soundboard shims & glue

[John W. McKone]

Here at our shop we (Richard Raskob and Myself) have been discussing the
proper glues to use when installing soundboard shims.  I thought I'd post
some of our ideas in the hopes that we could get some feedback from all of
you, and maybe find someone who has done some real quantifiable testing
along these lines.

The standard wisdom for glues whan working with soundboards seems to be
"the harder the better".  When we glue maple to maple on bridges, or maple
to spruce, the harder glue will more efficiently tranfer the energy to the
soundboard where we want it.

So far so good.

For years we have applied the same concept to soundboard shimming.  Hard
glue = good transmission.  Soft glue = bad sound transmission and should be
avoided at all costs.  We mostly have used quick set epoxy on our shims
because it is very hard, and  can be colored easily for a good match at the
seam.  We have also tried Garret Wade gap filling glue (good stuff) and
traditional hide glue as well.  The problem with these water based glues is
that they tend to swell a well fit shim to the point where it may become
impossible to install after the glue is applied.  The fact that epoxy
doesn't effect the shim much dimensionally is a big plus for us.  The big
minus with the quick set stuff is that it gives you a very limited amount
of time for glue and clean-up during  a job that calls for great care.  I
would like the luxury of more time.

So...

We have been trying a 2 hour epoxy lately and have been very happy with the
results, *except* it ends up being significantly softer when it sets.

This brings us to the point, and the question for discussion:  Is soft glue
in soundboard work such a bad thing?  Our conlusion at this point is that
it is in fact a GOOD thing.

OK class, here are the discussion points for the next time we meet...

1. Isn't the best glue to use one that is of similar hardness to the soft
spruce that it is gluing?  wouldn't a soft glued board react more like the
original board?

2.  Wouldn't a soft glue be more likely shift and flex with the board with
changes in humidity, thus being less likely to crack over time?

3. Isn't a hard glue joint a high impedence structure, more likely to
reflect sound back, than to pass it on to the rest of the board

4. doesn't most of the sound get from the bridges to the outside of the
board through the ribs anyway?

This is where we're at...

Comments please




John McKone, RPT
Operations Manager
Haugen's Pianos
Twin Cities