Compression Question

[Delwin D Fandrich]

----- Original Message ----- 
From: "Bill Ballard" <yardbird@vermontel.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: August 30, 2003 9:35 AM
Subject: Re: Compression Question


> At 8:49 AM -0700 8/30/03, Delwin D Fandrich wrote:
> >I wasn't referring to film thickness. I was referring to the
effectiveness
> >of various materials as vapor barriers. Each panel was coated with the
same
> >number of coats of finish and tested for rate of shrinkage and
expansion.
>
> Stop me if you've heard this one before, but......is there a direct
> correlation between permeability and stiffness.

Not that I am aware of. At least not with the application proceedures
and/or film thicknesses normally recommended by the typical finish
manufacturer or formulator. Logically I would think that the thicker the
material (i.e., the more coats of something that you apply) the more vapor
resistance you would have though I have made no attempt to either prove or
disprove this. The only finish we found--following the coating proceedures
recommended by the manufacturer--that provided an effective vapor barrier
was epoxy (quite a bit of it). And, yes, multiple coats of epoxy did have
an effect on the stiffness (and the mass) of the panel. But then so would
multiple coats of most any finish material, I should think.


>
> And in this, is there
> not a trade-off between how well the finish operates as a vapor
> barrier, and the nature of its sound-carrying ability? In this case
> because of increased stiffness, resulting in a board which takes the
> string's energy and delivers a longer but quieter tone, rather than a
> shorter but louder tone. You did say quiet clearly BTW that the same
> number of coats of different materials yielded no significant
> differences in permeability, which may moot this question.

Yes. But we did not measure film thickness nor for variations in stiffness
or tone quality. We coated them, stuck them in the soundboard conditioning
room--which, at the time, was set to take them down to an nominal EMC of
4.5%. We then measured their sizes and put them out in the warehouse. We
periodically remeasured them to see how much they were changing over time.
We were trying to determine if we could improve the moisture-related
stability of the soundboard panel without increasing the cost of either
materials or labor. We did try some relatively exotic new finish materials
(for the time, about 1988 or so) that were rather costly but we stuck to
the same number of coatings in each case. (Well, except for one or two
that, for chemical reasons, could not be recoated--they were one-shot
finishes. One of these was a conversion varnish of some sort. I think there
was another but I don't remember what it was off hand.)


>
> This matter of choice of finishing materials is well beyond my
> situation of an under-two-year-old board with compression damage.

Yes, it is.

Del