Luthiers Mercantile International, Inc.

1) Choose the right glue for the job! This is a big topic, and we have covered it in a previous blog, but please bear in mind that you do not want to substitute the wrong glue just because you've run out of the appropriate one. The results could be somewhat disastrous, even though they might not creep up for many years. Remember we are building (and repairing) instruments that are meant to last a lifetime. 

2) Meeting parts. Nothing is more critical than making sure that the parts you are gluing together fit perfectly. You never want to assume that it is okay for glue to fill any tiny gaps between the two parts you are gluing together. Over time these little 'glue bubbles' can destabilize the whole joint by inviting cracks to form and spread in the glue. If you are using clamps to press together parts so that they move into contact, then you are introducing stress that may backfire later on. You don't want to get comfortable with glue lines showing between mitered parts like binding and purfling. They should be essentially invisible. These joints are the first thing people look for when evaluating the "fit and finish" of a guitar. 

3) Surface preparation. There is a debate about whether wood pieces should be 'scuffed' prior to gluing or not, but the science is clear. A flat, smooth surface is best because the glue does not need to be thick to work well. Scuffed surfaces will have microscopic voids and ridges that collect glue. If you can get there with a plane or chisel blade, that is the best surface for gluing. Otherwise, be sure you have sanded past a 220 grit paper. Be sure you do not try and glue a burnished surface, which can sometimes appear shiny like a cleanly planed surface. 

4) Clamping. Visit any professional luthier's shop and you will inevitably find a cornucopia of clamps. It seems that you can never have too many! Good, even pressure over the entire joint is optimal. Cauls (can't have too many of these either!) help distribute pressure, but there is no substitute for direct pressure from the clamp or clamps. Always do a 'dry run' before any sensitive glue-up to make sure you fit your clamps on and around the workpiece and get them placed quickly and efficiently. Of course, this is especially true for glues with a short open time like hot hide glue and cyanoacrylate. Clamps should go on firmly and tightly, and there should always be a little squeeze-out (excessive squeeze-out is just messy and wasteful). If you tighten too much then too much glue will be forced out, 'starving' the joint. If your parts are will mated then you should not need much pressure at all. 

5) Environment. When you are gluing parts if is important that you are cognizant of your environment. Extremes of temperature and/or humidity can cause your wood to warp and move, sometimes in subtle ways that will not be apparent until it is too late. Some cyano glues want to be refrigerated when you are not working with them, but in general, you want to store your glue in a well-sealed container inside of a cool, dark cabinet. 

Happy shopping!

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