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Whether you want to emulate the passage of time or guard against it, following are some helpful tips for exterior finishes.
Protection Against Weathering
You can't completely stop or reverse the weathering of exterior wood, but you can slow the process dramatically by using the right type of finish.
There are two basic types of finishes used to protect wood surfaces from weathering – those that form a film or coating on the wood and those that penetrate the wood surface and leave no distinct layer. Film-forming materials include paints and varnishes. Penetrating finishes include water repellents, preservatives and pigmented semitransparent stains.
When selecting an exterior finish, consider appearance, durability, cost, surface type, ease of application, and maintenance.
Paints
Of all finishes, paints provide the most protection for wood against ultraviolet degradation and simple erosion. A painted surface retards the penetration of exterior moisture, blocks out damaging ultraviolet rays, and seals into the wood the natural resins and other oils that can otherwise be weathered out.
It's generally accepted today that a good acrylic latex exterior house paint will perform as well as or even outlast a good oil-base house paint. The fact that latex paints are more porous and can breath slightly – while still shedding water – may contribute to their longevity. In addition, oil-base paints tend to become brittle while latex paints retain their flexibility.
In general, painting is best done on a fairly smooth and stable surface, such as low-density, vertical-grain and edge-grain woods that resist shrinking and swelling.
Varnishes
What makes clear varnish finishes popular is that they accent and enhance the grain and color of the wood surface. Unfortunately, all types of varnishes – oil-base, alkyd, urethane and acrylic – require frequent maintenance. Ultraviolet light from the sun eventually breaks down the varnish. To avoid frequent scraping and recoating, the best approach to using varnish for exterior surfaces is to provide reliable shade, such as recessed openings, roofs and overhangs.
Penetrating Finishes
Penetrating finishes are designed to be absorbed into the wood, saturating the surface fibers and partially or completely filling the surface pores. Many finishes contain water repellents and wood preservatives. The preservatives control the growth of mildew and other fungi, and some even discourage insect infestation.
When inorganic pigments are added to clear penetrating finishes, the mixture is classified as a semitransparent penetrating stain. The pigment in the stain greatly increases the durability of the finish by absorbing much of the ultraviolet light. However, it's the total mix of pigment, resin, preservative and water repellent that ultimately determines the durability of any stain system.
The protective qualities of penetrating stains are best used on rough-sawn, weathered or coarse-textured woods – the very type of surface that won't take paint well, including siding, trim, exposed decking, fences and roof shingles.
Creating a Weathered Look
Timeworn, aged, lived in, rustic, natural, nostalgic, antique. All descriptors conjured up by the look of weathered wood. If you prefer a rough, gray and weathered look for your exterior, like the kind provided by Mother Nature, it can be obtained in two ways.
You can do nothing until the wood turns gray, then treat it regularly with a water-repellent preservative or a gray semitransparent stain to extend the life of the wood.
If you want to obtain a weathered look immediately, apply a gray stain; as the stain weathers, the wood will turn gray naturally. Be sure, however, to continue to protect the finish with a stain or water-repellent preservative.