Canadian Woodworking

Two-part epoxies

Author: Carl Duguay
Photos: Carl Duguay
Published: October November 2022
Epoxies
Epoxies

When you need an adhesive with exceptional shear and peel strength, good gap-filling properties and long assembly time, epoxy is the way to go.

For a lot of day-to-day wood adhesion jobs PVA (poly­vinyl acetate) glues, such as Titebond Original, are an excellent choice. They’re easy to use, don’t require mixing, dry fast, have great bond strength and don’t emit fumes. Type 1 PVA glues, like Titebond III, and polyure­thane glues, such as Original Gorilla Glue, are also highly water resistant.

But for those situations where you need to bond wood to metal, plastic, glass, ceramic or tile; where gaps are too wide (over .005”) for PVA glue to bond effectively; where you need a completely waterproof bond; or where you want a higher resistance to a range of common household chemicals, epoxy is a better choice. As well as having superior gap-filling properties, epoxies cure with­out shrinking and have a clear, transparent appearance. Once fully cured, epoxy can be sanded, drilled, routed or even painted.

Squirt, Mix and Use
Epoxy is generally available in a two-part format. Although each manufacturer has different guidelines, a 1:1 or 2:1 ratio of resin to hardener is common.

Epoxies

Slice, Knead and Use
Epoxy in putty format can be useful to fill holes or cracks that liquid epoxy will flow out of while uncured.

Epoxies

Syringe Style
Many syringe format epoxies set in five minutes and are available in larger containers, too.

Epoxies

Many Uses
Epoxy adheres to many different materials, which is one of its many strengths. Here, wooden handles are fixed to metal blades for a long-lasting bond. (Photo by Chad Martin)

CA Glue
Cyanoacrylates (CA glue, super glue or instant adhesive) are made from cyanoacrylate monomer compounds that cure within 10 to 60 seconds when exposed to moisture. They aren’t epoxies but they have some simi­larities. You can use them on just about anything – wood, metal or plastic. But beware, they aren’t flexible so any amount of bending, expanding or contracting can cause the glue joint to break. And compared to epoxy and PVA glues, they’re very expensive, though only a small amount is needed. They come in a variety of thicknesses that set at different speeds. (Photo by Rob Brown)

CA glue

Casting and Coating
When you want the epoxy to be thick – for example when making a river top table – you need to use a casting epoxy (casting resin) such as EcoPoxy’s FlowCast. Casting epoxies are specifically formulated for pours up to a couple of inches thick. They cure more slowly so you have a longer working time in which to apply the epoxy. For pours 1/2" or less, such as when you need to fill cracks and voids or for a super-hard tabletop finish, use an epoxy resin specifically designed for thin pours. These epoxies have a thicker viscosity, shorter work time and quicker curing time than casting epoxies. (Photos by EcoPoxy)

Epoxies

Stabilizing wood
Renovators and homeowners sometimes encounter partially rotted banisters, rails, window and door frames, and the like. Rather than replace the entire component you can use a special water-thin epoxy solution like LiquidWood to stabilize and harden the rotted sec­tions and then build up the missing sections with an epoxy putty, such as WoodEpox. (Photo by Lee Valley Tools)

epoxy

Epoxy is available in a range of formulations for various applications. Most consist of two parts – an epoxy resin and a hardener. When mixed, a chemical reaction causes the epoxy to set up (harden). The time it takes to set up depends on the chem­istry of the hardener. Some epoxies set up within one to five minutes while others can take many hours.

For woodworkers and DIYers, the most popular formats are the two-part, five-minute (quick-cure) and 30-minute (slow-cure) epox­ies, often referred to as general-purpose epoxies. As the name implies, once the resin and hardener are mixed, they set in five or 30 minutes. They reach handling strength in about one hour (up to about eight hours for some 30-minute epoxies) and are fully cured after about 24 hours. Bonding strength varies. With a bonding strength of around 4,500 psi, 30-minute epoxy is about 10 per cent higher than Titebond III glue, while five-minute epoxy has about 3,500 psi bond strength. However, both epoxies are more than adequate for most jobs wood­workers are likely to encounter in the shop.

Two main formats

Five- and 30-minute epoxies are available in dual containers or a syringe container. Dual containers consist of a resin and a hardener you dispense and mix by hand. Some are dispensed in a ratio of 2:1 resin to hardener, others in a 1:1 ratio.

Syringe containers come in a dual car­tridge with an integrated syringe that’s especially convenient for small jobs in the shop and for DIYers. You simply press a plunger to expel the correct ratio through a nozzle and then mix the two components by hand.

Using epoxy

Using two-part epoxy isn’t overly com­plicated. The resin and hardener are very sticky and have a noticeable odour that can cause an allergic reaction such as an itchy rash for some people. If you have dermatitis or if you’ll be mixing a large amount of epoxy, it’s a good idea to wear a respirator and gloves. When using dual containers mix only as much as you can use at a time. Once mixed, the epoxy will quickly and irreversibly harden. Using a thin piece of wood or a plas­tic spatula stir the mixture thoroughly as per the manu­facturer’s instructions. This is an important step as most epoxy failures result from inadequate mixing.

For small projects, such as adhering a steel bolt into a wooden door handle, you only need to apply epoxy to one surface. Don’t apply too much otherwise it will ooze out of the assembly. It takes a bit of trial and error to get a sense of how much epoxy to use. For larger assemblies spread the mixture on both surfaces and then lightly clamp the setup for at least 30 minutes to ensure the bond sets correctly. Wipe up any excess epoxy before it hardens using a rag moistened with isopropyl or acetone. An overly tight bond between the two parts being epoxied isn’t necessary and is usually not recommended.

Epoxy also comes in a putty format that looks and feels like modelling clay. It’s useful for bonding and filling jobs around the home. You won’t get your hands all sticky using this stuff, though if you have dermatitis wear gloves. Simply cut off as much as you think you’ll need for the job and knead the putty by hand, making sure to wash your hands after kneading the epoxy. The epoxy is then ready to use. It’s pliable enough to push into holes, cracks and crevices, and around leaky pipes. Once cured it’s hard as rock.

There are several brands of two-part epoxy on the market. All are viable products distinguished primarily by bonding strength, delivery format (dual containers or syringe cartridge), container size and price. Once you get into larger epoxy pours for river tables there are some other properties you’ll want to consider. Most big box stores carry epoxy in the syringe cartridge format while specialty retailers primarily carry dual containers. And, of course, you can order just about all these products online.

I use epoxy for securing wooden handles to chisels, for bonding small bolts to shop-made door and drawer handles, for bonding metal nuts on shop-made jigs and fixtures, and whenever I need to bond dissimilar materials. You’ll undoubtedly find your own uses for this incredibly useful shop accessory.


Carl Duguay - [email protected]

Carl is a Victoria-based furniture maker and the web editor at Canadian Woodworking & Home Improvement.

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