Coating Woodworking Projects with Epoxy Resin

Learn the basics of applying epoxy coatings to woodworking projects, including safety recommendations, surface preparation, sealing the wood, pouring the resin and varnishing.

By Jim Hingst @hingst_jim

Many years ago I had become intrigued with epoxy coatings after watching a sign maker coat a plaque. The result was absolutely beautiful. While most of us have seen table and bar tops finished with these durable coatings, only a limited number of craftsmen have ever worked with epoxy. If you have never used these coatings but are willing to give it a shot, I have compiled a few helpful tips to help you through your learning curve.  

After carving the aromatic red cedar log, three coats of System Three Silver Tip epoxy resin were applied to the sign pictured above.  For the finish coats the sign maker/master woodworker used Z Spar Captains Varnish from West Marine. The first varnish coat was brushed, followed by a sprayed coat. The letters were gilded with 23 karat gold applied over Rolco Slow Dry Size. Photo courtesy of Ben Van Houten of Wensco Sign Supply.

How Epoxies Work

Epoxy coatings are thermosetting polymers. When you buy them, they consist of two parts: an epoxy resin and a hardener. After equal parts of the epoxy resin and hardener are thoroughly catalyzed, a really hard finish, that resists scratches, water, chemicals and solvents, is produced.

The keys to achieving a coating, that is thoroughly catalyzed, are good surface preparation, an acceptable working environment and, above all, mixing the ingredients according to the instructions. If you control your process, you will control your outcome. Of course, that’s slightly easier said than done.

When you mix the epoxy resin and a hardener together, an exothermic chemical reaction begins. Exothemic means that the chemical reaction generates heat. In this chemical process the mixture undergoes a transformation as it cures from liquid to gel to solid.

Curing takes time. How much time depends on several factors. These include the type of hardener used, the ambient temperature, the temperature of the substrate and the amount of heat generated in the exothermic reaction. As you can guess, a major factor is heat. The warmer the epoxy is, the faster it cures.

Safely Working with Epoxy Resin

Prior to working with epoxies, carefully read the manufacturer’s Safety Data Sheet (SDS) and the safety precautions in their technical bulletins. The major health risk in working with these materials is skin irritation. Repeated contact can result in your body becoming sensitized, which can cause rashes and dermatitis.

Always wear disposable gloves when handling epoxy resins and hardeners. If you get any of the epoxy mixture on your skin, never wash it off with a solvent, such as lacquer thinner. It is one of the worst possible things that you could do.  The solvent will drive the harmful chemicals into your body. Instead, you should wash any of the solvent from your skin with soap and water. Then moisturize you skin with a lotion. If a bad rash appears, consult your doctor.

Potentially, after the two components are mixed together, you could burn yourself.  Combining the resin and hardener initiates a chemical reaction that generates heat and creates toxic fumes. Large amounts of the mixtures require safe handling. High heat can melt Styrofoam containers and shatter glass jars. Instead use polyester plastic cups for mixing, work in a well-ventilated area and avoid breathing fumes.

To learn more about the health risks associated with working with epoxy resin, read Safety Tips for Epoxy Resin Systems.

Epoxy Resin Hardener Types

Different types of hardener are available for curing the epoxy resin. Each type allows the mixture to cure at different rates and works best at different optimal conditions. When selecting a hardener type, consider the curing rate that best suits the type of project that you are working on. For example, you may choose to use a hardener that cures slower and has a longer pot life, when working on a big project.

Generally, the pot life of the resin/hardener mixture is much shorter than the open time that you have to work with it. Remember, that the pot life is affected by heat. Higher shop temperatures and a greater volume of the mixture (which generates more heat) will not only shorten the curing rate but also the pot life. On the other hand, if you are working in a cooler environment (temperatures lower than 70 degrees F), you will probably want to select a faster hardener. Because a resin/hardener mixture has a  limited pot life, you should pour the epoxy  on your project shortly after mixing.  

Sealer Coat

The  porosity of wood is a benefit and a challenge. The very tiny holes on the surface of the wood, provides it some "tooth" for the epoxy to bite into. That’s  the good news.  The bad news is that those tiny holes also trap air when you are coating the surface. The result is air bubbles in the epoxy coating.

To avoid entrapping bubbles, allow the wood to warm up in your shop, if you have just brought it in from a cold warehouse. Warmer temperatures will allow the air to release more readily after you start your pour. Then coat the wood with a very thin sealer coat. This coating serves two purposes, much like a primer. First, it creates a barrier that prevents the wood from outgassing and forming bubbles. Second, it anchors the flow coat to the substrate. After the sealer coating is in its gel stage and is no longer tacky, you can pour  a thicker flow coat.

Some people will thin the sealer coat with a solvent, such as lacquer thinner.  Others advise against thinning the epoxy mixture. The proponents of thinning of the mixture believe that thinning the epoxy with up to 10% lacquer thinner allows the sealer to  penetrate the surface of the wood better. This improves the anchorage as well as strengthening the wood. Thinning also improves the flow out of the epoxy on the surface, resulting in a smoother, more bubble-free finished appearance.

By sealing the surface first, you will minimize the chance of entrapping bubbles. Any bubbles that appear in the sealer coat, are easily eliminated with heat.

In heating the applied mixture, people have used a variety of heat sources including hair dryers, heat guns and propane torches.  What the heat does is lower the viscosity of the mixture. This allows any bubbles to more readily rise to the surface. The heat also allows the mixture to more readily level out. Better flow out of the mixture is the good news. The bad news is that on vertical surfaces a mixture with a lower viscosity can sag.

While there are blush-free hardeners, after the epoxy cures a waxy blush may appear on its surface. High humidity and cooler  temperatures during curing may contribute more to the appearance of blush. Abrading the surface with a Scotch-Brite pad or wet-sanding can remove the blush. After abrading or sanding the surface, you must wash the surface  to remove any residue. Failure to remove the blush or the residue, before you apply a second coat of resin, can prevent good intercoat adhesion.

Surface Preparation

Good adhesion to the substrate is contingent on several factors. The surface must be clean and dry. It must also have some tooth or roughness, so the resin can mechanically adhere to the substrate. Prior to sanding, any grease, oil, waxes or other contaminants must be clean using solvents. To ensure the substrate is dry prior to the application of the epoxy, you can heat the surface using a heat gun or heat lamp.  After the substrate is clean and dry, sand the surface with aluminum oxide or wet/dry sandpaper.

If you are coating wood, you should sand most hardwoods and softwoods with 80-grit sandpaper. The coarse grit will rough up the surface, giving it some tooth for the epoxy to bite into. Oily woods, such as teak, should be wiped down with acetone just prior to epoxy coating to remove any oils.

Vacuum off the dust or use a brush  to dust it off. Do not wipe the surface  with a tack cloth. While this is an acceptable practice when you are painting, the sticky residue from the tack rack can cause epoxy adhesion problems.  Instead, it is better to wipe the surface clean with a lint-free cotton rag dampened with solvent.  

After sanding, resist the temptation to touch the surface with your hand to check its smoothness. It’s true that your sense of touch is much more acute than your sight. You will feel imperfections that you may not be able to see. Oils from your skin, however, can contaminate the surface and prevent good adhesion of the epoxy. If you are coating a metal surfaces, salts from you skin can accelerate corrosion of the metal, which can ultimately result in adhesion failure.

Staining

In most cases, you can apply epoxies over stained wood. There is, however, only one way to be sure that the stained substrate and the resin are compatible.  Whenever you try some new application, it is best that you Test, Don’t Guess.

While many different types of stains have been used for projects, oil-based aniline dyes are often recommended.  Regardless of what you use to stain the wood, the dye or stain must be completely dry before coating with epoxy. After your project has been epoxied, you cannot stain the surface.  

Project Planning 

You have probably heard about the 6 Ps: “proper prior planning prevents poor performance”.  Proper planning is essential to achieving great results when working with epoxy. Before you start mixing anything, think through your manufacturing process. Then assemble all of the tools and materials that you will need. Remember that an epoxy has a limited pot life and open time. Having everything at hand allows you to comfortably work within those time frames.  Working efficiently is important on larger projects that require a large volume of epoxy. The larger volumes of resin and hardener mixture will generate more heat which will shorten the pot life and accelerate the curing time.

Test, Don’t Guess

Working with any new material involves a learning curve. If you have not worked with epoxies or a particular brand or series of product, be sure to read the manufacturer’s technical bulletins and safety data sheets (SDS).  To learn what you can and cannot do with epoxy resin requires a little practice. Before coating a project, start with a test batch. By testing the product first, you will discover how fast the mixture cures and its workability.

Measuring & Mixing Epoxy Resins

Carefully read the instructions that pertain to the epoxy series that you are using. Pay attention to the mixing ratio. While some resins and hardeners are mixed at a 1:1 ratio, not all epoxies use that mix ratio. You should only mix the resin and hardener according to its recipe. Using a higher ratio of hardener to resin will not accelerate curing times. Failure to measure the ingredients precisely will result in a failure of the epoxy to cure properly.   In mixing resin and hardener, you will generally measure by volume not by weight.

It is critical that you mix the resin and hardener thoroughly. Incomplete mixing may result in areas of the coated surface not curing to a hard finish and remaining tacky in certain places.

Pouring  Epoxy Coating

If you don’t want to epoxy to spill over the edge of your project, you need to create a dam around the perimeter of the surface. To do this, you can tack a temporary molding along the edge. Before you start pouring epoxy, make sure that the plaque or tabletop that you are coating is completely level. Otherwise, the thickness of your  epoxy coating will not be uniform.

Refer to the manufacturer’s instructions to calculate a sufficient amount of epoxy that will be required to cover your project. The optimal temperature for working with epoxy is 75°F.  The ambient temperature affects the pot life, working times, gel time and the curing time. As temperatures rise, resins cure faster. Conversely, as temperatures fall, resins take longer to cure.  The warmer temperature will reduce the viscosity of the epoxy so that it will flow more readily. At this temperature the resin should be tack free in about six to eight hours after the pour.

After thoroughly mixing the epoxy, begin pouring the mixture in the center of the surface creating a spiral  or zig-zag pattern to evenly distribute the epoxy.  You may need to repeat the  pouring in the spiral pattern until the epoxy runs together. The mixture should continue to level out for five to ten minutes after the pour. 

The amount or mass of the epoxy mixture also affects the rate at which it gels and cures. That’s because a greater mass will generate more heat. Consequently, a thicker coating will gel faster than a thin coating. This is probably the opposite of what you would have guessed.  The thicker coating holds more heat, which accelerates the curing process. Conversely, a thin coating dissipates any heat produced, which retards curing.

After you first mix the two parts, the mixture is in liquid form for enough open time for you to work with it. Until the epoxy thickens and begins to gel, you can rework the material, squeegeeing it to the desired thickness. After the mixture begins the gel stage, it will become as firm as hard rubber. At this point the epoxy is no longer tacky, but it still has not completed the curing process. You can tell if the epoxy is still curing, if you press your thumbnail into the mixture and leave an indentation. 

After the mixture is spread onto the surface and begins to level out, bubbles will appear in the epoxy. To eliminate these air bubbles, you will need to heat the mixture with a heat gun or propane torch. If you decide to use a torch, you will need to exercise some care to prevent damaging your project.

Whether you use a heat gun or torch, keep the heat moving over the entire surface. The heat from a torch will cause the air bubbles to rise to the surface and pop. As you are working, inspect the surface for any contaminants that may have fallen into the epoxy. Using tweezers,  remove any foreign matter while the epoxy is in its liquid state.

During the gel stage, you can apply another coat of epoxy without sanding, after it has achieved a “soft set” and up to 72 hours after coating. During this period, the second coating will chemically bond with the first coating.   After the mixture hardens, a second coating will not adhere to the initial coating. In this solid stage, you can wet sand the surface to give it some tooth. By roughing up the surface, the second coating will mechanically bond to the initial layer of epoxy. 

Wet Sanding to Remove Amine Blush

Some epoxies develop what is referred to as an “amine blush” on their surface. While some of the blush removes by simply washing the surface with water, wet sanding is the recommended method. Sanding must be done 24 hours after the final cure but before 48 hours, at which time the epoxy become extremely hard. 

Varnishing to Provide UV Protection

While some epoxies contain UV inhibitors, they only provide limited protection from intermittent exposure to the sun. Epoxies will typically degrade rapidly with prolonged exposure to ultraviolet light. Outdoor life for unprotected epoxy could be as short as 12 to 18 months. To protect the epoxy form degradation, outdoor projects should be varnished with a two-part polyurethane for the maximum protection.  

One-part polyurethanes and two-part polyurethanes are really not in the same family of coatings. In fact, one-part polyurethanes most likely are polyurethanes at all. Instead, the so called “one-part” polyurethanes are typically modified alkyds.

For outdoor projects, multiple coats of varnish are often recommended to build up a thick protective layer. Wet sanding between coats with 320 grit sandpaper levels each coat for the subsequent coatings and promotes  good intercoat adhesion.  Varnish will not last forever. It functions as a sacrificial layer. Periodically, when the UV inhibitors have become exhausted, you will need to lightly sand the varnish and recoat the surface. This annual maintenance will ensure long term durability of the finish.

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About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.  Vinyl Sign Techniques is available at sign supply distributors and at Amazon. 

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© 2016 Jim Hingst