Polyurethane glue (a very popular brand is Gorilla Glue) is a great all purpose glue. It bonds anything to almost anything else. Specifically for woodworkers, it will bond wood to wood, or wood to metal very well. It can be used as an interior as well as an exterior glue. Once it has cured, it is not affected by changes in temperature. It dries prety much the same tan or brown color as you see it in the bottle (or in the picture here).
Another great benefit to polyurethane glue is that it is completely water proof. In fact, water helps it cure faster and stronger. The moisture content in the wood actually helps this glue work better. If bonding to a surface like plastic or metal, lightly spray the surface you are gluing with a mist of water to get a stronger bond.
Polyurethane glue has an open time of between 15 to 30 minutes depending on the brand. This is great when assembling a complicated piece of furniture. It provides ample time to get the clamps on properly and to adjust them into position. It will take at least ten hours to cure completely, so it is a bit slow in this regard.
Careful Of The Polyurethane Blob
When polyurethane glue cures, it expands in a foaming manner. If you have never used this glue before, it is surprising how much it foams up. You can wipe off the excess glue as it bubbles up with a dry cloth or thinner. Usually howver, if the proper amount of glue has been applied, the excess is not too bad and you should wait till it has dried and then scrape it off. A card scraper is a good tool to have for removing dried glue.
I caution you to check up on your glue-up every five or ten minutes for at least an hour to make sure the glue isn't expanding too much and running down your assembly. Otherwise, once this stuff dries, it is extremely hard to remove. It sands and stains well after it has dried, however, its best to get the runoff before, it'll save you loads of time.
Another caution about this glue is to make sure your joints are tight. Just because you see it foaming and filling gaps does not mean it is supposed to be used this way. In fact, it has no structural gap filling properties, as the foam is full of tiny air bubbles.
Its shelf life is a few years when the bottle has not been opened, less than one year after it has been opened. To make it last longer when storing an opened bottle of glue, try to squeeze some air out, and keep it away from humidity and moisture. Otherwise you will find that the glue will completely harder over time.
Is Polyurethane Stronger Than PVA Glue?
Don't necessarily believe the hype about it being the strongest glue neither. This is just marketing. When tested against the strength of PVA yellow glue on maple blocks, the polyurethane glue came out about the same for strength. So for wood to wood bonding, you might as well use the less expensive basic PVA white or yellow glue, unless the final product will be exposed frequently to water.
Why Use Polyurethane for Woodworking?
A very common use of this glue is on cutting boards (it is food safe after it has cured), or where getting moisture is common on the final product will not affect the glue up. It can't be used in glue-ups in underwater applications however, but this is not usually an issue with woodworking. Because it can glue dissimilar materials it is also a good choice for pen kits, where you are gluing wood to metal.
Overall, it can be used on almost all woodworking projects like PVA glue can. It is more expensive than PVA glue so it's most efficient use cost wise is for outdoor or high moisture applications.