Wood vs. Wild An Outdoor project survival guideComments (0)
This article is from Issue 22 of Woodcraft Magazine.
None of us would ever dream of sending our children out into the snow without a proper coat, boots, and mittens. Yet some people callously shove their woodworking creations out the door to face Arctic blasts, blistering heat, and torrential rain. And while cruelty to wood won’t land you in prison, it will definitely peel years away from your project’s life expectancy and serve a death sentence to its good looks.
To help save your outdoor projects from abuse, we developed a condensed survival guide covering wood and manufactured panels, fasteners, adhesives, and finishes.
Tough-Enough Wood Products
Pressure-treated lumber involves injection of a chemical. Chromated Copper Arsenate (CCA), a previous choice, has been replaced by Alkaline Copper Quaternary (ACQ), with slightly different formulations tailored to various wood species, the most common being pines. The end of each board should have a stapled tag that identifies the treating company, chemical, year and whether the amount of chemical retained in the wood makes it suitable for ground contact or only above-ground use. Be sure to select fasteners that can stand up to ACQ’s corrosive properties.
EXTERIOR grade plywood is excellent for many outdoor projects. If plywood qualifies for exterior use, it will have the word “exterior” as part of the stamp that also identifies the face veneer grades. This indicates that exterior-rated adhesive bonds the plies. Pressure-treated plywood is another great choice for outside projects.
Tropical hardwoods such as teak and ipe have high density and natural decay resistance, two factors that recommend them for outdoor furniture. But these woods can be hard to find and usually have eye-popping price tags. Genuine mahogany (not lauan) is another decay-resistant wood that is lower in both density and price.
Domestic softwoods with natural decay resistance include western red cedar, cypress, and redwood. Shipping costs usually dictate which species is available in your area. All are lightweight and easy to work. However, realize that these species are particularly soft softwoods and therefore are susceptible to surface damage by a stray hammer blow or collision with brick, concrete, and other unforgiving surfaces.
Long-Lasting Exterior Adhesives
Modern adhesives routinely perform chores that would have been considered nearly miraculous a generation ago. In fact, Titebond III, which debuted just a few years ago, is the first one-part waterproof glue that offers water cleanup. It’s suitable for many outdoor projects but inexpensive enough that many woodworkers also use it for their indoor projects.
The chart below gives you fast facts on the key types of adhesives suitable for outdoor projects ranging from birdhouses to boats. In addition, you’ll find a wide range of products within the epoxy family to suit specialized applications. Epoxy putty, for example, is useful both an adhesive as well as a gap filler that can plug a knothole.
Some other useful outdoor bonding products include construction adhesives: PL and Liquid Nails are two well-known and widely-available brands.
Weather-Tough Exterior Finishes
With exterior coatings, the term “finish” is somewhat of a misnomer because even the most durable types require periodic renewal. So, you’re never really finished. The two most punishing elements your outdoor projects face are ultraviolet (UV) radiation and water.
Invisible UV rays beam right through clear finishes, causing a layer of wood cells (and the finish attached to them) to fall away in the same way that your skin reacts to a sunburn. As you’ll see in the chart below, pigment is your best ally in combating UV rays, with an opaque paint enjoying up to 20 times the longevity of a clear spar urethane. Be sure to use a quality primer under paint.
Water protection is important because it helps limit checking (cracking) of the surface. This condition is not merely unsightly, it also permits further water penetration, which then accelerates the destructive cycle. End grain is very susceptible to water damage, so you’ll want to keep the ends of legs and posts away from soil and wet surfaces. Seal them with a preservative or epoxy as an extra precaution. Plastic glides are a great choice for elevating furniture projects,
as shown in photo lower left. For larger-scale projects such as trellises, keep posts out of soil with anchors that you drive into the soil or attach to concrete piers. Both strategies keep vulnerable end grain dry.
Of course, there’s one more choice for outdoor projects: simply omit the finish and let the wood take on a natural weathered patina.
Hard-wearing plastic glides elevate outdoor furniture legs above water so it can’t wick into the end grain.
The chemical properties that make certain woods naturally rot-resistant can also attack fasteners, causing corrosion that can weaken joints and cause unsightly staining. The powerful chemicals employed for pressure-treating lumber can be even more reactive with fasteners, making your choices even more critical.
There are two broad categories of outdoor-rated fasteners. One type uses a corrosion-resistant material for the fastener itself, and the other relies on a protective plating or coating to shield a steel body.
Using a metal that resists corrosion, such as stainless steel or brass, offers more dependable performance than galvanizing and other surface treatments that can be easily damaged by abrasion or impact.
Stainless steel screws are generally suitable for outdoor projects, but not all alloys are truly stainless in every application. As a general guide, select the 316 stainless alloy for marine applications, but choose among the less-expensive 302, 304, or 305 screws for general exterior projects. Screws in the 200 series use alloys that cut back on the expensive metals, saving money but compromising corrosion resistance. To be on the safe side, choose screws that are clearly identified as compatible with your project’s building material. Stainless steel nails are available but can be tough to find.
Silicon bronze screws have corrosion resistance that is tough enough for boat-building chores: the 651 alloy, for example. As with stainless steel, there are various alloys, so it pays to read carefully and ask questions before you buy.
Aluminum screws as well as those made from solid brass have excellent corrosion resistance but both of these materials are quite soft, so drilling pilot holes is an absolute necessity. You’ll also need to carefully monitor your torque when driving to avoid snapping the screw in two.
Zinc-plated screws as well as coated screws offer good exterior performance at a budget-conscious price. Kreg Blue-Coat Screws claim a rust-resistant performance that’s 400% better than their former zinc-coated screws. However, these screws are not recommended for use with ACQ-treated lumber.
Epoxy and ceramic coatings that resist corrosion are typically used for deck screws, and you’ll often find a range of colors to help them blend into their surroundings. Read the box before you buy, especially if you’re working with ACQ lumber.
Galvanized nails, bolts, and lag screws utilize a zinc coating to resist corrosion. Electro-galvanizing produces a smooth plated surface for air-driven nails as well as the manually-driven version. Hot-dip galvanizing produces a thicker but rougher surface coating on hand-driven nails. Deck and siding nails with a twisted or ringed shank offer excellent pull-out resistance.
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