Spring: Time to clean-up the yard, refresh the garden and perhaps build something new. Despite the warm breezes and sunny days, what we build for the outdoors will eventually face Arctic blasts, blistering heat, and torrential rain – all bent on peeling years away from your project’s life expectancy and good looks. To help save your outdoor projects from abuse, this edition features a quick guide to choosing the right wood, fasteners, adhesives, and finishes for your outdoor project.
PRESSURE-TREATED lumber is the builder’s choice for ground contact. In large dimensional lumber (6×6, 4×4, 2x, etc.) it is well suited for most outdoor tasks, but it has a tendency to crack, making detail work difficult. The chemical properties of the wood are also extremely corrosive to fasteners.
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 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.
Contrary to its typical usage, White Oak is an excellent domestic hardwood for exterior projects because it has closed cell structure, which inhibits moisture absorption. This is reason enough to make this classic building material the first choice of wineries, distilleries and breweries worldwide.
|Top Choices for Outdoor Wood|
|Mahogany||Medium Hard||Easy to work|
Dulls cutting tools, pre-drill
Wipe down with Naphtha prior to finishing
Dulls cutting tools, pre-drill
Wipe down with Naphtha prior to finishing
Works well with hand & power tools. Accepts a variety of finishes.
May dull cutting tools
|White Oak||Hard||Moderately dulls cutting tools. Iron-based fasteners will stain surface black|
|Western Red Cedar||Soft||Iron-based fasteners will stain surface black|
|Redwood||Soft||Use sharp cutting tool to minimize splintering. Pre-drill. Iron-based fasteners will stain surface black|
|Pressure Treated Pine||Soft||Requires corrosion-resistant fasteners. Pre-drill. Wash hands after working and wear dust mask when sawing|
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 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. Birdhouses to boats, 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.
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 pullout resistance.
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.
Of course, there’s one more choice for outdoor projects: simply omit the finish and let the wood take on a natural weathered patina.
|Choices for Finishes|
|Finish||Re – Application||Appearance||Notes|
|Polyurethane||NA||Clear||NOT RECOMMENDED. Hard coating action doesn’t work well with outdoor projects (frequent expansion and contraction with the temperature swings). Clear coating allows 100% UV Penetration.|
|Latex Paints||2-3 years||Obscures natural look.||NOT RECOMMENDED for furniture. Ordinary latex house paint is engineered to have an elastic film – which works good for siding, but it will feel rubbery on chairs and tables. It is not resistent to abrasion.|
|Urethane (Modified Acrylic Varnish)||2 Years||Clear||
Water-based finsish w/soap and water clean-up. Can be recoated in 1 hr.
Moderate UV filtering.
|SPAR Urethane||1 Year||Clear||
Marine-grade finish against abrasives and water. Scuff-sand worn surfaces before re-coats.
Moderate UV Filtering.
|Oil-based, semi-transparent, penetrating stain||3 to 7 years||Grain partially obscured.||Check capatibility if top-coating with clear finish. Moderate UV filtering.|
|Exterior-grade Paints||7 to 10 years||Woodtexture may show throuugh but natural look is hidden.||Greatest UV protection.|
|Pentrating Oils||1 to 2 years||Clear||Thin finish requires multiple coats. Virtually no UV protection.|
A little extra protection
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.
Information for this article is sourced by permission of Woodcraft.