In my very limited instrument repair experience consisting of two violins and a harp I used hide glue. Depending on the age of the instrument - if fairly new I'd try a new product out there which is hide glue advertised needing no pot or mess, pretty much straight from the botttle. I'd have to thumb through some back issues of my mags to find the ad, but hope someone recalls seeing this also. Hide glue I believe has more give and flex needed with the more obtusly or cross grain construction of many instruments. I do know you don't want to use epoxy (to rigid), or anything that will expand and displace stresses. Keep in mind your dealing with larger but thinner solid woods that need to move. I wonder if this movement is in part why the back and top (face)aren't flat.
For fine instruments, hot hide glue is really best. Oddly enough, bowed instruments are designed to come apart relatively easily by working a thin blade between the back and sides (they call the side assembly the garland).
So the proper way to fix it would be to remove the back, wash off the old glue by hot water and Q-tips and judicious scraping, and then reglueing.
For small repairs it is possible to do it without removing the back and then cleaning the glue as best you can.
Clamping is necessary, particularly on less expensive instruments that might be made out of unseasoned wood that have warped and were put together under protest...(fine instruments are often made of wood that has been seasoned for 25 - 50 years!)
I would recommend the hot hide glue, it's actually easy to work with and very strong. I recently read on the musical instrument maker's forum (Google MIMF) that Knox unflavored gelatin is essentially food grade hide glue in handy packages.
If you decide to do the repair with hide glue you might ask for tips at the MIMF.
Ask about using gelled CA glue if you don't want to use hide glue, CA glue is used in guitar repair (which I'm familiar with).
Good Luck with it!
P.S. Luthiers spool clamps are easy to make out of closet rod, 1/4-20 threaded rod and wing nuts.
Actually the stresses on some of the major glue joints on a guitar are far greater than on bowed instruments. It's a function of their construction. Guitar strings originate on the bridge, which is glued to the surface of the guitar soundboard directly and must resist the considerable string tension trying to pull it (and the soundboard) off.
On bowed instruments the bridge sits (unglued) atop the soundboard, the strings pass over the bridge and apply their force downward due to the fact that they are fixed to the tailpiece which is in turn looped around the endplug.
So string forces are trying to pull the guitar apart and are trying to press the violin, viola, cello or base together. The adhesive requirements correspond. One exception, Archtop (Jazz) guitars are made in the same way as bowed instruments.
Old guitars and some classicals still were/are made with hide glue. But guitar construction is not as bound to tradition as bowed instruments and new materials and adhesives are always being tested.
Also, it is uncommon to remove the soundboard or back of a guitar as many repairs can be accomplished through the soundhole, whereas with a violin etc, if any of the linings or the bassbar become loose, there really is no access to fix them unless you pop the back off.
I believe [link:www.tools-for-woodworking.com/index.asp?PageAction=VIEWPROD&ProdID=6449&HS=1|thi s] is the 'ready' made glue people are talking about. I talked to the guy who makes it, and he told me that it has a long open time, and it has to be clamped for something like 48 hours :o I decided against using it on my last project.
As for the type of glue, I really don't think it matters much. If this is a cheap Chinese made bass, they probably used whatever glue was cheapest when they made it :)
The reason most instruments are made with hyde glue is that they are under a lot of stress, and are very expensive. Therefor, being able to take them apart and repair them is more important than initial strength.
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