Use the best glues in your next workshop project

Many of the advances we enjoy today in the workshop ultimately spring from innovations in glue chemistry. Sheet goods depend on it, biscuit and dowel joinery couldn't happen without it, and woodworking everywhere is simplified when you can rely on glue to hold parts together forever. Today's top glues are long-lasting, very strong and comparatively inexpensive. Here's what you need to know to stock up on the best.

Carpenter's glue is the king of benchtop adhesives. White glues come from a chemical family abbreviated as PVA (short for polyvinyl acetate). Yellow or brown formulations are usually aliphatic resins. They're generally easier to sand and somewhat stronger. Either way, you can't beat ordinary carpenter's glue for typical wood-to-wood joints on interior applications. It's strong, cheap, easy to work with, cleans up with water, and is non-toxic, fume-free and available everywhere.

As good as it is, carpenter's glue doesn't work well around moisture-its Achilles heel. Manufacturers are fond of describing their wood glues as “water-resistant” but don't be fooled. All this usually means is the product won't dissolve immediately after getting wet. If you're building outdoor projects, you'll need something more than just water resistance, because prolonged periods of wet weather will turn ordinary PVA and aliphatic resin glues to mush.

The three most practical choices for weatherproof outdoor duty include epoxy, polyurethane glue and, my general-purpose weatherproof favourite, cross-linking PVA. Strictly speaking, most versions of these glues are incapable of standing up to prolonged exposure below the water line. Strength in the face of wet weather is their thing.

Epoxy comes in different formulations that offer curing times from five minutes to 24 hours. It's strong and completely weatherproof. On the downside, epoxy is relatively expensive and messy. You have to stir two goopy components together to trigger the curing action. Despite these annoyances, I always keep tubes of five-minute epoxy on hand. You can't beat it for quick repairs on wood and non-porous materials.

 

You'll find polyurethanes in everything from wood finishes to weather sealants. Polyurethane glues are single-part adhesives that look like extra-thick maple syrup. Poly glue appeared on this side of the Atlantic about 10 years ago, and it's here to stay for three reasons. Polyurethane does a terrific job of bonding wood and non-porous materials such as metal, plastic and glass. It also foams as it cures, offering a small amount of gap-filling power in slightly sloppy joints. Strength drops off quickly as joint gap increases, though with small gaps the bond is still more than tough enough. Hardened squeeze-out from polyurethane glue also sands away without clogging abrasives and even takes stain passably well.

Cross-linking PVA is another weatherproof glue. It cleans up easily with water as long as it's still wet. But this water solubility is a one-way street. Once cross-linking PVA cures, it be-comes more or less insoluble. It's currently the cheapest, most widely avail- able weatherproof wood glue. Look for versions that carry Type II and Type III water-resistance ratings.

So why does anyone bother with ordinary PVA when cross-linking versions are so good? Shelf life and cost. The chemical subtleties that make cross-linking glues weatherproof also make the glue go stale in the bottle sooner than carpenter's glues.

There are at least 40 kinds of glue in the world. Luckily, you only need a few to make your workshop projects stay together for a very long time.

Type II and Type III adhesives are ideal for projects exposed to exterior conditions. This is the latest formulation of single-part weatherproof adhesives made specifically for wood. White carpenter's glues have been around for decades and they still deliver strength, ease of use and water cleanup. Extra-thick, gap-filling wood glues are the premium choice for interior wood projects.

Polyurethane glue is a single-part, weatherproof adhesive that also sands well and takes stain better than others. Five-minute epoxy is a workshop must-have because it combines strength, rapid cure times and the ability to bond a wide range of materials.


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