The Many Types of Wood Glues...
With all those other great glues, why should I bother having wood glue in my workshop?
Good question! You would think that with the chemical marvels described above a water-based wood glue would have gone the way of the dinosaur. You couldn't be more wrong. In its own element... that favorite of termites cellulosis dinnerosis (a.k.a. wood)... good ol' wood glue... is as strong or stronger than those other fancy-schmancy adhesives.
Why? A number of reasons...
There are three basic types of wood glues to be concerned with. There are of course variations on these and new glue products and formulations are popping up like mushrooms. So experiment if you like, but these three are recognized as the premier woodworking glues.
PVA (polyvinyl acetate)
PVA's are the traditional wood glues, and some of the most common adhesives. They are also called "aliphatic resins". They provide a strong wood-to-wood bond but will not adhere strongly or consistently to nonporous materials, such as metal or plastics. It is important to use clamps to hold the wood parts firmly while the glue is drying. Any movement after the initial "set", usually occurring in a few minutes, will cause breaks in the glue bond that will make it much weaker.
PVA's do not stick well to other glues, so they are of little value in furniture repair where old glue failure is the culprit... loose chair spindles immediately come to mind. Though the "pros" tell you to remove the old glue to give the new glue a chance to absorb into the wood, in reality it is often difficult or impossible to remove the old glue sufficiently to make a lasting repair unless the entire piece is disassembled. Instead, use a solvent-based adhesive, such as GOOP (see article on GOOP for more info).
Though widely used by woodworkers, PVA's are not really suitable as a primary adhesive or for edge gluing... attaching boards together to make wide surfaces such as tabletops or other furniture. They are "plastic", so without other supporting fasteners such as screws or nails, metal reinforcements or dowels, the glued joints will eventually break apart. For pro-quality wood gluing where extra fasteners might get in the way, read the sections in this chapter on polyurethane glue and hide glues.
One of the more well known PVA adhesives... for all parents and anyone who was ever a kid... is Elmer's White Glue... effective on paper but not very strong on wood. Yellow wood glues offer more strength and greater water resistance. Note that PVA glues are not completely waterproof, however.
Water-resistant PVA glues have all the benefits of regular PVA glue but have increased tolerance to moisture and extra mildew resistance. Because of these additional benefits, I use them for all applications PVA glues are suitable for rather than keeping both standard and water-resistant PVA in my larder.
Polyurethane glue is one of the best waterproof glues available, but until recently was not available outside professional circles. It is a one-part adhesive that will adhere to wood, metals, stone, ceramics and many plastics.
Polyurethane glue does not dry like PVA glues, but instead chemically reacts with moisture in the objects being glued or even in the air. This reaction causes an expansion of the glue, filling all voids and giving an exceptionally solid glue joint. If the material is dry, spraying a light mist onto it before gluing accelerates the curing process.
In many ways, polyurethane may be the best wood glue. It both accepts wood stains and sands well in thin coatings, neither of which are true for PVA wood glues. Most other adhesives act as a sealer on the wood surface. And removing these other adhesives can be difficult because they dry to a "gummy" texture that resists removal from the wood by sanding... the second strong reason to consider trying polyurethane glue for your next project.
Polyurethane glue has a longer setting time than PVA glues, so you have more time to adjust misaligned clamps and other errors before you reach the gluing world's version of the "point of no return".
Nothing is perfect and polyurethane glue is no exception.
Remember poly glue's expansion factor I mentioned a few paragraphs back, HMM? I wasn't exaggerating... overgenerous application of polyurethane glue can cause a messy situation as it emerges like the Blob from your joint!
And you can't just wipe it off with a sponge like PVA wood glue. Polyurethane glue is not water soluble, so always have your trusty can of denatured alcohol handy for the cleanup of your work piece and your sloppy hands! Use of protective gloves is not a bad idea if you are doing a large job. Thin latex gloves... like your beloved dentist uses to protect herself from the blight of your yucky cavities... are far and away the best gloves for this task because you still have some dexterity and "touch".
One other annoying characteristic of polyurethane glues is their proclivity to harden in the container.
This is associated with moist air becoming trapped in the container, beginning the setting process. This is not as much of a problem for small-time users but is an expensive nuisance for quantity buyers. Suggested remedies are pouring a small quantity of mineral spirits in the container when stored for long periods. This keeps the glue young and supple... just don't forget to pour the solvent off before using the glue!
Once dry, this stuff will not come off anything easily.
Most solvents are ineffective on dry polyurethane glue, leaving mechanical removal by either cutting or sanding as your only recourse. On hands or other unfortunate body parts, it will wear off eventually (as your epidermis sloughs off yesterday's cells) or you can accelerate the shedding process with an abrasive soap such as Lava. Or just leave it on your body as a badge of courage... "Me handyman, ugh!"
(Note: I know my editor has breathed a sigh of relief that I chose "dentist" as an example of latex glove use. Actually, I have always thought of dentistry as very similar to construction and "handymanship" in techniques, materials and goals... do it right the first time, use the best materials available, and make lots of money. OK... two out of three ain't bad!)
Hide glue is not a type of glue that is used where no one can see it (peek-a-boo, I glue you)... really! It is called "hide" glue because it is made from animal products. If you have ever heard the jokes (though not funny to everyone) about the failed race horse going to the "Glue Factory", this is the glue recommended by more jockeys (and cabinetmakers) than any other!
Hide glues are the "glue of glues" for cabinetmaking and furniture for a few good reasons. The glue can be formulated to have varying degrees of initial tack... the stickiness that keeps the glued parts in place. Some types of furniture assembly, for example, need a longer setting time so assembly can be completed before the first joint dries. Chairs are a good example of this. In other cases, such as the installation of reinforcing cleats under tables or edge gluing, a quicker set may be preferable.
Secondly, hide glue does not "creep". Wood pieces glued with hide glue stay put! And is it long lasting? Virtually all antique furniture is held together with hide glue. In fact, the use of hide glue goes back thousands of years to ancient Egypt!
Hide glue, unlike most other adhesives, can be reactivated with moisture. This is a mixed blessing. For furniture restoration purposes, it is a treat to be able to steam off the old glue... try that with most other adhesives and you will have little success. This is especially important for the restoration of wooden musical instruments such as pianos and guitars. Of course, this sensitivity to moisture can also cause furniture stored in damp locations to begin to fall apart... though certain formulations of hide glue and the finish of the furniture can slow or stop this sort of glue failure.
You will probably not find a true hide glue in your local hardware emporium. In our 'hurry up and get it done" society, hide glue has lost favor because it is not an easy product to use. It requires heating and mixing to get the right consistency and tackiness for the job. Consequently, the other adhesives mentioned earlier have displaced hide glue in most do-it-yourself applications.
There is a hide glue for the unwashed masses, manufactured by Franklin International, called Titebond® Liquid Hide Glue. Unlike the traditional hide glues, this is a one-step product... squeeze it out and use it! It shares many of the characteristics of regular hide glue... relatively long setting time, non-creeping grip, and can be loosened with moisture for furniture repair. I have used it myself and found it to be an excellent wood glue, with one problem... once opened, it does not have a long shelf life.