For several years, the economy has been changing our daily habits and spending patterns. One effect is the increased attention on the comfort, appearance and entertainment value of the home. This involves the outdoor environment, including decks, pergolas, gazebos, cooking and dining areas, Adirondack chairs, Kennedy rockers and other furniture often of fine design, construction and well-figured woods.
Left uncoated, wood absorbs water, expands and dries out and shrinks. The grain raises, the wood becomes rougher and often fuzzy or splintered. Over time, these stresses lead to checking, cracking and splitting. Unfinished wood is also a nutrient source for fungi (molds and mildew), algal or lichen growth that discolor and degrade it. Ultraviolet radiation, a component of sunlight, can cause color changes and lead to finish failure. Everyday use and abuse are also a nemesis.
The best defense is an appropriate finish. Here are a few options:
A film finish extends above the surface of the wood, while penetrating finish soaks into the wood. Film finishes can be paint (colored coating containing the film-forming resin, pigments and solvent, at a minimum) or a clear finish (film-forming resin and solvent, at a minimum).
Paint provides the most protection as the pigments block UV light and the thickness of the film slows down the moisture exchange as well as providing protection against abrasion, impact and microorganisms. It also hides the color, figure and depth of the wood.
A clear finish allows us to see the characteristics of the wood, even to accentuate those desirable features such as figure, color and depth with stains and other products. The film should be designed with enough flexibility to move with the wood without checking and enough hardness to resist abrasion and impact.
Clear film finishes for outdoor use should have UV blockers or inhibitors in the formulation to minimize degradation of the wood, although they will not stop color changes due to reaction of the wood extractives (that give color to the heartwood) to visible light. Pigmented, transparent or semitransparent varnishes offer better service at the cost of adding some color to the wood by the agency of nanopigments in particle sizes similar to the wavelength of UV radiation (300-400 nm) that partially block the UV and blue range of the spectrum while allowing the rest of the visible light to pass through.
UV radiation can also degrade the finish resins constituting the film former, resulting in an erosion of both the film and the pigments within. Don’t use dye products on exterior wood, as even with UV inhibitors and blockers the dyes will eventually fade — usually badly and fast.
Drawbacks to clear film finishes for exterior use, especially on horizontal surfaces, are that they will eventually discolor, check, crack, flake and peel. Periodic maintenance, such as cleaning, sanding, recoating and application of waxes or maintenance oils can extend the service life of the finish. Depending on the wood, the item, finish preparation, the particular finish used and the specific environment in which the item is used, the service life could be from a few months to three years or so.
With a maintenance schedule that includes cleaning, sanding for tooth and recoating before serious degrade has occurred, the service life could be extended indefinitely. Of course, follow manufacturer recommendations for applying the coating, but generally the wood should be clean, dry and sanded shortly before finishing. For most woods, 150 to 220 grit is sufficient. Finer grits might decrease adhesion of the coating.
Penetrating treatments can consist of single-purpose liquids to provide water resistance or waterproofing and resistance to insects, decay and growth of fungus, algae and lichens, UV blocking or conversion. Or they can be a multipurpose treatment to provide any combination of those properties.
Most penetrating protective coatings used on outdoor furniture, fixtures and architectural features is a mixture of drying or semidrying oil with a solvent to lower viscosity and help the oil to penetrate into the wood and possibly a resin to increase the hardness and durability of the mixture. Chemicals and semitransparent pigments to block UV light might be added as well as biocides to prevent growth of molds, mildew, algae and lichens.
Penetrating oils without resin added have little to no build on the surface. When applied, care must be taken to ensure good penetration of the oil into the wood. Any excess oil must be wiped off before it dries. Failure to do so will leave a gummy surface in many cases.
Many of the penetrating oil finishes can be used as a treatment under a paint or varnish, extending the service life of the final coating. A multipurpose oil finish can be easier to maintain than a film finish because more coating might be applied without extensive sanding. A light sanding, followed by an oxalic acid or a deck cleaner (that might contain one or more of these chemicals: phosphoric acid, alcohol, polyethylene glycol, sodium percarbonate, trisodium phosphate and sodium hypochlorite) and reapplying the oil might be all that is needed.
Because the oil soaks into the wood and because finishes could have varying amounts (or none) of the resin, the sheen might be quite flat to somewhat satiny, but generally not glossy.
Making the choice
As the finisher, you will have to educate the client on the advantages and disadvantages of each type of finish. And there is no one right answer. The desired look, ability and willingness of the customer to provide the necessary maintenance, along with the right environmental conditions, all play a factor.
The Department of Agriculture’s Forest Products Laboratory has done extensive testing on finishing products for outdoor furniture use and test results are available online (www.fpl.fs.fed.us). Some general conclusions from the testing include:
• The amount of finish material applied doesn’t seem to matter as much as the amount of solids and pigment remaining in or on the wood after the solvents evaporate. The type and amount of pigment or other UV stabilizer had a greater influence on performance than water repellency. Solvent-based products performed better than water-based products.
• With both film finishes and penetrating oils, the finisher must take care to make sure that all wood that might be exposed to moisture or liquid water — including joints, screw and nail holes, end grain of feet or legs and any cuts, scrapes, splits or other sites of moisture entry — are fully sealed at the application of the original finish and any future refinishing. Unlike interior wood furniture, which does require waterproof or resistant joints, exterior furniture might fall apart after a few seasons of the excess movement of an unsealed joint.
• Both film and penetrating finishes last longer on wood that is not sanded too finely. Saw-textured wood (as opposed to smooth-planed wood) will absorb the penetrating oil and hold a film finish better than smooth-planed wood or wood sanded too finely.
• The average moisture content of the wood at the time of finishing should approximate that of the moisture content the furniture will maintain through its service life — about 12 percent for most of the U.S. and Canada — to minimize stress.
Don’t be afraid of putting a clear finish on exterior wood. Just be clear about your expectations and experiment and take advantage of your finishing supplier’s expertise.
This article originally appeared in the August 2012 issue.