Repairing Cracks Caused by Dryness
on Acoustic Guitars

A washboard texture which looks somewhat like corduroy is one of the first signs of dryness.
Take heed, your instrument is giving you a warning that it needs attention.
This soft spruce top has lost a great deal of moisture and the dark grain lines now stand prominent, noticeable to the touch.
Cracking is likely if the problem isn't addressed and it's dry season continues.

Picture of Dry Spruce Guitar Top

You can see how the top crack on this guitar follows along the grain, a common scenario when cracks are caused by dryness.
 This one split from bridge to end block.
 

Close Up of Open Top Crack

After Dead Flat Comes Concave...

Concave Top with Crack

In extreme cases the top can become concave as seen in the first photo.
Though the majority of instruments that are truly dry will crack the top first (assuming it's made from spruce or a soft wood), very dry instruments can also split along their sides or seam lines.
On one of those, a good rap or bump to the side can result in the side splitting wide open, not a pretty sight.

AFTER

After weeks of humidity and patience this top eventually began to return to it's normal, slightly arched shape. Once properly humidified I glued the top crack and placed 4 small spruce cleats along it's length to further reinforce the glued crack. I tried to get as close as possible with the camera at the least flattering angle to show you the end result. Even though this crack is difficult to see you can still make it out.

While some cracks are relatively easy to repair it's important to realize that most times it is a one shot deal. When repairing cracks it is essential that the separated halves are well aligned and the work is done cleanly.  Reversing a bad repair is far more difficult and expensive than having it done correctly to begin with. Because of the heat necessary to disassemble poorly performed crack repairs we often create more finish work and fall short of the cosmetic goal that was far more possible if done properly.
 

Finish Repair Challenges

The real challenge to repairing most tight cracks is primarily with the finish. Obviously we are not going to strip or refinish the top on this guitar for one simple top crack.
The crack in the finish leaves behind a small valley which I then fill with lacquer (assuming the original finish is lacquer) and it is then leveled and polished. However, the newly added lacquer will begin to shrink almost immediately and a small "depression" is likely to appear, even after weeks of patient waiting. Lacquer is very high in solvents and it shrinks quite a bit.  For this reason small chips, sink marks and cracks can reappear after a flawless repair days or even weeks after it's been done. I try and educate my customers to the nature of wood and  lacquer so they understand what is possible. It also requires more repair time.
When finish touch up will not render a far better looking repair I usually avoid it.

Cracks That Will Not Close

More often than not, cracks that have spread open and will not close with humidity are spliced. This delima is usually only found on older, very dry instruments, perhaps something that was left in a very poor environment. But there are other causes that can create the need for a splice, like something that has been damaged and wood is now missing or an area that is under so much stress that closure is not possible.
A splice is an insert of wood, like a filler strip, that is inserted into the area that is open. If the crack that is being repaired does not follow a straight line but runs across the grain the splice is much wider.
I have used several different tricks for splicing top cracks but regardless of the method, this tends to be a very particular, very time consuming repair.
Starting with the top (or back) that needs repair, a symmetrical section is removed and prepared for the insert. Using a donor top or raw materials an insert (splice) is made from the same kind of wood and shaped to fit. Here's the tedious part...the more exact the fit, the better the outcome....the more closely the grain in the splice matches the original top, the better the outcome...the easier the finish is to work with...the better the outcome. Splicing a sunburst top...let me give you someone else's number! ha ha
There are some repairs that can drive a perfectionist mad, this one could make the list.
What about just filling the crack with putty, wouldn't that be easier? Oh, my, would it! However, I don't do it. Putty/wood filler is not an acceptable repair method in most cases. While there are obviously some repairs that may require it, wood should be replaced with wood. Putty will shrink, doesn't look right etc. but on something very inexpensive, splicing or wood replacement can easily exceed it's value.
 

Related Articles:
Instrument Care & Humidity
Crack & Damage Repair