How to Treat Wood for Outdoor Use

Using wood for outdoor projects has been done for millennia. Wood left exposed to the elements will weaken and deteriorate quicker than if it is sealed or treated. If you’re wondering how to treat wood for outdoor use, we’re here to help.

Use a clear sealer, preservative, tung or linseed oil, marine varnish, or polyurethane to preserve the look and character of wood. They will protect the wood from moisture, insects, and other rot-causing agents. The more opaque the treatment, the greater the protection from UV rays, so in high exposure areas or for pine or plywood, use stains and paints.

In this article, we’ll discuss the use of untreated wood outdoors, the benefits of sealing wood, and how to treat wood for outdoor use. We’ll explore how long untreated wood will last outdoors, and identify the best way to protect wood for outdoor use. Our goal is to provide you with the best information for your next project.

Can You Use Untreated Wood Outside?

Untreated wood has been used for outdoor projects for longer than recorded history. Before pressure treated lumber came into use in the 1940s, the only choice was untreated wood. Untreated wood will gray and weather, though, and dry out or rot depending on the environment.

Consider the number of ghost towns throughout North America in various stages of decay. Most contain untreated wood exposed for a century or longer, and some buildings are in surprisingly good condition.

There are even old wooden barns that have been in continuous use for centuries too. However, they’ve probably been repaired and maintained and had a protective coating applied periodically.

Benefits of Sealing Wood for Outdoors

Sealing wood for use outdoors has been an ongoing challenge for thousands of years, and continues for the same reasons today. Applying a protective coating can have many benefits. Here are some reasons why wood should be sealed or treated for outdoor use.

Protection Against Moisture and UV Rays

Sealing wood helps protect it from moisture and UV rays by filling voids between fibers and within cells, reducing the absorption of moisture. Adding color or tint to the sealant also helps reflect UV rays.

Both UV rays and moisture cause wood to gray or fade, crack, cup, split, and warp. They cause cells to expand and contract, thus damaging their bond and altering the wood’s texture, resulting in wood deterioration.

Prevents Rotting

Wood that is in contact with soil or other surfaces, covered with dirt, leaves, and debris, or doesn’t receive much sunshine, often stays damp or wet. The moisture in the wood can lead to the growth of mold, mildew, fungi, and moss, trapping even more water.

The trapped moisture will cause wood to rot. Sealing outdoor wood and keeping it clean will reduce moisture buildup and allow the wood to dry, reducing damage and rot.

Protection Against Insect Damage

Sealing wood helps close pores and openings and keep wood dry, thus reducing or preventing insect damage. Insects tend to prefer wet or damp wood but will also bore into and create tunnels throughout dry wood.

Damage caused by insects can affect the structural integrity of wood components and cause them to fail. Plus, insect damage can decrease property value.

Increased Lifespan

Sealing outdoor wood will help extend its lifespan by 20% or more and keep it looking good. Sealers, stains, and paint can reduce moisture, insect, and UV damage, and slow the wood’s natural deterioration and rot.

Protecting and cleaning the wood on a regular maintenance schedule increases lifespan and decreases the replacement cycle and expense.

Reduce Splitting

Wood planks or boards are commonly cut so the grain runs lengthwise, not from side to side. As a result, splitting tends to originate at the ends of the boards.

Applying a sealer to the board ends helps reduce the potential for the wood to swell and crack or split by keeping moisture out of the end grain. Sealing the back, front, and sides will decrease moisture absorption which can cause the wood to swell, cup, check, or warp.

Easier Maintenance

Sealing wood makes maintenance easier. Sealing decreases splitting, checking, cupping, UV damage, and wood grain lifting or fuzziness. It also inhibits the growth of mold, mildew, moss, and rot.

Sealing not only helps the wood last longer, but it keeps the wood looking good. The result is that the wood is smoother and easier to maintain, plus, applying sealer is easier than replacement.

How to Treat Untreated Wood for Outdoor Use

Treating untreated wood for outdoor use will protect the wood from the elements and make it last longer. Here are some ways to protect raw wood depending on location, purpose, and use.

Wood Sealer / Wood Preserver

Wood sealers and preservers penetrate the pores of untreated wood and seal or close them so the inner wood stays dry. Both protect the wood from mold, mildew, woodworms, algae and rot.

They are available in both water-based and oil-based formats, with the latter being the more durable. Preservers and sealers are often clear and preserve the natural look of the wood and protect it from moisture but not so much from UV rays.

Sealers can, however, be tinted or combined with stain to create both a moisture and UV barrier. The amount of color added can highlight wood grain and characteristics or mask imperfections and graying of older wood.

Sealers or preservatives can be clear, semi-transparent, or opaque. They can be applied by brush, roller, paint pad, sponge, or sprayer, making them an easy solution for treating untreated wood.

Paint

Exterior paint is an excellent way to protect untreated wood from moisture, UV rays, insects, and rot. It also helps prevent warping, splitting, and checking. Available in thousands of colors in both oil and water-based formats, with oil being the more durable choice.

Paint not only protects the wood but can complement any color palette. It is also longer lasting and more durable than many other options.

Paint is ideal for outdoor furniture, fences, structures, and even decks. However, paint hides the natural beauty, color, and look of wood, so for some, it isn’t an option. Paint can be applied by brush, roller, sprayer, and paint pad, making it an easy DIY or professional choice.

Marine Varnish

Marine or spar varnish is similar to urethane or polyurethane-based sealers. It doesn’t penetrate the wood so much as coats it with a durable, impermeable, clear, protective waterproof coating.

Varnish is usually oil-based and is applied in layers, resulting in a smooth hard finish. Marine varnish can be used with stains or tinted for extra protection from UV rays too.

Marine varnish is ideal for boats, paddles, oars, masts, furniture, decking, fences, and other surfaces. Commonly applied by brush or sprayer, it is available in glossy, semi-gloss or satin, and matte finishes. It shows the wood grain and characteristics but leaves a smooth finish, not a wood-textured one.

Oil-Based Stains

Exterior oil-based wood stains soak into the wood to protect the core wood from moisture, rot, and insects. Oil-based stains are more durable, longer lasting, and moisture resistant than water-based stains. They won’t peel or crack like paint, and allow the grain and texture through. Stains are available in transparent, tinted, or semi-transparent, and in opaque formulas.

Oil-based stains can be applied with a brush, sprayer, or roller to decks, fences, outdoor furniture, and other surfaces. The main drawback of oil stains is that they usually produce VOCs that can be harmful, however, they tend to dissipate quickly in outdoor environments. Oil-based stains are easy to maintain but can develop mildew if applied too thickly or used in damp environments.

Pine Tar

Pine tar is a natural wood preservative created by slowly burning resin collected from the rootstock of pine and other pitch-producing trees. The tar penetrates into the wood, protecting it from UV rays, moisture, mildew, mold, fungi, and algae.

Pine tar has been used for millennia to protect ship hulls, decking, and rigging, as well as decks, fences, cabins, barns, and other structures from the elements.

Pine tar needs to be warmed or thinned with linseed or other natural oils in a 1:1 ratio to make it more spreadable. Apply two or more coats of pine tar with a stiff bristle paintbrush. It will absorb and dry faster at warm temperatures but can take from 1 to 14 days or more to cure.

Tung Oil or Linseed Oil

Tung and linseed oils are natural oils used to protect untreated wood from weathering, moisture, and rot. The oils absorb into the wood grain and tend to highlight its natural color, grain, and texture. The oils are commonly used on outdoor wood furniture, fences, and decks made of more exotic soft and hardwoods.

Linseed and tung oil are easy to apply with a brush, pad, or cloth. They often need to be rubbed into the wood and buffed. They can take a day or weeks to cure. The natural oils require more maintenance and reapplication than other methods of protection.

Foundation Tar

Foundation tar or sealer is a thick, non-fibrate bituminous compound often made from coal tar. It is commonly used to damp-proof concrete or cement block foundation walls.

The tar forms a barrier-like membrane that helps keep moisture and insects from penetrating. It is also often used to coat the underground portion of fence and deck posts to prevent moisture and insect damage and rot.

Thinning foundation tar produces a creosote wood treatment that has also been used for centuries. It is used to coat railway ties, exposed wooden bridge supports, piers, deck supports, exposed exterior wooden walls, and much more.

Creosote can extend the lifespan of wood by between 40 and 75 years, or longer. The tar is usually applied with a heavy-duty sprayer, roller, or 3-knot brush. A 5-gallon pail will cover approximately 100 sqft.

How to Seal Wood for Outdoor Use

 

Sealing untreated wood for outdoor use isn’t difficult, it just takes time and effort, and, depending on who does the work, money. The type of product may require using specific manufacturer’s instructions, but the following step-by-step instructions should work for most products.

Step #1: Surface Preparation

New wood should be sanded smooth if so desired and cleaned of any construction dirt and sawdust. Make sure there are no boot prints or other marks as they are difficult to remove once the wood is sealed. Pay attention to any holes, cracks, or knots and patch, fill, or address them as required.

Depending on location, the back of the boards and any cut ends or notches may need to be treated prior to installation. If the wood has been sealed in the past, it may require some washing, scraping, sanding, and/or stripping to prepare the surface for a new coat of sealer.

Step #2: Choose an Appropriate Sealing Product

Depending on the wood species, its purpose, and location, plus the desired finish or look, the product selection may be more limited. Additionally, climate and exposure should also factor into the selection process.

Extreme temperatures, UV rays, and wet or dry locations can necessitate certain sealers over others. Clear sealers will highlight the wood grain and color of exotic woods like cedar or Ipe used for decking, furniture, or focal points. Whereas pine and plywood may look better with a stain or paint finish.

Some situations may even require a primer or a combination sealer with stain. If the wood is used for utilitarian purposes such as walls, posts, or supports, a longer-lasting pine tar or creosote may be more appropriate.

Step #3: Tools

The type of sealer and the location of the wood to be sealed can affect the tools required for the task. Protective drop sheets to catch drips and spills, tarps to protect plants and sidewalks, and paper or plastic to tape to windows or trim are often forgotten but usually necessary items. As are rags, paper towels, or towels, to wipe up or off excess. A ladder or step stool may be needed or even scaffolding.

Select appropriate application tools too – brush, sponge, paint pad, roller and tray, sprayer, and extension pole. Don’t forget protective clothing. Some manufacturers recommend masks or respirators due to off-gassing, safety glasses, and a hat if working overhead.

Rubber gloves help with grip and make cleanup easier, as do disposable coveralls. Also, remember solvents for cleanup and containers for proper disposal too.

Step #4: Application

Once the previous steps are complete, you’re ready to apply your chosen sealer – take a before picture. Depending on the magnitude of the task and location, start high and work your way down going from one side to the other.

If there are trim, windows, rafters, or other obstructions, consider edging first and then working on larger areas. Keep an eye out for drips or globs, as well as overlapping lines. For decks, it’s common to coat two or three planks from end to end to avoid overlapping lines.

Work to apply thin even coats, and remember, don’t paint yourself into a corner. Let the first coat dry as per the manufacturer’s instructions before applying a second or third coat.

How Long Will Untreated Wood Last Outside?

Untreated wood has been used for millennia and artifacts and even structural remains thousands of years old have been found in varying states of decay. The remnants of the oldest wooden structure are located at Kalambo Falls in Zambia and are estimated to be at least 476,000 years old.

Much depends on the type of wood, exposure to the elements, moisture, proximity to the ground, and UV rays. The climate and how the wood was treated and used, along with wood density and natural preservatives inherent in certain species also impact longevity.

Untreated wood will begin to change color within months of exposure to the elements and begin to show signs of decay within a decade. Wood fastened into structures tends to hold their shape and purpose longer than random pieces left on or buried in the ground.

The blistering heat or frigid dryness of some regions sucks the moisture out and helps preserve the wood. Insects and moisture can accelerate rot and decay.

Untreated wood that is periodically maintained and sealed or painted will last decades or longer. The numerous ghost towns and abandoned structures attest that untreated wood can last centuries without care.

Best Way to Protect Wood Outside

The best way to protect outside wood often depends on wood species, where and how the wood will be used, as well as the environmental conditions it will be exposed to. Wood used near or in contact with the ground should be sealed or protected with pine or foundation tar, or other similar products.

Support structures should be sealed with wood preservatives like creosote, oil-based stains or sealers, or other long-lasting preservatives. You don’t want to crawl under decks or buildings yearly to reapply protective coatings.

Pine is a common species used for wood siding or plywood and should be painted or stained depending on aesthetics and use. Cedar, Ipe, oak, and other hardwoods used for decking, walls, trim, furniture, and decorative pieces should be clear sealed with sealers, preservatives, tung or linseed oil, or marine varnish.

Opaque sealers like paint will protect exposed wood longer and better from UV rays than transparent or semi-transparent coatings. Wood used in high traffic or wear areas like decks, stairs, railings, and furniture often requires more frequent maintenance too.

Hopefully, we’ve provided you with some helpful information on how to treat wood for outdoor use.