There’s no question that advances in adhesives technology over the years have made life easier and more predictable for woodworkers. A byproduct of that, however, is a confusing variety of types and brands of adhesives and not a lot of clear information on how wood glues work and which ones work best for a given woodworking project. That’s too bad, because nearly every woodworker is utterly reliant on sticking things together with adhesives.
If you really want to learn how glue works, there’s a readable – but not dumbed-down article in the Woodweb Knowledge Base. Take a few minutes to read through Charles B Vick’s, "Adhesive Bonding of Wood Materials", and you’ll all of the important concepts along with a fun facts like:
“Physical forces of attraction composed of three intermolecular attraction forces are believed to be important to the formation of bonds between adhesive polymers and molecular structures of wood. Generally called van der Waal’s forces, these include dipole–dipole forces, which are positively and negatively charged polar molecules that have strong attractions for other polar molecules…”
Woodworking forums are a great source for information on woodworkers' actual experience using various glues. This Woodworking.com forum discussion of the merits of two popular waterproof glues, for example, contains this no-nonsense comparison of Gorilla Glue and Titebond III:
"Gorilla Glue is a polyurethane glue and TB III is an aliphatic resin glue.
Gorilla glue will adhere to more substances but TB III gives a better wood to wood bond.
Gorilla Glue cannot be washed off your hands…it must wear away while TB III cleans up with water.
Gorilla Glue is messy as it foams excessively upon curing. The foam is gap filling but has no strength. TB III does not foam and any excess can be wiped off with a damp cloth or sponge.
Unless you specifically need to fill a gap, TB III is probably the better choice as it's just easier and cleaner to work, especially on wood to wood gluing.
If you need to simply fill a gap without any strength required, use the Gorilla Glue. If the gap filled portion will be stressed, use Epoxy."
The most important question most of us ask about a wood glue is, “How strong is it?” The answer may be different than you think. Fine Woodworking magazine recently tested the core group of woodworking glues for strength in holding together a typical open mortise joint and published the results in their August, 2007 issue. It would be worth keeping a copy as a reference; the tests cover a good, representative range of common gluing variables, such as joints that fit a little too tight, joints that have a little slop, and difficult to glue tropical hardwoods.
Interestingly, epoxy and polyurtethane, two glues with a reputation for being super tough, turned out to be a disappointment in some situations. Others that you might rate as pretty wimpy, like liquid hide glue, held up very well. As it turns out, though, there’s a reason why good old yellow PVA wood glue and the more advanced type 1 waterproof glues, like Titebond III, are the glues of choice for so many applications – they're inexpensive, easy to use, and in just about every situation, the strongest.