Using epoxy and poly glues

Above: All prepped for two-part epoxy, here the West System is in use.

Words and photos: David Luckensmeyer

Increasingly I’m leaving white and yellow glues behind (along with short open-times and associated glue-up stress) and reaching for two-part epoxy and polyurethane glue. Let me tell you how I use them.

Epoxy

Unlike white and yellow glues, epoxy does not grab on contact, and its longer open-time (approx. 20–30 mins) makes for more peaceful glue-ups. It exhibits outstanding waterproofing and gap-filling characteristics. I use it for all kinds of joinery, from chairs to tabletops, drawers, boxmaking, and so forth.

I particularly appreciate being able to control the viscosity of epoxy by using fillers designed to enhance gap-filling and strength. These fillers usually consist of microfibres mixed into the epoxy before use.

Perhaps the most pertinent downside is the perception that a two-part glue is ‘trouble’. Mixing is not as ‘easy’ as grabbing a bottle of ready-to-go glue. And care is required to dispense and mix correct quantities of resin and hardener.

But proprietary dispensing pumps, reusable mixing cups and stirrers take nearly all the pain out of the process. Use a variety of spreaders including disposable sticks, rollers or brushes. Make sure to use plenty of glue and only moderate clamping pressure. Excessive clamping leads to a glue-starved joint which is prone to failure.

here are two other downsides that matter. One is the very long curing time of most epoxies (usually eight hours or more). There are additives that can speed curing but they reduce open-times. The other is joint movement or ‘cold- creep’. Cold-creep is the mechanical movement of a joint over time, and yellow glues and epoxy exhibit more cold- creep than many other glues.

Polyurethane

Polyurethane glue does not suffer from mixing requirements or cold-creep. And it is also gap-filling and waterproof. I love that I can dispense it from a bottle just like white and yellow glues, and it has a relatively short curing time (2–4 hours). And because it forms a very rigid joint, it is excellent for laminating applications.

Poly glues foam and expand. This product is made in Brisbane by Boatcraft Pacific.

Poly has a similar open-time to epoxy (up to 30 mins). As a result, it is a joy to use for long or complex glue-ups. Because it is not grabby, joints can be unclamped, adjusted, and reclamped without any of the normal ‘hurry up’ stresses.
Alas, it does have several downsides to manage. For starters, polyurethanes are more toxic and makers must wear gloves and use adequate ventilation.

And then there is the foaming phenomenon. Unlike epoxy with its chemical curing process, poly cures through the introduction of water. The moisture content of timber is enough so it does not take much. But as it cures, poly foams and expands. Consequently, it is important to use less glue and strong clamping pressure. Squeeze-out is easily cut away at the bandsaw or scraped away by hand.

I now prefer to use epoxy and polyurethane over white and yellow glues in my furniture projects. Both have outstanding open-times, with epoxy edging poly for clean gap-filling joints (including tinted applications like resin-filled tables), while poly is the glue of choice for laminations because of its shorter curing time and lack of noticeable cold-creep. Consider pre- finishing before glue-up, at least partially. Neither epoxy nor poly likes to stick to freshly oiled surfaces, and the scraping and sanding of joints after curing is straightforward.

David Luckensmeyer is a Brisbane based woodworker and furniture maker, see www.luckensmeyer.com.au and Instagram @luckensmeyer