Panel drying and finishing

[David Love]

Quite a treatise.  Actually my question was much simpler and I was just
wondering about putting a warped panel into a rib press and more
specifically would it make any difference in how the panel reacted
ultimately.  This following a suggestion someone made a week or two ago
about finishing the ribbed side before ribbing so that the glue could be
wiped off more easily.  On the surface, so to speak, it seemed like a good
idea, until I started thinking about the warp problem if the panel wasn't
preconditioned before finishing.  As for my own method, they are RC&S boards
so the crowning issue is a non issue.  I don't finish either side first
because I don't really care about a few minor glue smears on the bottom of
the board underneath a finish but I'm always open to easier ways to clean
up.  I do finish the bottom of the board after ribbing before it's glued in
(of course masking off the areas of rim contact) and I while I have
conditioned the board again prior to installing it sometimes, I don't
always--another area where there is some disagreement--I haven't noticed any
difference.  As for squeeze out, I think if you don't have squeeze out along
the entire length of the rib on both sides, you're not using enough glue.
More danger there than in having a too much.  

David Love
davidlovepianos at comcast.net 
www.davidlovepianos.com

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Kendall Ross Bean
Sent: Friday, October 24, 2008 6:30 PM
To: 'Pianotech List'
Subject: RE: Panel drying and finishing

David~

Prefinishing before ribbing seems to me to be a rather complicated way to
avoid having to clean up/deal with glue squeeze-out. The quick and dirty
answer to that problem for me would be: "Don't use so much glue!" ;-). Of
course, easier said than done...

But back to your original question...

I think the issue here is really what sort of EMC changes the board is going
to be exposed to between the time you finish one side and the time you
finally get around to finishing the other side, and how you can either make
that work in your favor, or against you.

As you are probably already aware there are a number of different ways, and
orders of operations, of gluing up a soundboard, and finishing it. Some
bellymen (excuse me, bellypersons) are very particular about the order of
refinishing of the soundboard sides. (Others say it doesn't matter). A lot
depends on the order of the other operations, and how careful you are to
keep the board at a constant EMC.

(Believe me, when I was setting up our soundboard operation I talked to a
LOT of soundboard people... I ran into some weird theories, like "When you
spray or brush finish on a soundboard, that side expands because the finish
is absorbed into the pores and they swell..."  Well, maybe with water based
finishes... ...I don't think it is the case with solvent based finishes
because the solvent evaporates too fast...)

As you have observed, wood finished on one side but not the other does tend
to warp when the EMC changes. (Obviously, if the EMC stayed the same, this
would not happen.) But your question was, why?

While the lacquer or shellac or varnish used to finish the soundboard (or
any other piece of lumber) is not an impregnable moisture barrier, the fact
is is that it slows down moisture absorption significantly on the side of
the panel it happens to be on. If you were to finish the convex side of the
soundboard first (i.e. the bridge side) and didn't finish the concave side
(the rib side), AND THEN LET THE SOUNDBOARD SIT AROUND OUTSIDE OF THE HOT
BOX for a long time, obviously the concave side (the rib side) is going to
take on moisture faster than the convex, and as a result, crown will tend to
diminish.

How much, is another question.  

The fact that there either are, or are not, ribs on the convex side of the
board at this point (depending, again, on order of operations), will have a
lot to do with how severe this loss of crown is, because naturally the ribs,
with their grain crossways to the soundboard grain- (-on conventional
boards! Have to qualify everything here-)-will tend to resist that loss of
crown. If there are no ribs on yet, definitely the loss of crown/warping
will be more severe. (-Depending on the moisture differential, of course.)

(Another related question we won't get into here is: how might finishing one
side of the board beforehand affect the pressing or crowning operation? Will
the panel bend as readily? Will subsequently bending/crowning the panel harm
the finish just applied? Etc. Etc. But as I said, we won't go there...)

The whole issue is really "which way would you rather the soundboard "warp"
(or bend or crown up)?" Put the finish on the side first that you want to
expand the least WHEN EXPOSED TO MOISTURE CHANGE (which in this case should
be the rib side...)

If you are reading between the lines here, you will note that all of this is
pretty much dependent on EMC change, which is why soundboard makers try to
keep the board in the hot box as much as possible (unless they are making
the boards in Colorado, Arizona, Utah, Nevada or New Mexico, in a desert
environment... ...where often they don't even need a hot box...)

If you can keep the EMC constant, I guess it really doesn't matter that much
the order of finishing of the soundboard sides.

The way that we installed soundboards, we relied on the board "crowning up"
additionally after it was glued in the grand rim. This will happen much more
readily if that side of the board doesn't have a finish on it yet (as in
finishing the bridge side of the board after it is glued to the rim, rather
than before). But you can have a finish on both sides of the board before it
is glued in the rim, and it will still crown up after being installed in the
rim, just  more slowly. (Like I said, the finish isn't impregnable, but the
fact that it slows the moisture absorption down significantly, relative to
an unfinished side, is what makes the wood bend. 

This would be the main reason for finishing the rib side of the soundboard
first, if in your operations you anticipated the soundboard subsequently
being exposed to an EMC change outside the hot box and tending to warp (or
crown) one way or another depending on which side you refinished first. The
unvarnished/unfinished side will always absorb moisture more readily or more
quickly than the finished side. Just make sure that the order in which you
finish the sides or surfaces gives you the results you want: More crown, or
less. Obviously many other factors about the soundboard making process must
be taken into consideration.

More often it's simply a practical matter: It's simply lot easier to finish
the rib side of the soundboard before it's installed in the piano, behind
all those beams or backposts.

There are several factors that contribute to soundboard crown:

1)Laminating it in a curved mold (i.e. gluing on the ribs or bridge(s) while
the soundboard is pressed into or over a curved dish or cauls). (Depending
on whether a hot box is used or not, this may or may not be dependent on EMC
differential) (Using EMC differential alone to crown a board, soundboards
have also been pressed on perfectly flat surfaces...)

2)Expansion against the confines of the grand rim (This is dependent on EMC
differential). (You could add to this the beveling of the inner rim and the
careful fitting of the board to the rim, so that the mechanical mounts help
support the soundboard crown or arch. The latter is not really dependent on
EMC differential.) 

3)Expansion of the soundboard (cross grain) against the ribs (with the
grain) (i.e. different rates of expansion due to grain orientation)(This too
is dependent on EMC change)

4)One surface (finished side) absorbing moisture less readily than the
unfinished surface (also dependent on EMC) An instance of this would be if,
say, the board were allowed to sit after gluing in the rim for a couple of
weeks, with no finish on the bridge side of the board, but the ribs side
finished.

5) Downbearing on one part of the board causing it to "arch up" in another
(Such as strings being tensioned in the treble part of the bridge first, to
cause there to be more crown in the area of the bass bridge.) 

Generally in my experience it is much more practical to finish the rib side
of the panel and ribs together after they (the ribs) have been glued on. (We
did however mask the border of the soundboard that would be later glued to
the inner rim and belly rail.)

I had a friend who had one of the more recent Mason Hamlin BB's (about 15-20
years old). He was having trouble with it going out of tune. It was worst
when the lid was up, for some reason. Ultimately he discovered that the
piano had left the factory with the underside (rib side) of the board
unvarnished/unfinished. He got under there and put a finish on it, and it
greatly alleviated the problem.

Hope this helps, just sort of thinking out loud here.~

Flame suit in place...

Kendall Ross Bean

PianoFinders   

-----Original Message-----
From: David Love [mailto:davidlovepianos at comcast.net] 
Sent: Friday, October 24, 2008 2:06 PM
To: 'Pianotech List'
Subject: Panel drying and finishing

I have heard of some individuals who prefinish the side of the panel to be
ribbed first masking off the area that will be in contact with the inner rim
as well as the ribs.  I have not done this yet the idea of slightly easier
cleanup of glue without it soaking into the wood near the ribs is tempting.
However, I have found that when a piece of wood only has a finish on one
side and you change the EMC, it tends to warp.  First, why is that and does
that tendency suggest that the one side finishing process prior to ribbing
is a bad idea?

David Love
davidlovepianos at comcast.net
www.davidlovepianos.com