Reducing Checking in Green Wood with Wax Emulsion Sealers

Wax Emulsion Sealer Overview: Few surface coating products have been as beneficial to woodturners trying to successfully season green timber, as modern green wood sealers. These sealers are specifically formulated to control the rate of moisture loss in green timber by forming a durable wax membrane between the exposed end/side grain and the surrounding ambient atmosphere.

The goal is not to prevent moisture from moving through the wax coating, but instead to retard the rate of moisture evaporation, thereby reducing drying defects like end grain checking and warping.

Commercial Wax Emulsion Sealers

Two widely available commercial wax emulsion sealers are Anchorseal, by UC Coatings Corporation, and Mobil-Cer M, by ExxonMobil Oil Company. Anchorseal is paraffin based colloidal solution for logs and lumber.

It contains paraffin, water and a surfactant and is milky-white in appearance. Mobil-Cer M is a microcrystalline wax based coating. It contains microcrystalline wax, water and a surfactant and is also milky-white in appearance. Winter formulations of these products may also contain anti-freeze agents.

Anchorseal, a popular wax emulsion sealer for logs and lumber (5 gal winterized formula)

In the woodturning workshop, wax emulsion sealers serve an important function by controlling the rapid loss of moisture through the end grain/side grain of logs, blanks, turning squares, or roughed out bowls. These sealers prevent the rapid onset of surface checking for a limited period of time, until the logs can be further processed into turning squares, or blanks. Few species of timber can successfully reach Equilibrium Moisture Content (EMC), when left in the full round log form, regardless of the type of end sealer used.

High figured areas such as crotch feather are good candidates for sealing to prevent checking

Turning squares, blanks and bowls of many species can be successfully air-dried by applying the wax emulsion sealer to exposed end grain surfaces and any high figured areas on the blank. Once the sealer has dried, the sealed pieces can be dried on wire racks in the open air. Several years ago this was my primary method for seasoning production salad bowls and turning squares in my studio.

To understand how end grain wax emulsion sealers help prevent drying defects in green timber, it’s important to first understand the basic physical processes green timber undergoes to reach EMC.

How Wood Dries – In a Nutshell

Numerous factors affect how wood dries in the air. These include vapour pressure, relative humidity, ambient temperature, amount and velocity of air movement over the piece, moisture movement in the wood itself and amount of any supplemental heat (electric, gas, solar, etc) that may be added to the drying space.

Saturation vapour pressure is achieved when the ambient air holds the maximum amount of water vapour possible. When the existing water vapour is less than this maximum, the air is capable of taking up more moisture. Relative humidity is the ratio of actual vapour pressure to the saturation vapour pressure, expressed as a percentage value.

When you cut a piece of green wood and expose it to the air, surface moisture will evaporate from its surface if the relative humidity is less than 100%. The actual rate of evaporation is dependent on several factors, including the ambient temperature and the difference in vapour pressure between the air that is very near the surface of the wood and the air that is just above this surface zone.

The temperature of the ambient air and the wood itself, also have a significant impact on the rate of evaporation from the wood surface. Fresh cut logs stored in the direct sun will have surface evaporation rates higher than those stored under cover, or in full shade. Artificially induced heat, such as that in found in a drying kiln, can also increase the drying rate of green timber.

Air movement over and around the wood, as well as its velocity, also impact the evaporation rate of surface moisture. The velocity of air in direct contact with the surface of the wood will be slower and will contain a higher vapour pressure, than the faster air moving just above the main airstreams. This characteristic is known as the “boundary layer effect.”

Supplemental heat, if any also affects the movement of water in the wood and the amount of evaporation from its surface fibres. Logs, or blanks stored in heated buildings will generally dry faster than those left in outdoor spaces. Obviously, wood dried in kilns will dry much faster than traditionally air-dried timbers as well.

End grain sealers will not prevent mold from forming underneath the sealed surface

How Wax Emulsion Sealers Work

The inside and outside end grain portions of this 32" White Ash bowl were sealed with Anchorseal wax emulsion before air-drying

Wax emulsions help to equalize the rate of evaporation from the surface of the wood, with the rate at which the interior moisture can travel to the surface. When this is achieved, many drying defects are eliminated and the maximum amount of timber can be utilized in every log. When the moisture inside the blank has reached equilibrium with the ambient moisture, the piece is said to have reached Equilibrium Moisture Content, or EMC.

How Water Moves Through the Timber

The water in wood moves from zones with higher concentrations to zones with lower concentrations. For this movement to occur, the outer portion of the wood must be drier than the inner portion. The movement of water occurs in two separate but distinct phases, movement from the interior to the surface and evaporation of this water from the surface of the timber.

The fibres on the surface of most timbers reach moisture equilibrium with the ambient air very quickly after cutting. A moisture gradient is soon formed by the difference in moisture content between the inner and outer portions of the timber. Reducing the rate of surface moisture loss by the use of wax emulsion sealers prevents the formation of steep moisture gradients and subsequent differential stresses that contribute to surface checking.

When water on the surface of the wood evaporates, the moisture lowers on the surface. Moisture diffusion, in conjunction with capillary flow causes the moisture contained on the interior of the timber to move towards the drier exterior surface of the wood. If the evaporation rate on the surface of the wood exceeds the rate that interior moisture can move to the surface fibres, a steep moisture gradient is formed.

Wax emulsion sealers may be applied with various tools including brushes, rollers and spray equipment. In addition, they are also easily applied from small dip tanks

If this steep gradient continues, the outer surface fibres may drop below the fibre saturation point. Stresses may develop as the drier outer fibres tendency to shrink, is resisted by the moister interior fibres. These stresses may cause a host of drying defects including checking, warping and twisting to name a few.

Application Methods for Wax Emulsions

Wax emulsion sealers can be applied by various methods including brush-on, dip tank, roll-on, spray-on and vacuum assisted. The decision on which method to choose should be based on the quantity, size and location of the logs, or blanks to be processed. Correct application procedures are essential to obtain maximum protection from the wax emulsion coating.

For best results, the cut surfaces should be as smooth as possible before applying the wax sealer. The log on the left is ready to be sealed; the log on the right should be re-cut before application

TIP: Wax emulsions must be applied as soon as the log is cut for maximum protection. Application of the sealer to dried log surfaces will reduce the overall protection level achieved and may not stop the continued progression of any existing surface checks.

TIP: For best results, cut surfaces should be as smooth as possible. For very rough surfaces, spraying will generally produce a more uniform coating than brushing.

TIP: If the cut ends of logs have dried, or are already showing surface checking; a fresh cut should be made to eliminate surface checks, before application of the log sealer for best results.

When numerous checks are evident, the end of the log should be re-cut past the point of any end checking before application of the wax emulsion sealer

Brushing On Wax Emulsion Sealers

Wax sealers can be easily brushed onto the exposed end grain or side grain surfaces of logs, turning squares or bowl blanks. For an optimum surface coating, the cut surfaces should be as smooth as possible. If the exposed end grain is very rough, jagged, or dirty, a fresh crosscut should be made before applying the wax emulsion. The sealer is initially white but dries to a crystal clear dry wax finish. Excess sawdust or debris should be brushed off before applying the sealer.

Brushing sealer on the
end of a Mesquite log

Because even smooth surfaces cut on a chainsaw or bandsaw still have a ridged surface, you should brush the sealer on in a crosshatch pattern (first up and down, then back and forth) to insure uniform coverage. The sealer should be applied onto the end grain until it nearly runs off. Applying too thin a coating will reduce the protection level and may compromise the successful seasoning of the timber.

If any bubbles are present in the surface, brush across the area again until the bubbles are completely removed. Bubbles left in the wet film will burst when the coating dries, which may cause differential moisture gradients to form between thicker and thinner areas in the surface coating.

The same Mesquite log shown above, once the sealer has dried on the end grain. White areas visible in the photo are thick areas of sealer that have not fully dried in the checks

These differential moisture gradients cause uneven drying zones across the face of the end grain and subsequent surface checking may develop.

TIP: If you wish to apply two coats to the surface, do not allow the first coat to fully dry before applying the second coat for best results.

TIP: For brush-on applications, simply decant a small portion of the sealer into a small bucket with a tight fitting lid. Cut the handle of an inexpensive brush to fit inside the bucket. When you have finished using the sealer, replace the brush in the bucket and secure the lid. There is no need to clean the brush! The tight fitting lid will prevent the remainder of the sealer from drying and the brush will remain ready for use.

When brushing the sealer on the end grain portion of resawn turning squares or bowl blanks, also coat 1/2" to 1" of the exposed adjacent side grain. This insures a better coating on the area where the end grain and side grain areas meet. Brushing becomes inefficient when the logs are more than 12" in diameter. When applying the sealer to larger logs, it’s far more efficient to use a paint roller, or on very large logs to spray it on.

Rolling On Wax Emulsion Sealers

Standard 9" paint rollers make excellent applicators for medium to large sized logs and blanks. My favourite roller is the type sold for painting in tight areas. It is approximately 4" long and works exceptionally well for any but the largest of logs. Rollers can apply the coating much faster and more uniformly than hand brushing. In addition, because the nap on the roller is available in different sizes, rough cut log surfaces can be more easily and uniformly covered.

For most applications, it is easier to decant a
small portion of sealer into an airtight
container when brushing or rolling small amounts

TIP: To eliminate cleanup of the roller, leave the roller submerged in a small amount of the wax sealer after each use. As long as the application bucket has a tight fitting lid, the sealer will not evaporate and the roller will remain ready for use at all times.

Using Dip Tanks to Apply Wax Emulsion Sealers

When applying the wax sealer, care must be taken to insure the end of the log is totally covered. The end of this log has numerous areas that are not properly covered

When processing large amounts of smaller blanks, turning squares, or pen blanks, dip tanks are simple, economical and efficient. Almost any shallow container can be used for making a dip tank. Old pie pans work well, as do small plastic bowls. Simply decant a small amount of the sealer into the tank and dip each end of the blank into the sealer. The level of the sealer inside the dip tank should be adequate enough to coat the end grain and the small amount of adjacent side grain, in one step.

Small dip tanks can be used to effectively seal pen blanks and turning squares

When using this method, it’s quite easy to process several hundred squares per hour. To insure proper drying of the coated blanks, stack the wet blanks in an alternating crosshatch pattern. This allows maximum airflow around the blanks, whilst insuring a stable drying stack.

To speed the drying of the coating and increase overall throughput, a high velocity fan can be directed towards the drying stack. Adding forced air circulation to the application protocol is particularly useful when processing blanks on days with high humidity.

Spraying On Wax Emulsion Sealers

If you will be processing a large amount of squares or logs, or you are working with very large diameter (2’ to 6’ diameter) logs consider spraying the sealer. Many types of spray rigs are available in electrically powered, petrol powered and manually powered with sizes and price’s to fit any budget.

Because most of my timber harvesting is done during the hot and humid summer months in remote field locations, I prefer to spray the logs before transporting them to my studio. Spraying the logs in the field insures the end grain will be protected during transport back to the studio and is the fastest and most efficient method of applying the sealer on large logs.

Manual sprayers are excellent for spraying in remote locations that do not offer electrical service. Trombone type slide sprayers are available that draw directly from 5 gallon buckets of sealer. In addition, backpack type sprayers are also available that are manually pumped.

When there is a significant amount of logs to be sealed in remote locations, simple pump-up garden sprayers can be used

However, I have found that the simple pump-up garden sprayers available in most garden or home centres, work very well and are much easier to use after a long day chainsawing logs, than trombone or backpack style sprayers.

When I’m back at my studio, I prefer to use HVLP, or airless style sprayers for applying the sealer to turning blanks or logs. Most of the electrical and petrol powered sprayers used for applying log sealers will need to be cleaned after each day’s use. Trombone, backpack and garden style pump-up style sprayers should be cleaned at least once per week when used regularly.

Vacuum Assisted Application

When sealing high figured, unstable burrs, or crotch figured pen blanks, vacuum assisted impregnation of the surface is useful. This method applies sealer to the side grain and end grain simultaneously, producing a well sealed outer surface. My use of this method is limited to pen blanks, but it can be adapted to larger turning squares quite easily.

The pen blanks are loaded into a Mason style jar and the sealer is added until it covers the blanks. Using a special adapter that fits the top of the jar, a vacuum is pulled on the jar. The blanks are allowed to remain under vacuum for a few minutes, then they are removed and allowed to drain on a wire grate inside a closed bucket. When the excess sealer has drained off, the blanks are transferred to another wire grate and allowed to dry in the open air. This method is particularly useful with unstable timbers, as it produces a very uniform and well-sealed surface.

Wax Emulsion Sealer Storage and Handling Considerations

This roughed out Box Elder burr nested bowl set was sealed with Anchorseal on all cut surfaces prior to air-drying

Wax emulsion sealers must be kept in containers with tight fitting lids to prevent evaporation, or crusting. In addition, unless the particular formula was ordered with and anti-freeze agent, the sealer should be protected from freezing temperatures.

When using wax sealers in your studio, care should be taken to clean up any spills on the floor. When dried, these sealers produce a nearly clear, waxy coating that is quite slippery. Soap and water can be used to clean any un-dried sealer. Fully dried sealer requires a chemical solvent, or steam cleaning.

When spraying these sealers or when working with heated emulsions, consult the manufacturers MSDS sheets in the U.S.A, or COSHH sheets in the U.K., for full safety and handling instructions. Avoid prolonged skin contact, or wear impervious gloves. Eye and respiratory protection is required under certain conditions, consult material safety sheets for full information.

If you wish to dilute the sealer, always add the water to the wax emulsion, never emulsion to the water. Dilution water should not exceed 100 ppm CaC03, or the wax may separate from the emulsion. Avoid storage areas that have abnormally high heat levels (such as near a radiator, or furnace), as this may cause separation of the product.

When sealing large logs that contain very irregular or rough surfaces, an additional cut should be made to provide as smooth a surface as possible for sealing

Buying Wax Emulsion Sealers in Bulk

Typically, these log sealers are available in 1 gallon, 5 gallon and 55 gallon drums. If you are a member of a turning club, consider having your club purchase a 55-gallon drum and divide the sealer among your club’s members. When purchased this way, the cost is substantially reduced over the 1 and 5-gallon container sizes.

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Safety Note: Always follow all manufacturers safety instructions before working with your lathe, or any of the tools or products you may use. If you are unsure about any operation, obtain competent professional instruction before proceeding. Use and wear all necessary safety devices during turning and observe safe woodturning practices to prevent accident or injury.

Steven D. Russell is a professional studio woodturner, teacher and writer. He has written numerous articles for international woodturning magazines, which have been published in more than 78 countries around the world. Steve has demonstrated in numerous cities across the United States. His studio, Eurowood Werks, specializes in bowls, platters and hollow forms with unique visual and tactile treatments.

Steve is also the current and founding President of the Lone Star Woodturners Association, Inc., an AAW member chapter. The LSWA is a 501(c)3 non-profit educational organization dedicated to teaching and demonstrating the art and craft of woodturning.

Steve is also a featured writer for the Guild of Master Craftsman's "Woodturning" magazine, published in London England. Woodturning magazine is the world's leading magazine for woodturners. Look for his articles covering technical topics, or project based articles in an upcoming issue.