The Best Exterior Wood FinishesPaul S
Inside our homes we’re surrounded by wood furniture, cabinets, millwork, and other wood items with beautiful finishes… and without much thought we simply expect them to last a lifetime.
The finished wood items we have outside aren’t so lucky. They’re exposed to the extremes of solar radiation from the sun… moisture in the forms of dew, rain, and snow…. high temperatures… freezing cold… fungal attacks… and in some cases foot traffic. Good exterior finishes protect the wood from these harsh conditions, but will certainly fail unless they are renewed on a scheduled basis. Maintenance is a necessity for outdoor wood finishes.
Choosing an Exterior Wood Finish
There are a variety of exterior wood finishes with different characteristics and properties. To choose the best one, you have to match the product with the project and make some decisions as well. These are the key factors and considerations to select the best finish;
- Function – which finish is the best choice for the project you are working on? For example, the finish you use on your deck is not necessarily the best choice for your new solid mahogany entrance door.
- Life Cycle/Maintenance – some finishes last longer than others, but none last forever. How often are you willing to clean, scrape, and/or sand and recoat the finish (i.e., weeks, months, or years) and how easy do you want the maintenance and repair process to be?
- Appearance – should the finish be clear and bring out the beauty and depth of the wood, lightly colored and semi-transparent, opaque like paint, glossy (shiny), matte (dull), or look “natural” – nearly invisible so that it’s not obvious the wood has a finish?
- Application – given a choice, should the finish be relatively easy to apply or are you ready for a product that requires more work and advanced skills?
- Cost – how important is the price tag?
Unfortunately, no finish scores well in all categories – you have to choose a product that fits you and your project the best. There’s give and take in the selection process – for example, the finishes that are easiest to apply and maintain don’t last as long as others and the ones that last longest are more work and more expensive. The one thing they all have in common is that they have to be recoated every so often to maintain their protective qualities.
Is a Finish Really Necessary?
If you like the look of silvery grey weathered wood, you may be considering leaving your project bare and avoiding the time and expense associated with applying and maintaining a finish. If the wood is naturally resistant to decay and the climate is just right, there’s a chance leaving it bare will develop the weathered look in time. There’s a better chance the wood will get dirty, grow mildew, and turn black and green.
Weathering and Decay
In the outdoors, bare wood is destroyed by the forces of weathering and decay. Weathering alone is a slow, deliberate process. Exposure to sunlight and water erodes the surface of the wood. As it erodes, the grain raises and checks and cracks develop causing the surface to become rough. The cracks expand and become larger as the boards cup, twist and warp – pulling or eroding away from fasteners. The roughened surface will change color and collect dirt, especially on the horizontal surfaces. This is a slow process and produces results as shown in the photo.
Decay is caused by fungus and breaks down the wood much faster than weathering. Mildew is an airborne fungus that lives on organic materials like pollen, dirt, and wood. In most of the U.S. the climate has the right blend of warmth and dampness that allows mildew to thrive. If the wood stays damp, it will attract and host other fungi and develop rot. In climates that are predominantly cold and dry, decay is much less common or non-existent.
Decay Resistant Wood Species
Finished or bare, the best wood for outdoor projects is the heartwood from a species that naturally withstands decay. Some woods that fit the description are accoya, catalpa, cedar (Spanish, western red, eastern white, or Alaskan yellow), chestnut, cypress (old growth is best), ipe, juniper, locust (black), mahogany (Honduras or African), mesquite, mulberry, oak (bur, white), redwood (old growth is best), sassafras, teak (old growth is best), walnut, yew, and pressure treated lumber.
Combined with a properly maintained exterior wood finish, these species will look great and last a long time outdoors. All exterior wood finishes fall into two general categories – penetrating finishes and film forming finishes. Let’s explore their characteristics and properties.
- Do not blister and peel
- Do not have to be scraped or sanded – they wear away
- Let the wood breathe and dry out
- Easiest to apply and recoat
- Most natural looking
- Offer little protection from dirt and wear
- Need maintenance more often than other products. Penetrating finishes typically last three months to a year on horizontal surfaces and twice as long on vertical surfaces.
- Do not bring out the depth and beauty of the wood
Penetrating finishes are made to soak into the wood surface and seal it from water. They do not offer any protection against wear and only a little protection from the sun, if any. However, penetrating finishes are the easiest to apply and maintain and come in an assortment of formulations that includes water repellents (WRs), water repellent preservatives (WRPs), colored WRPs, teak oils and tung oils, and semi-transparent stains. Manufacturers seem to be blurring the lines between these finishes which can make it difficult to determine what exactly is in the can. A general rule of thumb is the more natural looking the finish, the less protection it offers and more often it will need to be renewed.
Water Repellents and Water Repellent Preservatives
Water repellents (WRs) (note – not water-proof) and water repellent preservatives (WRPs) leave the wood with a natural look (it may not be apparent the wood has a finish – especially a few weeks after it’s applied). They are clear/transparent and help to reduce warping and cracking by limiting water absorption. The traditional ingredients for WR finishes are a solvent, paraffin wax, and a drying oil or varnish resin. The solvent helps the wax and resin soak into the top layer of the wood before it evaporates. Adding a mildewcide and/or wood preservative to the mixture makes it a WRP and protects the wood from fungus. Some WRPs use a paraffin oil as the solvent which also serves as the preservative. The non-drying oil makes the surface of the wood oily for a time. Some of the newer WRPs on the market include a small amount of pigment that adds a little color and extra protection.
To help reduce the amount of volatile organic compounds (VOCs) released into the atmosphere, some of the newer finish formulations are based on waterborne technology. If the directions say to thin or clean up with water, it’s a waterborne finish. A drawback of waterborne finishes is that they don’t soak into the wood as well as the solvent based finishes which causes them to form a very thin film on the surface.
Some WRPs can be top coated with paint (check the label) and help to preserve the wood better than the paint alone. It’s a good choice on horizontal surfaces and the first few vertical feet (e.g., painted window frames, door frames, and doors).
Water repellent preservatives (WRPs) are the finish of choice for wood decks. They are brushed on wet and given time to soak into the top layer of the wood before the excess is wiped off. The end grain of the wood will soak up more than the flat grain which has the benefit of protecting it longer.
Teak Oil, Tung Oil and Tung Oil Finishes
This category of exterior wood finishes brings out the color of the wood gives it a natural look for a short time (before it starts to weather and turn grey). They are popular because they are easy to apply and refresh (though the wood preparation will take some effort if you wait too long between maintenance cycles). The higher quality products need to be refreshed every 3-6 months depending on the climate and exposure conditions. The lower quality products will need to be refreshed more often. In cases where the finished items are under cover from the elements, the finish can be expected to last longer than it would with direct exposure.
Teak oil does not come from teak trees – it’s simply a name manufacturer’s use for a type of exterior finish they make. Because teak wood is decay resistant, it’s a popular choice for outdoor furniture and boat decks and trim. As a result of the woods’ popularity, finish manufacturers developed various products for the market and named them Teak Oil. Similar products include Danish Oil, Antique Oil, and Velvet Oil. Like the water repellents, some Teak Oil finishes contain a small quantity of pigments to help them last a little longer.
Tung oil (and linseed oil) is a vegetable oil that absorbs oxygen and cross-links to form polymers. Because it converts to a [rubbery] solid when exposed to the air, Tung oil is classified as a drying oil and is used as stand alone penetrating finish indoors or as an ingredient to manufacture oil-base varnishes and oil-varnish blends. When cooked with resins to create oil-base varnishes, drying oils are completely transformed and the end product is far more durable.
By itself Tung oil provides very little durability and protection from the sun, water, or wear and when used outdoors it turns a milky color and becomes food for mildew. Because Tung oil is more water repellent than linseed oil, it’s the better choice when formulating exterior varnish. Depending on the product, you may see dehydrated castor oil in place of the Tung Oil. It’s a synthesized drying oil with similar properties minus the discoloration (yellowing).
Teak Oil and Tung Oil finishes are usually a blend of drying oil and varnish along with some additives to help protect the wood from fungus and the sun.
Semi-transparent stains have the similar ingredients as water repellent preservatives (WRPs) with the addition of a substantial amount of inorganic pigments (clay and ground up rocks) which change the woods’ natural color. The pigments are not affected by ultra-violet (UV) light and do a good job of blocking it from the wood. The less UV that gets through to the surface of the wood, the less damage it can cause (that’s why paint does such a good job).
The pigments (and preservatives) are held in place by a thin resin (called a binder) which acts like glue. As the binder breaks down over time (mainly from UV damage), the pigments wear off and increasingly expose the wood surface. Ultra-violet (UV) light from the sun damages the wood by breaking down the lignin. Lignin gives the wood its natural color and serves as the glue that binds the wood fibers together. As the lignin breaks down, the wood turns silver grey and erodes.
When inorganic pigments are ground extremely fine they allow visible light to pass through making them nearly invisible. But they are large enough to block UV light which has a shorter wavelength. These pigments are either transparent iron oxides (transoxides) or titanium dioxide. The pigments help to protect the binder in the stain which keeps the pigments in place longer and extends the service life of the mildewcides and preservatives.
Though cost is not a guarantee of performance, top products are comparatively expensive. High quality ingredients – pigments, resins, and preservatives – are costly and necessary for maximum longevity.
Semi-transparent penetrating stains perform best on coarse sawn or weathered wood like wood siding or on fencing and deck rails and posts. They are not a great choice on the walking surface of decks because they show wear paths where people walk. If used on smooth fence boards, they should be power washed or liberally wetted and allowed to dry a couple of times to open up the pores before staining.
If the wood is dirty or has mildew, clean it well before applying the stain (use a deck cleaner – not soap). If the wood is weathered but clean, you can apply the stain without any preparation (unlike paint).
Follow the directions on the can and apply the stain with a brush, spray, or roller. The directions may require that you back brush if you use a roller or sprayer – that’s to make sure the stain is worked into all the cracks and crevices. Cool cloudy days are best for applying stain so it has a chance to soak in before it dries.
Film Forming Finishes
The second major category of exterior wood finishes is the film formers. These include paints, solid color stains, varnishes and polyurethanes.
- Offer very good protection against dirt and wear.
- Clear film formers bring out the beauty and depth of the wood.
- Available in a variety of sheens.
- High quality products provide the longest lasting protection of all exterior finishes. Clear film forming finish can be expected to last twice as long as water repellent wood preservatives (or more). Paints can last a decade or more.
- More demanding to apply and maintain than penetrating finishes.
- Can blister or peel if water finds a way into the wood and gets behind the finish.
- Must be chemically stripped, or scraped and sanded, if it blisters, cracks, or peels.
Paint provides the longest lasting protection – it blocks the UV completely and seals the wood from water and microbial attacks. It’s a good choice on wood siding, trim, and doors as well as outdoor furniture that doesn’t get wet too often. Siding and trim should be caulked to prevent water from getting behind the paint and causing it to blister and peel.
The ingredients of paint are the clear finish (called a binder), pigments, and additives. The binder forms a thin film on the surface of the wood and serves as the glue that holds everything together. The pigments provide the color and make the film opaque which blocks UV. And additives like biocides improve the performance and longevity of the paint. The film forming resins in the binder slow the rate of moisture transfer into and out of the wood, but the wood is still vulnerable if it’s exposed to the conditions that promote decay. When water gets trapped behind a film forming finish it causes blistering and peeling.
The best choice for exterior wood paint is acrylic latex. Good quality acrylic lasts longer than oil-base paint because it has better resistance to UV. Acrylic latex is also more porous than oil-base which lets the wood breathe and shed water. Lastly, acrylic latex is more flexible than oil-base paint and doesn’t become brittle and crack.
NOTE: When painting horizontal surfaces, or any vertical wood near a horizontal surface, it’s a good idea to apply a water repellent preservative (WRP) a couple of days before the paint (make sure it’s one that can be painted). This will extend the service life of the wood by protecting it from the water that splashes on the ground, door jamb, or window jamb causing rot.
Be sure to sand the wood before applying the primer to ensure you get good adhesion. Wood that’s weathered for a day or longer should not be painted without sanding first. Follow the primer with two coats of paint in accordance with the manufacturer’s directions. You’ll know it’s time for a new coat when the paint weathers away and the primer starts to show. Don’t repaint too often too avoid making it too thick.
Solid Colored Stains
Solid colored stains fall between paint and semi-transparent stains in terms of protection. They have more pigments and binder than a semi-transparent stain but are thinner than paints and need to be recoated more frequently. The advantage they offer is they are easier to apply and recoat than paint and because they are not as thick they allow some of the natural texture of the wood to show. Like paint, waterborne acrylic stains have a longer service life than oil-base stains.
Solid colored stains are a good choice for outdoor furniture, deck rails and posts, fences, and cedar siding, shakes, and shingles. Note that applying stains requires a good technique to avoid creating lap marks (stripes).
Clear Varnishes & Polyurethanes
Clear film forming finishes are the best choice if you want to maximize the depth and beauty of the wood, bringing out its natural color, grain, and figure while protecting it from wear, weathering, and decay. The trade-off is that clear film forming finishes are demanding to apply and maintain. If you don’t recoat the finish before it cracks and peels, it will have to be stripped off and replaced.
Clear exterior varnishes are formulated differently than their interior counterparts to allow them to perform well in harsh outdoor environments. The typical challenges for exterior varnishes include;
- The finish has to be flexible to avoid cracking as the wood expands and contracts with the wide variations in moisture content (MC) and temperature. Varnishes are made by dissolving resins (e.g., phenolics, alkyds, and/or urethanes, etc.) into drying oils at high temperatures in an oxygen free vessel. The ratio of oil to resins determines how flexible the final varnish will be. Exterior varnishes have a higher ratio of oil which makes them more flexible (drying oils form rubbery polymers) so they don’t crack as the wood expands and contracts.
- The finish has to prevent liquid water in the forms of dew, rain, frost, and snow from coming into direct contact with the wood. Although the moisture content (MC) of wood varies with relative humidity because all finishes allow water vapor to pass through them, we need to seal the wood from excess water which can lead to decay.
- In the same way that ultra-violet (UV) radiation gives us sunburn and ages our skin, it damages the binder in stains, paints, and clear finishes as well as the surface of the wood. Clear finishes need additives that protect both the finish and the wood surface from damage from UV radiation.
- The exterior finish must contain a biocide additive to prevent the growth of fungus, mold, and algae that live on organic materials.
Top quality marine varnishes are often used as the “gold standard” for exterior clear finishes. That’s because they are exposed to a great deal of sun and water, the two greatest threats to wood degradation. Traditional high quality marine varnishes are formulated with phenolic resins, tung oil, UV inhibitors, and biocides. Varnishes based on alkyd resins tend to oxidize and fail too fast when used outdoors.
More recently, uralkyds (also known as oil-modified urethanes) have become popular because they provide greater durability and water resistance and are less expensive. However, traditional varnishes are easier to maintain because they have better adhesion properties and don’t always have to be sanded between coats.
Protecting the Finish and Wood from UV Light
Through a process called photo-degradation, ultra-violet (UV) radiation damages the wood and the finish. At the surface of the wood UV breaks down the glue (called Lignin) that holds the wood fibers together. In the finish the UV breaks down the chemical bonds in the polymers that make the clear finish and paint binder. The signs of this damage include loss of gloss, chalking, fading, cracking, and physical changes like brittleness and cracking. Paints provide the best protection because the pigments that give them their color also block the UV from reaching the surface of the wood or beyond the surface of the paint itself. For clear finishes, we need another solution or the finish will fail quickly, creating a major repair effort.
To counteract photo-degradation when using varnish, it must contain additives that effectively block UV from reaching the wood or breaking down the bonds in the finish itself. In the 1970s, coatings formulators began using UV additive in finishes called ultraviolet light absorbers (UVAs) and hindered amine light stabilizers (HALS). These UV additives have substantially improved the durability and longevity of clear finishes used outdoors. Needless to say, chemists have, and continue to improve the performance of UVAs and HALs. Today it’s possible for clear finishes to last multiple years in exterior applications.
UV Additives – UVAs and HALs
Together, ultraviolet light absorbers (UVAs) and hindered amine light stabilizers (HALS) additives do a great job protecting the wood and the finish from solar radiation damage. And, unlike pigments and transoxides, UVAs and HALs do not affect the transparency of the finish – the wood grain and texture are not masked at all. For optimal performance, they are added to coating in small concentrations (e.g., 0.1% – 2.0%).
UVAs and HALs work together – but each has a separate job. Ultraviolet light absorbers (UVAs) have the task of protecting both the wood and coating by absorbing the harmful wavelengths of light and converting it to heat. Unfortunately, this process slowly destroys the UVAs and they don’t last forever. To achieve the greatest level of protection, the longest lasting compatible absorber is added to the finish and multiple coats are applied which builds up the depth and concentration of the absorber.
The second additive, hindered amine light stabilizers (HALS), have the job of protecting the coating at its surface where UVAs are at their lowest concentration and depth. HALS work to prevent damage to the polymer at the surface which causes loss of gloss, chalking, and cracking. HALS don’t absorb the UV energy, instead they inhibit photodegredation of the polymer by scavenging free radicals which helps prevent surface defects that lead to failure of the coating. They work extremely well and are actually regenerated as they do their job instead of being consumed by it. As a result, HALS provide UV protection for a very long time.