Advanced sound board repair information by Delwin Fandrich

Advanced sound board repair information by Delwin Fandrich

Posted by Delwin Fandrich on 24th Nov 2019

By Delwin D. Fandrich, RPT
Puget Sound, WA Chapter


In Parts 1 and 2 of this series I wrote of the characteristics of the soundboard that change
with time and exposure to the natural elements, specifically their loss of stiffness as the
crown originally built into them dissipates due to wood fiber compression damage and
compression set over time. Many have observed a decrease in tone sustain time as a
soundboard loses this crown. I introduced the idea that the loss of sustain time is not
really due to the loss of crown, but to the decrease in across-grain stiffness that causes that
loss of crown. Finally, I also introduced the possibility of restoring at least some of that
stiffness by saturating the soundboard surface with a thin penetrating epoxy. This month
we'll continue that discussion and then describe in detail the process involved.

One difficulty with wood – especially a softwood such as
spruce – as a soundboard material is that it is not at all
good at resisting compression across-grain. Wood fibers,
regardless of species, are only capable of withstanding even
short-term compression levels up to approximately one
percent without sustaining some damage. Even at this
relatively low level of compression, damage in the form of
compression set will occur if the compression level is
maintained over a long period of time. In the compression-
crowned soundboard panel the compression level
frequently goes well above two percent (assuming the
relative humidity in the air surrounding the piano gets high
enough – and it often does) so it shouldn't surprise us that
the wood fibers in soundboard panels inevitably end up
exhibiting various signs of compression damage. Once
wood fibers have been damaged by compression there is
nothing we can do to reverse that damage.1Sometimes soundboard compression damage is visible
to the naked eye in the form of compression ridges and the
resulting visible cracks. Often, though, fiber compression
damage is not itself readily visible but its effect can easily be
seen. Wood fibers that have been deformed by compression
no longer have the capability of developing the stress
interface against the ribs required to form and/or sustain
soundboard crown, especially crown that must work against
string downforce.

Since we can't restore the wood fibers to bring back
crown we're going to approach the problem of stiffening the
panel from another angle. We're going to saturate the wood
fibers near the surface of the soundboard panel with a very
thin epoxy resin. In a sense, we're going to turn the
soundboard panel into a fiber-reinforced sandwich.
The physical characteristics of coating epoxy resins are
ideal for this purpose. It is exceptionally strong in compression
– the compressive strength of the recommended epoxy
is approximately 15,000 psi. Coating epoxies are designed to
penetrate the surface of wood and saturate the wood fibers at
and near the surface – just the physical characteristic we
need for our purpose. By saturating the top surface of the
soundboard panel with a very thin epoxy resin we are going
to actually give the wood panel some of the physical characteristics
of a laminate. We will be greatly improving its
structural integrity by filling and repairing the maze of
cracks inherent in these soundboards, we will be at least
slowing any further compression damage, and we will be
making the soundboard assembly considerably stiffer. It is
ironic that those areas of the soundboard panel that have
sustained the greatest compression damage will end up being
both stronger and stiffer than other areas of the panel. And
we will do all of this without adding much weight to the
assembly. (See Figure 1)

Figure 1:



The Technique

I first used this technique some 25 years ago. A client
found herself in the position of having to care for and
support her very young family using the only marketable
skills she had available. She was a good pianist and an
excellent teacher. Her piano was a recently inherited
Steinway Model L. While she had been saving to have the
piano fully remanufactured, all of the work planned wassuddenly no longer economically possible. Unfortunately,
the soundboard was a maze of compression ridges and
cracks – so many that opening them and filling them all
was quite impractical. There was no discernable positive
crown; indeed, through the main part of the board some
amount of negative crown was evident. I had been investigating
the wooden boatbuilding techniques mentioned in
Part 2 and wondered if something like this couldn't be
applied to the piano soundboard. I described my idea to
my client and, with her encouragement – she was desperate
– I ended up treating the soundboard in roughly the
manner described below. The results were far better than I
expected and the performance of the piano continued to
exceed my expectations until I eventually moved from the
area and lost contact with the owner.

I have subsequently refined the technique and have used
it on a number of other pianos in similar condition. The
process does restore at least some of the stiffness lost in
compression damaged soundboards. While most of these
boards probably should have been replaced, in each case
circumstances ruled this out; either there was simply no
budget for the new board or the piano simply wasn't worth
the investment. More recently I have also used the technique
on soundboards that still had measurable crown and
seemed to be in good physical condition but which exhibited
the acoustical symptoms related with a general lack of
stiffness.

My partner and I have discussed the possibility of
treating only certain parts of the board, such as just the treble
area, as a localized treatment for specific tonal problems. We
have not yet tried this but I'm sure it's only a matter of time.
The technique described below is reasonably simple and
obvious once you are familiar with the material used. While
each piano may present its own unique variations, the
following steps should get you through the process without
catastrophe.1 – Remove the original finish.Remove all of the original finish – preferably by scraping,
though chemical strippers can be used. I'm not concerned
about the effect strippers might have on any real or imagined
acoustical properties the wood may possess. For all
practical purposes, having already suffered many years of
compression at various levels, the wood fibers making up
the original soundboard panel are already pretty much shot.
I am, however, concerned with the ability of the wood
fibers to absorb the epoxy that is going to be applied later
and the residue of chemical strippers may interfere with that.
If chemical strippers are used, you must get rid of all traces
before epoxy-saturating the wood surface. Thoroughly wash
the surfaces with lacquer thinner – or even better, with
straight acetone – to remove all traces of the stripper.2
2 – Scrape or sand the surface smooth.When thoroughly dry, scrape or sand the surface to expose
clean, bare wood. Again, scraping the surface is probably
best – it leaves a cleaner and more open wood surface – but
we have also had good success with sanded surfaces.
Much has been written about the benefits of a scraped
surface as opposed to a sanded surface. The scraper is
supposed to leave the pores more open and more able to
accept whatever chemical mixture we're putting on,
whether it be glue, some finish material, or epoxy. At least in
the case of epoxy saturation I have found no practical
difference between the two techniques. Use whichever is
the most practical for you and for the particular soundboard
on which you're working.

It is not necessary to fine sand the surface at this point.
In fact, from my experience so far, it appears a final block
sanding with 100–grit or 120–grit paper is better than going
all the way to 220 or finer. A random orbital sander will
make the job go reasonably fast.

3 – Dry the board and force in as much crown as possible.

The easiest way to dry out one of these old boards is to
simply drape blankets around the outside of the piano and
place a small space heater under the board. Turn up the
heat and let it cook for a few days. I prefer a baseboard
style heater of the oil-filled variety because it won't get as
hot as the more common wire element or ceramic heaters.
This is not for the soundboard's sake as much as for my
own piece of mind – a fire is much less likely if something
inadvertently falls directly on the heater. How long it will
take to dry out the board will depend on your environment,
how well insulated the area around the piano is, the
original moisture content of the soundboard panel, etc.
You'll have to use your own judgment; it could take from a
day or two up to a week. Any longer would be overkill
under any circumstances.

Once the board is as dry as you're going to get it, insert a
series of wedges between the ribs and belly braces as close to
the bridge line as possible. You want to force as much
upward curvature into the board as you can without damaging
anything. You should be able to end up with the board
standing about 10 to 15 mm proud at the center of the
soundboard. Obviously, this will taper out to somewhat less
than this away from the center. Try to end up with a reasonably
uniform curve. You won't be able to force much of a
curve into the treble area so don't try overly hard. Fortunately,
it's not really necessary. It's easy to damage the board
and/or ribs if you go too far with this; use good judgment
and be gentle.

4 – Ensure structural integrity.Check to be sure all of the soundboard panel's boards are
glued securely to the ribs. Make any repairs necessary at
this stage.

5 – Repair any obvious and/or serious cracks.All really wide cracks get filled with an epoxy matrix made
up of liquid epoxy and micro-fibers and colored with
fresco powders or universal colorants. To get a better idea of
color, wet out an area near the cracks in question with
acetone or lacquer thinner.

Using a good quality masking tape, tape off the
soundboard immediately adjacent to the crack and, using a
flexible putty knife, work in as much of the colored epoxy/
fiber matrix as possible. Don't worry if it comes out the
bottom of the crack. In fact, if the crack goes all the way
through the board you'd like it to go all the way through.
Allow this epoxy fill to semi-cure, remove the tape, and
scrape the surface clean – top and bottom. If you can get a
smooth surface at this stage you are ready for the coating
epoxy. Otherwise you'll have to wait until the epoxy in the
cracks has fully cured so you can block sand the surface flat.
If you have predicted the color of the finished soundboard
closely the finished repair will blend in fairly well.3
6 – Coat the soundboard surface with coating epoxy.Coating epoxies are simply clear epoxy resins that are
formulated to be thinner and have better penetrating
characteristics than are typical with normal structural
epoxies. Several companies make epoxies that are formulated
expressly for coating purposes. The product we use
most often is System Three Coating Epoxy.4 Several
others are available, but whatever you use should be a true
coating epoxy. Avoid epoxies that have been thinned using
solvents. The formulator may not tell you if solvent
thinners have been used, but they will nearly always tell you
if they have not. Pick a coating epoxy that does not use
thinners.
Caution: Whenever working with epoxies you must protect
your hands and skin from contact with the uncured mixture.
You can do this with either latex (surgeon's) gloves or
one of several creams on the market. Read the directions!
With your hands protected, mix the epoxy according to
the manufacturer's instructions. I know this should be
obvious, but it bears repeating: read the instructions before
mixing! It seems that each and every epoxy formulator
insists on using a different mix ratio. And just to keep you
alert they will sometimes change the mix ratio of a material
you are already familiar with. Once, when we had run out
of our System Three Coating Epoxy we picked up a substitute
from a local marine supply store. To make mixing easier
we also purchased the handy little pumps labeled to go with
the brand, epoxy type and can sizes we purchased. Unfortunately,
the mix ratio specified on the cans was 3:1 while the
pumps were set up for a 2:1 ratio. Check the mix directions
on every batch just to be sure.

Don't ever attempt to speed up the cure time of epoxy
by adding extra hardener. Rather than accelerating the cure
you may end up with a mix that will never fully cure.
Besides, for this procedure to work best you want the cure
rate to be very slow. This is one of the advantages of using a
coating epoxy – it cures very slowly and penetrates quite
deeply into the wood surface. The whole idea is to allow the
epoxy to penetrate the wood surface as much as possible and
saturate and fill the entire maze of tiny, mostly invisible,
cracks that have developed over the years. During the first
hour or so you should go over the surface with more epoxy
as it saturates the surface. Any areas that look dry should be
brushed over with more epoxy until the entire surface has a
good wet look.

7 – Apply a second coat.With System Three Coating Epoxy I automatically apply a
second coat. The additional build adds stiffness to the final
system. But, unless your timing is right, a second coat can
take a lot of extra time. The first coat should have gone on
in the morning. You'll want to check the cure progress
through the day and, just as the first coat gets thick and
tacky to the touch, paint on the second coat. At this stage
you can simply brush on a second coat and it will chemically
bond to the first. Otherwise the first coat must fully
cure and be scuff sanded to provide a mechanical bond
between the two coats.

The initial coat of epoxy will leave the soundboard
surface quite rough. Even applied directly over a partially
cured and quite rough first coat, the second coat will flow
out considerably better and leave the surface reasonably
smooth.

With some epoxy formulations you may be able to skip
the second coat and at least some of the cleaning mess. MAS
epoxies do not leave the common waxy film on the surface
as they cure. With MAS epoxies you can simply paint epoxy
on the surface as you see it being absorbed into the wood
surface until you have a fairly smooth and uniform coating
over the entire surface and you see no more evidence of
epoxy/wood penetration. MAS coating epoxy is a bit
thicker than the System Three formulation, but its cure time
is quite long. It still penetrates reasonably well though not as
well as the thinner System Three Coating Epoxy. When
using MAS epoxy we simply flow on the one coat, let fully
cure (this takes about a week to ten days), and dry sand it flat.
Next month, in the final article of this series, we will
complete the soundboard repair and draw conclusions.

Notes

1.For a more complete discussion of the strength characteristics
of wood in general and wood fiber damage due to
compression in particular, see "The Mechanics & Strength
of Wood & Wood Structures – Part 1," April, 1996 and
"The Mechanics & Strength of Wood & Wood Structures
– Part II," June, 1996 in The Piano Technician's Journal.
2.It is assumed that you will take appropriate measures to
protect yourself from the fumes of this stuff. Solvents
such as acetone and lacquer are very damaging to your
lungs and to your body in general. Use them only in
well-ventilated areas. And that means forced ventilation,
not just an open window.

Most of these products are also potentially damaging to
your skin. Wear protective clothing and gloves.

3.You can, of course, use wood shims and various inserts to
repair these cracks if you wish. Personally, I have more
confidence in epoxy fillers. Remember, these repairs are
simply cosmetic. Regardless of the method chosen, and
whether or not the surface is epoxy-saturated, repairing
soundboard cracks will have little or no acoustical effect
on the finished project.

4.There are now a number of different epoxy systems
available. While I expect that most of them are excellent,
we have used – and, hence, can recommend from personal
experience – only a few of them. Those that stand out
are:

For general coating applications we use System Three
Clear Coat epoxy resin. Clear Coat is an extremely low
viscosity, clear (almost water-white), solvent-free penetrating
epoxy system. It is especially formulated for the clear
coating of wood. It has a very long pot life and its very
slow cure rate allows plenty of time for good penetration
into the wood surface.