Epoxy: Still Gluing Strong After All These Years

Before dazzling rivers and bar-top coatings, epoxy simply served as a humble, high-performance adhesive and repair resin. Epoxy manufacturers catered primarily to boat builders, providing a product that withstands the harsh conditions of a marine environment. You've seen epoxy at center stage; now learn how this versatile adhesive can play an important supporting role in your woodworking.

Epoxy bonds to a wide range of surfaces, making it the adhesive of choice for joining dissimilar materials. And cured epoxy forms a waterproof bond, so it's also a good option for outdoor projects, especially those consistently exposed to moisture. Because it requires a surface with "tooth" for bonding, when gluing plastic, metal, or other smooth, non-porous materials, scuff up the glue surface for best adhesion [Photo below].

Epoxy consists of two parts: a liquid resin, and a hardener that must be mixed in a precise ratio. Hardeners, available in fast and slow offerings, give you open times ranging from 10 minutes to 60 minutes or more.

If you need only small quantities, squeeze bottles or syringes work well for eyeballing the proper amounts of resin and hardener. But for mixing quantities larger than ¹⁄₂ oz. at a time, some manufacturers sell metered amounts with a single push [Photo below]. Or, in lieu of pumps, use a kitchen scale and measure the parts by weight.

The temperature of the work environment can influence your choice of hardener. Epoxy gives off heat as it cures (known as an exothermic reaction), and the ambient temperature either accelerates or slows down the reaction. A fast hardener produces more heat, good for cool ambient temperatures, and a slow hardener gives you more working time in a hot environment. Always refer to the manufacturer's instructions for the appropriate working temperature range of your epoxy.

It might seem like a small detail, but choose your mixing container carefully. Mixing epoxy in a tall, narrow container reduces surface area, concentrating heat buildup and speeding cure time. The built-up heat can also melt plastic containers, or even combust. A metal tray, or the hollow on the bottom of a soda can make good mixing containers.

After accurately measuring your resin and hardener, mix the two using a disposable spatula [Photo above]. Avoid the temptation to add more or less hardener to adjust your working time, as it decreases bond strength.

Spread epoxy with your spatula, brush it on with a stiff-bristle disposable brush, pour it where you need it, or even dip parts into the mix container [Photo above]. Freshly mixed thin adhesive will flow into cracks and crannies. For jobs requiring a thicker viscosity, let the epoxy set up partially before applying [Photo below].

If you drip epoxy where you don't want it, wipe it off before it cures, using denatured alcohol. Otherwise, wait for the epoxy to fully cure and carefully remove the excess using a scraper, chisel, or sandpaper. Spread leftover epoxy in a thin film so the heat generated dissipates quickly as it cures. Once cured, you can cut, sand, and shape epoxy using the same cutters you use on wood.