spar varnishes can also be used on wood trim, fences and patio furniture.  It is the longest lasting clear, when applied in multiple coats.  The one draw-back is it is not intended for use on flat horizontal surfaces such as patios or decks.  ALLPRO offers a private label product called SPAR PRO®.  Similar products are available from most national paint companies.