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Fixing Cracks

Q: The back is separating, at the bottom where the strings attach, on my Ibanez F5 knockoff mandolin from the 70's. Also the side is splitting diagonally as part stays glued to the top and part glued to the back and the binding most of the way around appears crazed, cracked  and generally unsound. Other than that it's in pretty good shape :-). I've taken the strings off. My playing skills probably don't warrant the expense of sending it out to be repaired, but I'm a carpenter by trade with decent woodworking skills and would be willing to try the repair myself. Do you think I'm dreaming or is it doable for a wannabe luthier?
    Thanks in advance, Tom
A: It is unclear, from your description, exactly where these cracks are, so let me speak of the universal concept of crack repair. Wood changes shape every time the humidity changes. Joints that were perfectly fit by the original luthier can have stress build up in them. Sometimes the stress alone can break a joint or the wood around it. More commonly, impact damage releases the stress. Once, many years ago, I was playing fiddle in a country band. Dashing to get his equipment set up, the bass player hooked his foot in my cord and pulled my fiddle off the top of my Twin Reverb. The violin fell a mere two feet to the cement floor, but when it hit, it exploded! Every joint came apart.
       Cracks, once opened, resist closing. And, if the crack is not fixed quickly, continuing change in the shape of the wood makes the crack even harder to close. So, you may have to add or subtract wood to make the crack close. Remember that if the clamping pressure required to close the joint exceeds the strength of the glue joint you have wasted your time. Wood shrinks a lot from side to side and not much from end to end. Areas where two pieces of wood whose grains are going opposite directions, for example, where the top back and sides are glued to the tail block, are especially prone to cracking. If your back cracked because it shrank in relation to the tail block, you must take the back loose from the tailpiece, refit that joint and then reglue.
       Titebond or Elmer's Carpenters glue are easy to use and clean up and are plenty strong to close a well fit joint. There are specialty glues that work better for specific jobs, but lets not talk about them here.  Remember that the fresh glue in the joint acts like grease. A joint that seemed perfect during dry clamping can slip before it grips. Wipe away the excess glue with a wet rag and check the joint under bright light.  If it has slipped, panic, fix it before the glue sets.
       Cracks should be reinforced with small patches of the same kind of wood, glued on the bare wood on the inside of the instrument, with the grain 30 to 45 degrees from the grain of the crack. 90 degrees looks stronger but doesn't allow the wood to move, making the joint more likely to fail eventually.
Steve Mason

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