Woodworker's Journal 2004 Summer, страница 12

Exterior Finishes that Weather the Test of Time

Regardless of which outdoor lumber you use, the ravages of sunlight and moisture will eventually have their way in the end. Although this might be decades from now, the best way to extend the life of your patio and yard furniture is to protect it with a tough surface finish. The best finish to use depends on the particular project, your maintenance efforts over time and the amount it will be exposed to the elements.

Jerry TerHark, who lectures nationwide on wood finishing, says outdoor wood finishes face two major natural challenges. Sunlight is an ever-present enemy. For paint and varnish finishes, sunlight initially helps the surface film polymerize and dry when it combines with heat and air. However, polymerization continues even after the finish dries, and as the finish continues to harden it eventually becomes brittle. Sunlight also breaks down the line where the wood and finish meet, which loosens the adhesive bond resulting in cracking and peeling.

Where sunlight leaves off, moisture continues the destructive work. Water penetrates under the coating of finish — either through the end grain, which is harder to seal, or through cracks and openings left by ultraviolet sunlight. Once under the finish, water enters the wood

Adhesives that Hold Tough, Rain or Shine

Several varieties of wood glue are formulated to stand tough against water exposure, temperature extremes and degradation from sunlight. The most user-friendly options are yellow PVA wood glues intended for exterior projects. Titebond® II and the new Titebond® III varieties, among others, clean up with soap and water and have good bonding characteristics on all but the oiliest exterior lumbers.

If you're building with teak or other exotic woods with high oil content, polyurethane glue is a better choice. It cures by taking a small amount of moisture from the wood or air and combining with it to form a permanent bond. Most formulations foam up as they cure, so clamping is essential to keep glue joints from separating. However; Franklin International's HiPURformer poly glue dispenses from a heat gun similar to hot-melt glue without foaming or expanding.

Two-part epoxy, a third weatherproof adhesive, is also a good option for bonding oily woods. It doesn't expand as it cures and even fills gaps on misfitting joints.

12 Outdoor Projects Summer 2004 £

pores. Add oxygen and moderate temperature to the equation, and conditions are right for fungus and mildew to grow. Blue-black stains on the surface are sure signs that mold is setting in. Moisture not only initiates rot but further loosens the bond between the wood and finish, exposing more bare wood. It also attracts termites and other wood boring insects.

Finish Options

For an exterior finish to do its job, it needs to slow down the degradation of UV light, form a water-resistant seal and offer at least some degree of protection from nicks and scuffs. So, your choices of finish amount to three options: marine spar varnish, a penetrating wood preservative or a good coating of exterior primer and several coats of paint.

Marine spar varnish has made its way into woodworking from the boatbuilding industry. The best spar varnishes are combinations of tung oil and phenolic resin with a high oil-to-resin content. The more oil the varnish contains, the more flexible the surface film remains as it polymerizes. Your choice of a spar varnish should also

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