Hairline Crack

[Ron Nossaman]

>Hello All,
>
>I'm in the middle of this, my first restringing job, and the soundboard
>has a
>very thin crack. Actually, it's more like a very slight separation
>between two of the
>panels. They're not pulling away from the ribs at all. Is there a simple
>way to
>handle this?
>
>Thanks, guys.
>
>Martin

Yessir, that's a crack. I don't know if it qualifies as simple or not, but
there are at least a couple of ways to handle it. The first way is to
ignore it. Since the acoustic effects of a hairline crack are not going to
be audible in the finished product, it's a possibility. Alternately, since
the strings are off and you'll never get a better chance, and since you
probably want the finished job to look like you actually showed up and did
something to try to improve the situation, you probably want to fill the
crack. 

The two approved, acknowledged, and generally agreed upon ways of filling
soundboard cracks are spruce shims or epoxy.  

First, dry the soundboard assembly down with heat to below 6% equilibrium
moisture content. That's below 30% relative humidity at 70° Fahrenheit.
Prop it up  with a few wedges between the ribs and the beams underneath to
hold up a crown of sorts. I can't say if this actually helps or not, but it
isn't likely to hurt, and that's the way most folk do it. 

For spruce shims, you need to scrape the crack out with a "V" shaped knife,
or scraper, to match the angle of a spruce shim you either make yourself or
buy from one of the supply houses. Even if you buy a shimming knife from
the same source as your shim stock, you will probably have to overhaul and
sharpen the knife to match the shim angle. The crack should be scraped out
so the shim goes the full depth of the panel (notch the shim over the ribs
if necessary), and fits as snugly and precisely as you can manage. There
isn't a trick to this, or an easy way to do it. It just takes plain old
hard headed work and care, whittling away until the shim fits. Then glue
the shim in (I use Titebond), mop up the excess glue, and trim it flush
with the panel when the glue is dry. That's DRY! If you trim the shim too
soon, it will shrink below flush to the panel surface when it finally
dries. Give it overnight at least. Before I started making my own shims, I
got them from APSCO. Shaff's were too wide an angle for my liking, and
wouldn't drive into the scraped out slot and wedge in as well.

The second method is epoxy. I don't do them this way, but I don't see a
problem with it. I think the procedure is to scrape the crack as if you
were installing a spruce shim, but you don't need to be as careful to make
the width and depth as uniform. Tape underneath to keep the epoxy in the
crack, mix up the epoxy and trowel it in. Some say they mix wood dust into
the epoxy, some don't mention it. Either way, you trim it flush when it's
hard, just like a spruce shim.

Either way, it's strictly a cosmetic repair and isn't doing a thing to
improve the function of the board, so either method is OK.

Hope this helps,  
Ron N