How to Fill Knot Holes and Cracks with Epoxy

By Jim Hingst

Some wood has imperfections, such as cracks or knots, which should be filled before sanding, staining and varnishing. The easiest way to fill these holes is to run down to your local hardware store and buy a can of wood putty. It is the simplest solution, but probably not the best. The reason is that wood putty can fall out over time. A much better solution is to fill the cracks in the wood with epoxy colored with pigment. It takes a little more effort, but it is much more effective. Here’s why. Epoxy penetrates the pores of the wood and holds on tighter. The epoxy that you use as an alternative to the wood putty is the same epoxy that you use as epoxy glue.

The process of filling cracks in wood with epoxy is not difficult. You do need to be prepared to work fast, however, because epoxy has a limited working time. The first step in working with epoxy is to assemble everything that you need in one place before you get started.

The next step may or may not be required. Check the wood for your project piece. If the crack goes all the way through the wood, you will need to do some taping. 3M Blue Painters Tape will work.  What works even better is clear plastic packaging tape.

There are a couple of ways to tape off a crack. One way is to tape the bottom of the wood and fill the crack from the top. That works for many woodworkers. Here’s the problem though. As epoxy cures, the chemical reaction creates carbon monoxide gas.  That means that bubbles form in the epoxy and float to the surface. To get rid of the bubbles, you can carefully heat the surface of the epoxy with a propane torch and that will burst your bubble. Another way is to tape the side of the board that will be exposed and fill the crack in the wood from the bottom. That way any bubbles float to the side of the board which will not be exposed. Both ways work. The choice is yours.

The next step is to mix the epoxy. In the sign industry one of the most widely used epoxy resins is the West System Epoxy. Because of its versatility and durability, old school sign makers frequently select this product for projects.

West System 105 epoxy resin is the key component in a variety of epoxy formulations including coatings, lamination and adhesives.

The West System is comprised of three components: the 105 epoxy resin, a hardener and filler. Depending on which hardener and filler you select, you can use it to as an adhesive, or as a clear coat or for laminating fiberglass or to repair damage to a surface.

If you use West System epoxy and use West System 300 mini pumps, metering and  mixing the correct ratio is easy. All you need to remember is to combine one full pump of resin with one full pump of hardener.

Measuring the resin and hardener in exact proportions is critical. Whatever you do, don’t be creative. Some people make the mistake of adding in a little extra hardener, thinking that they will aid the curing. It doesn’t work that way. In fact, adding more hardener creates problems with the epoxy curing completely. 

Also keep in mind that the proportions for mixing epoxy vary from product to product. Some brands require a 1:1 ratio. Other brands mix at a ratio of 5 parts of resin to 1 part of hardener. The lesson here is to always read the instructions and follow them to the letter.

Epoxy Resin Hardeners

West System offers many different types of hardeners. The major difference from one harder to another is the rate of speed at which the mixture cures. The 205 hardener, which is very popular, sets up fast. For filling cracks, this hardener should work just fine. In warm weather environments, you may need a slower setting product, such as the 206 hardener or the 209 hardener. The 207 hardener is a specialty super clear hardener, which you can use in a variety of applications, such as adhesives, surface repairs, fiberglass lamination as well as a finish coating.

Mixing the Epoxy Resin and Hardener

After dispensing the two components in a clean container, you must thoroughly mix the resin and hardener together. For mixing epoxy, I use a plastic cup, such as a Solo cup.     

Whenever you mix any components, make sure that you scrape the sides and the corner at the bottom of the container. For mixing I use the West System mixing plastic mixing sticks. These sticks are squared off which allows you to get into the corners of the cup.

After you have mixed the epoxy, it is time to add the pigment powder. Many woodworkers will use either a black charcoal pigment or a dark brown pigment. You don’t need to add much. For you older readers, it’s like Brylcreem – a little dab will do you. As an alternative to pigment powder, you can also use a colorant such as Mixol.

Working Time

After you mix the ingredients, the mixture will transform from a liquid to a gel and finally to a solid. You need to work fast because once the resin and hardener are combined the pot life is limited. In some cases you may only have as little as 15 to 30 minutes to work.  

The open time that you have when working with the epoxy will depend several factors, including the size of the batch that you are mixing and the ambient temperature of your shop. When filling cracks, you typically will only need to mix a small batch.

Epoxy also cures faster as the ambient temperature increases. The optimal temperature is 70°F. For hot weather applications, use a slower hardener. Do not use epoxy when the temperatures of your shop or the materials are below 50°F. Low temperatures can compromise adhesion.

When to Use Epoxy Filler

If you are filling an imperfection on a vertical surface, you will need to mix in a filler to thicken it up. The 405 filler is great for general purpose adhesive applications. Gradually add thickener to the resin/hardener mixture until you attain the desired consistency. For adhesive applications the epoxy should have the consistency of creamy peanut butter. 

The high density fillers, such as West System’s 405 or 406, change the structural characteristics of the epoxy, making it either stronger or thicker. These high density fillers become very hard after the epoxy cures, which makes sanding difficult.

By comparison low density fillers, such as West System’s 407, are designed for filling holes or to add mass to a surface which can be easily shaped in a  manner that Bondo is used for cosmetic purposes. Lighter weight filler are much easier to sand.

Pouring Epoxy into the Crack

When everything is thoroughly mixed, pour the mixture into the crack. A spoon works for this procedure. As I wrote earlier, bubbles will rise to the top. When these bubbles form, heat the epoxy with your torch to burst the bubbles. After the epoxy has dried completely, it is time for sanding.

Conclusion

For filling in cracks in wood, especially larger cracks, epoxy resin is a much better choice than the wood fillers in a can. Epoxy is waterproof, solvent resistant and chemical resistant. With respect to service temperature range, epoxy resins withstand extreme heat and cold, which make them great for either indoor or outdoor applications. 

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About Jim Hingst: Sign business authority on vehicle wraps, vinyl graphics, screen printing, marketing, sales, gold leaf, woodcarving and painting. 

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 500 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. 

Slides of Jim's Artwork

© Jim Hingst 2018