How to Protect Outdoor Wood
Protect the look and performance of outdoor wood with a coating of preservative. No matter what type of wood you have, from pressure-treated pine to exotic hardwood, staining and sealing the surface is the best way to protect it from the elements.
Types of Wood Preservative
The most common outdoor wood coating is an exterior-grade penetrating stain/sealer. These water-repellent preservatives include a "mildewcide," and some products contain ultraviolet light absorbers that protect from sun exposure. Available in both oil- and water-based formulations, the resins penetrate wood pores to provide pigment and block out the damaging effects of weather exposure. Available in a range of semi-transparent color tones, these stains allow the natural woodgrain appearance and texture to shine through.
The second category is a film-forming sealant that bonds to the surface of the wood similar to paint or shellac. Pigments are added to change the wood color and add UV protection. These products can provide a solid paint-like coating or, when applied to sanded surfaces, a semi-transparent high-gloss furniture look. They’re available in oil- or water-based finishes but can only be maintained with another coat of the same film-forming sealant. If you want to switch to a penetrating stain, the old film-forming sealant will first have to be sanded off. Film-forming coatings are often avoided in areas prone to foot traffic.
Outdoor wood coatings are typically formulated with either water or oil. Most water-based stain/sealants have tiny particles of pigment and resin that tightly adhere to each as the finish dries, similar to a patchwork quilt. With oil-based finishes, the tiny particles actually fuse together chemically into one large sheet-like substance, which usually achieves a harder finish that is less likely to develop an amber color over time.
Water-based finishes are popular because they’re easier to use, and that advantage goes a long way for DIY homeowner. Compared to oil-based formulas, they’re much easier to clean up, have a lower odor, and are often less expensive. However, most water-based coatings require more coats and still don’t last as long as their oil-based or “alkyd” counterparts, which have a reputation of providing more long-term, wood-preserving durability.
Although using a sprayer can speed up application, back-brushing the coating is still critical. When you brush the stain, the brush creates a friction that breaks the surface tension of the wood and works the preservative into the pores for the best protection.
Allow all wood to dry before staining or sealing. This is particularly important for pressure-treated lumber because the waterborne preservative leaves moisture in the wood, and that moisture will inhibit absorption and retention of the stain/sealer you’ll apply.
For best performance of paint and stain coatings, allow treated lumber to dry for 2 to 4 weeks prior to application. Estimating exact drying time is difficult, and a lot depends on how much time has elapsed since the treatment, the wood’s exposure to the sun, ambient weather, etc.
Wood with natural preservatives, such as Western Red Cedar, cypress and redwood, do not require as much drying time because the wood was never pressure-treated with a preservative.
For best results, clean the surface of new wood to remove any “mill scale,” which is grain compression from the milling process that can cause the stain to run off without absorbing. Clean the surface with an oxygenated bleach to remove mill scale.
Prior to staining, add extra protection by sealing or fill any serious knots or imperfections in the wood. A porous black knot or crack in a board makes a prime place for water to accumulate and lead to rot. Patch these areas with a putty knife and non-shrinking exterior wood filler (or similar repair product). Hardware stores sell tinted wood filler to closely match your wood stain of choice.
When wood is exposed to the sunlight, the ultraviolet rays can damage the wood fibers over time, causing the surface to turn gray, and this holds true for any wood species. The most direct way to renew the appearance is to sand or pressure-wash the surface. (Never use steel wool.) However, sanding can be very difficult and time-consuming.
Pressure-washing can remove the gray but cause the surface to fuzz or splinter, posing a “touch” hazard for areas such as deck surfaces where people may walk barefoot. When using a power-washer, limit your pressure to no more than 1,000 or 1,200 PSI to reduce wood damage.
On older decks or fences, some individual boards may be heavily weathered. Replace them completely, clean the existing boards, then stain them all to match the rest of the project.
Be sure to mask off any area you want to protect from any cleaning chemicals or the wood stain. Plastic drop cloths work well.
Oxygen bleach makes a good outdoor wood cleaner that won’t harm the grass or plant life as chlorine bleach will do. Scrub the surface with a stiff-bristle brush to remove grime. Rinse it off thoroughly.
Wood with Old Stain
It’s usually a good idea to remove old stain before applying new stain, especially if changing products or colors. Old stain will usually show through the new stain, leaving blotchy spots in the finish. However, if you plan to stick with the same color and type of stain (and your deck is in decent condition) then you can probably get by with a thorough cleaning and a fresh maintenance coat. As always, refer to the manufacturer’s recommendations for best practices.
If there is a buildup of old stains on the deck, then you may need to remove it with something stronger than oxygen bleach. Stain strippers are more caustic, but they remove most weathered stains in a single application. Small, stubborn spots of stain can then be removed with a hand sander once the deck has dried.
An often-overlooked step for outdoor staining projects is the application of a wood brightener. These chemical agents open the woodgrain to improve penetration of the stain and help restore the appearance of weathered wood to a like-new condition. The product can simply be sprayed onto the wood surface, given a few minutes to work its “magic” and then rinsed off, requiring very little labor.
After using any chemical treatment to clean and prepare the wood, use plenty of water to completely remove all traces of the products—and then allow the surface to dry prior to stain or sealer application.
The wood surface should always be clean, dry, and free of mold or mildew.
Always thoroughly mix the stain to evenly blend the solids and ensure a consistent color tone throughout the project. Stir the container every 15 minutes with a paint stick as you work.
Avoid applying the stain/sealer in direct sunlight, which might dry the product on the surface before it has time to adequately penetrate the wood.
For a vertical structure like a fence or deck rail, begin application at the top of the project and work downward, which allows you to catch runs in the stain as you go.
Most stain/sealers can be applied with a brush, roller, stain-application pads, or an airless sprayer, but always consult the manufacturer’s instructions for details.
For both horizontal and vertical structures, brushing is the best method for application because the bristles push the stain/sealer evenly into the wood to increase absorption. Work the brush in the same direction as the woodgrain. Maintain a wet edge to avoid lap marks.
Using a sprayer or roller can apply the stain more quickly, but both of these methods leave much of the stain on the surface without adequate penetration into the pores. If the product doesn’t adhere well, it can wear away unevenly, so you should still back-brush it into the woodgrain for the most consistent appearance and best protection.
After the required coats of the new stain/sealer have dried, water should bead off the wood surface, indicating that the project has been sealed.
On a recent fence-staining project, the HIR team tried out the Deck Defense “one coat” finish from Perma-Chink Systems, and found it to be very easy to work with. Deck Defense is a totally waterborne product but has penetrating properties like oil finishes, and it hardens into a lattice-like coating that strengthens the natural wood fibers from the inside out. Formulated to protect against the effects of sun, wind, and water, Deck Defense is available in seven colors and made in the USA. Find out more at www.permachink.com.