High quality CA glue (cyanoacrylate). This glue works very well in certain piano repair situations where a very thin glue is needed. The most common repair that I use this glue for is when I come across a piano that has tuning pins that are too loose to hold. You can put a few drops around the base of the tuning pin and because this glue is very thin it will wick down around the tuning pin and swell the wood in the pinblock around the tuning pin and then dry very quickly. The tuning pin will be ready to tune within a few minutes. Make sure that if you are applying this glue on a grand piano that you remove the action in case the glue drips down through the pinblock. If you are using this glue on a vertical piano make sure to tip the piano on it's back so the glue doesn't run down the plate.
Available in several different sizes: 1/2 oz bottle, 3/4 oz pocket bottle with screw on cap, 1 oz bottle and 2 oz bottle.
This glue also works well for hammerheads that have come loose from the shank. Apply a couple of drops of glue to the loose joint and within a few minutes it should be set.