(More than) a few words about glue....
Glue - it’s what makes guitarmaking possible! Without it, we wouldn’t get very far.
In my shop, I use many different glues for different purposes.
Each plays its role, and my job is to understand the specific strengths and weaknesses of each so I can apply them in the right places. Here’s a bit about the 5 different glues I mostly use - what they are, where I use each one, and why:
-Yellow Glue: the most common glue for most builders, and a very viable choice for many applications. Yellow glue (such as Franklin Titebond, the most common) is quick, easy to use, shelf-stable, and holds fairly well. It can have a tendency to ‘creep’, meaning that joints will shift under pressure over time, but nonetheless many perfectly good instruments have been built entirely with yellow glue. It can be disassembled with dry heat (very useful for the repair person), though it does not reactivate like hot hide glue. I use yellow glue for gluing tops and backs to acoustic rims, for bindings and purflings, and for a variety of other miscellaneous parts of the guitars.
(A word here about the Titebond family of glues.
-Titebond I (aka “Original”) is the best suited Titebond glue for guitarmaking. It dries hard and brittle, and thus does not “damp” vibrations between adjacent pieces of wood.
-Titebond II has additives which make it moisture-resistant for outdoor applications. Great for laminating curved deck railings, but not so great for guitar joints. These additives result in a rubbery, somewhat elastic texture to the dried glue, which will damp vibrations in the finished instrument. This rubbery glue line is also much more likely to creep or shift under string tension.
-Hot Hide Glue: the most traditional instrument adhesive, hot hide glue remains one of the best for its unique properties. Hide glue resists shear stresses and ‘creep’ better than many modern adhesives, and is less susceptible to the ill effects of dry heat (such as being left in a hot car). Old hide glue can be “reactivated” with moisture and heat, and will rebond to itself and amalgamate with new hide glue, which makes it the natural choice for conservation-grade repair and restoration. Finally, hide glue joints have a noticeable positive benefit for the sound of acoustic instruments, specifically when used for bracing. All my top and back braces are glued with hot hide glue. I also use it extensively for repairs, and it is the only adhesive I will use for neck resets due to its serviceability and strength.
-Fish Glue: A protein-based glue similar to hot hide glue, fish glue has the great advantage of being liquid at room temperature. Another benefit is the strong immediate “tack” which helps hold thin/small pieces in place, and the fairly long working time. Fish glue can dry to a perfectly invisible line, which makes it a great choice for tricky repairs where the assembly time may be too long for hot hide glue. The only downside to fish glue is that it is sensitive to high-humidity conditions and may come apart if exposed to excessive moisture.
-CA (“Krazy”) Glue: Fast, strong, clear and nearly universal in their adhesive compatibility (i.e.,they stick to almost anything!), the CA family of glues have become essential to most modern builders. Though not often used in major structural joints, CA glues have enormous utility for decorative details such as binding, purfling, rosettes and inlays. They are also the basis for a wide variety of repair techniques, both on wood and on clear finishes (lacquer and modern “poly” finishes can often be repaired almost invisibly using CA).
The watery, “thin”-viscosity CA glue is very useful for its ability to draw into cracks and porous materials. Unstable or fragile woods such as spalted maple and buckeye can be “stabilized” by soaking them in thin CA. In cases where cracks or splits are too tight to apply glue inside the break, water-thin CA is sometimes the only choice as it can “wick” into the crack.
CA family glues can also be used as sealers and even finishes for small parts or details. Woodtuners have long used CA glues for this purpose.
-Epoxy: Long considered a dirty word for instrument makers, epoxy is in fact an extremely useful and sometimes necessary asset for the modern instrument maker. Its moisture-resistance (proven by decades of use in marine applications) and considerable strength make it an obvious choice for certain joints which should never need disassembly. Epoxy also shines (ha!) as a pore filler and sealant. In my work, its greatest advantage is the ability to glue carbon fiber composite materials, which I integrate into many of my instruments.