Protecting exterior wood decks is an age-old challenge that does not have one simple solution. One option is Pure Tung Oil. The use of Tung oil to protect wood dates back to around 400 BC as a protective finish for wooden ships and still continues to this day.
Pure Tung Oil will produce a warm rich finish that effectively repels water. It will create an unfinished look, as it will penetrate into the wood surface to saturate the fibers of the wood and keep moisture out. It will not form a film coating and it will need to be regularly reapplied (lifespan will depend on sun exposure), but maintenance is pretty simple. Tung oil itself does not support mold or mildew growth, but it is not an active biocide either, so it may not completely prevent mold or mildew formation on its own.
This guide will cover the initial application and maintenance of Pure Tung Oil for decks, as well as some considerations to help your finish last a little longer.
Preparation
Pure Tung Oil needs to be able to penetrate into the wood surface. This means the wood surface should be clean and free of dirt, oils and other debris or additives. It is best to use Pure Tung Oil on raw wood, so that there are no known possible contaminants or chemical interactions with existing products. It is also recommended not to use Pure Tung Oil on fresh pressure-treated (aka wolmanized) wood. We recommend allowing pressure treated wood to age, or season, for at least a year before coating.
Cleaning
- Begin by sweeping or blowing off the deck as best as possible. Try to remove any surface dirt, leaves, pollen, etc. before washing the deck. This will prevent the absorption of contaminants.
- Remove any and all surface coatings. If there is an existing surface finish on the wood it will prevent the Tung oil from soaking in and protecting your wood. Any finish that dries into a film will need to be removed as best as possible. This includes paints, spar varnishes, spar urethanes, alkyd finishes and perhaps even deck stains. Use an appropriate paint or varnish remover or sanding sequence to remove the previous coating and express fresh wood. Follow proper sanding procedures to eliminate sanding marks.
- Wash and clean the deck to remove as much dirt, grime and oils as possible. There are quite a few options for deck cleaning and they can affect the overall look of the finished project; therefore, it is recommended to do research before choosing the right cleaner for your project. Consider how much “weathering” to remove or how the surface may lighten or darken when using certain cleaning products.
- Oxalic Acid (a.k.a. wood bleach) – This is a great option for cleaning wood and removing stains. This process tends to remove the gray from weathered woods, but it also may cause naturally dark woods to slightly lighten (less than liquid bleach). Follow the directions on the oxalic acid packaging for mixing, use and care instructions. As with any process, test prior to use.
- Soap and Water or other home-made cleaners – There are numerous recipes for wood cleaners that can be made with regular household cleaners. One common option would be powdered laundry detergent and water and a good scrubbing. Borax is also commonly used and it is generally considered safe for surrounding plants.
- Bleach – Liquid bleach or similar products (like OxiClean) will work well to remove graying, as well as kill mold and mildew and remove stains. When using, make sure to rinse well and also be aware of damage or discoloration to surrounding plants or structures.
- Power Washing – Use of a power washer is common for cleaning driveways, patios, houses and decks and can work very well at removing stains without the use of other cleaners and/or chemicals. Care should be used when using a power washer on wood as it can easily damage the surface and cause it to become fuzzy or rough. Always use as low as pressure as possible to clean the wood to prevent damaging the surface fibers.
- Commercial Deck Cleaners – There are numerous types of commercial cleaners available with a wide variety of application methods and final looks. Some may be spray and wet down, while others may require scrubbing or use with a low pressure power washer. Read the manufacturer’s instructions and test prior to use. Also check for compatibility and/or if they leave any residues that may interfere with the Pure Tung Oil or other penetrating finishes.
- Exotic or oily woods may contain a high amount of their own natural oils that may make it harder for the Tung oil to penetrate. Some common examples would be Ipe, Cumaru, Mahogany, Teak, Garapa and others. A good wipe down with paint thinner, mineral spirits, turpentine, naphtha or acetone will help to dry out the surface oils and better allow the Tung oil to penetrate.
- Allow the surface to dry out. Most of these cleaning procedures will be very water intensive, so allow the wood to dry for a few days before coating. Rain will not necessarily restart the clock, but wait for the wood to look and feel dry, so as to not seal water into the wood. Also check for good uniform color on the dry wood. Pure Tung Oil will richen the natural colors and also highlight spots and stains. This would be the time to address any missed spots and/or stains.
Pretreatment – Mildewcides and Fungicides
Once the surface is clean, the finishing process can begin. If mold or mildew was or is a concern, begin by using a wood preserver. There are many powdered or liquid options that can soak into the clean wood surface to help kill termites, ants, beetles, molds and fungi. Follow the manufacturer’s instructions for proper use and care. Some products may be able to be mixed into the first application of Pure Tung Oil to even save this step. Check for products that can be mixed in oil-based products (not water or alcohol based).
Stain – Interior vs. Exterior and Dyes
Stain can be used prior to applying Pure Tung Oil to change the final color of the deck. In fact, using a stain may help to extend the life of the overall finish. We recommend using interior stains as they usually contain fewer additives. Exterior stains may contain a variety of additives including waxes, silicones, fungicides or other resins that may interfere with adhesion or penetration of the Pure Tung Oil. Many exterior stains are intended to be standalone products, whereas interior stains are usually meant to be top coated. Stains are almost always made with pigment. Pigments are powders that are ground up and dissolved in liquids. The fact that they are naturally solids means that they won’t break down or fade at all or much slower than liquid dyes. Pigments will also block and absorb UV light to further protect the wood surface and keep the wood from greying out. In fact, most clear exterior finishes (including our Marine Finishes) use transparent iron oxides (usually red, yellow or brown) to boost their UV protection. Dyes should be avoided as they tend to fade with exposure to UV. As with fungicides, you can mix stain into your first application of Pure Tung Oil to add a little bit of color and protection as well. Again, look for oil-based stains for blending. We DO NOT recommend using our TrueTone Color Infused Tung Oil for exterior applications.
Oil based stains and dyes/tints that will dissolve in oil or non-polar solvents (Naphtha, mineral spirits, turpentine, citrus solvent, etc.) can be mixed in with Pure Tung Oil. If using a conventional stain, you can blend it and Pure Tung Oil up to 1:1. If you need more color, you should use the stain as directed without blending. If using tints or dyes, it is best to dissolve them in the appropriate solvent first before blending with the Pure Tung Oil. It is only recommended to apply color in the first application. At least two applications of untinted Pure Tung Oil should be applied after the color coat to ensure the color is sealed into the wood.
Please Note: When using tints or dyes, they may not hold up well to UV exposure. Tints and dyes are usually made up of extremely fine particles that can dissolve in the carrier solvent and then these absorb into the wood. Because they are so small they are usually not very light stable, meaning the color will fade quickly when exposed to sunlight. Stains traditionally contain actual pigments which are slightly larger particles made from a variety of compounds. These tend to sit on the wood fibers (instead of soaking in) and then they are trapped in the coating. The larger particle makes them much longer lasting when exposed to UV.
Application
Our Pure Tung Oil is sold as exactly that. It is Pure Tung Oil with no other oils, additives or solvents added. The application of Tung oil without cutting it in solvent can be difficult as it needs to be rubbed into the wood surface to ensure good penetration.
Number of Coats and Thinning
Tung oil will dissolve easily in non-polar solvents like mineral spirits, paint thinner, VM&P Naphtha, turpentine or citrus solvent. Do not thin with water, alcohol, acetone or lacquer thinner. Avoid VOC compliant and “safer” solvents as they may not blend well. You can always test by mixing the Tung oil and solvent together to see if they separate or stay blended. If they separate immediately then they are incompatible. Also, be aware that by adding solvent, you are changing the VOC content of the product. Be sure to check with your local authorities with regard to VOC limits for exterior architectural coatings.
The thinner to Tung oil ratio will depend upon the species of wood being finished as well as the amount of work to apply the mixture. Usually, the more thinner to the Tung oil ratio, the easier it is to apply and the easier it will soak into the wood. The solvent used to thin will evaporate off and leave the Tung oil behind. Therefore, the higher the thinner to Tung oil ratio, the less actual Tung oil is applied to the wood with each coat.
- A 50/50 blend of Tung oil to thinner is a good starting point. This usually yields a thin enough product that can be brushed or rolled on and it will soak into the wood nicely.
- For very dried out woods (e.g. reclaimed barn wood), full-strength Tung oil may provide good penetration.
- For dense and oily woods (e.g. exotics like Ipe or mahogany) a thinning ratio of 3:1 (solvent to Tung oil) will help with penetration.
- The Tung oil should soak into the wood surface and not leave a thick liquid coat on top of the wood. If it’s sitting on top of the wood, consider thinning it more.
Application Method
If thinned properly, the typical application method is to brush, wipe or roll on the blend of Tung oil and thinner to a clean and dry wood surface. Allow the Tung oil and solvent mixture to sit on the surface for 30-40 minutes or longer (maximum 60 minutes) to give plenty of time for penetration. After allowing it to soak in, wipe off all excess with clean rags, t-shirts, towels or pads. Most likely there will be areas of the wood that still appear dry or “starved” and will require reapplication. Allow the previous application to dry for 4 hours and repeat the above process until the wood looks evenly saturated. The number of applications will depend on how absorbent the wood is and the Tung oil to thinner ratio. A good method to check for project completion would be to drop a few drops of water on the surface. If it soaks in immediately, more coats of Tung oil are needed. If it beads up on the surface there is good saturation and the project is completed. Be sure to wipe off the water spots after a few minutes if they haven’t soaked in.
Cure Time
Tung oil is a drying oil meaning that it will chemically react with oxygen to crosslink into a dry film (it won’t feel oily). This can take many days or even weeks, but that is not a concern. Because the wood is being saturated, the surface is usable and walkable as early as 4 hours after the final application. Keep in mind that the first 7 days or so will be the least water resistant. It is best to avoid water as best as possible and to plan around a dry weather forecast if possible. Dew, humidity or a light rain after a day or two after application shouldn’t be a problem; however, some water spotting may become noticeable if a heavy downpour happens. If there is some water spotting, lightly scrub or scuff the surface, wipe with paint thinner and allow to dry. Rub in some more Tung oil to the spot to blend it back in and reseal it.
Maintenance
Tung oil will slowly wear off the surface. It may wear off through use or through exposure, but it is not a permanent solution. Look for signs of starved looking wood (dried out) or areas that begin to grey or lighten. Another test would be to apply a few drops of water to the surface (or watch during the rain) and see if the water beads up on the surface or if it soaks into the wood. If the water soaks in, a maintenance application is due.
The wood should be in relatively good shape and mostly protected, but address any small problem areas. Use bleach sparingly and immediately rinse, or avoid using it at all, as it will damage the remaining oil. Wash or scrub the surface with borax dissolved in water to remove any stubborn dirt or stains and rinse with clean water. Allow the surface to dry at least overnight.
Reapply the Tung oil as before. The wood should be mostly saturated, so you may want to cut the Tung oil with a little more solvent this time around as it should absorb less. Let the oil sit for 40 minutes to an hour and wipe off the excess. After 4 hours, repeat if needed. Again, it would be ideal to aim for 2-3 days without any rain after re-oiling the wood to allow it to cure.
