A guide to dealing with those pesky wood imperfections.
Let's face it: Sometime during the life of an average acoustic guitar, it will likely develop at least a small crack. You are carrying around a wooden box made of exotic hardwoods measuring anywhere from about .090" to .150" in thickness. That's a pretty fragile thing in and of itself. When you factor in that the strings are applying upwards of 180 pounds of force to the top via the bridge, it's really a wonder that guitar bodies hold together at all.
Cracks vary as much as all other things guitar, but there are some commonalities that can be addressed to give us a better understanding of them and how to proceed with a repair. Since cracks most often occur in the body woods, let's first look at the body, panel by panel.
Most of the worst and hardest-to-repair cracks happen on a guitar's top. One of the most common instances is actually not a crack at all, but a center-seam separation. This usually happens when the guitar is allowed to dry out, causing the two halves of the top to come unglued at the center seam. It most often occurs in the area from the bridge to the end block.
With all top cracks, timing of the repair is critical. The longer you wait to fix the crack, the less likely it can be done invisibly. Also, try and resist the temptation to run your finger over the crack to feel it. This puts oils and dirt from your hand into the crack, which makes it hard to re-glue and can leave a dark line that can't be removed.
If you know that you're not looking at a finish crack and the top is indeed cracked, remove the string tension and get the guitar to a good repairperson as soon as possible. Center-seam separations or other lengthy top cracks can be made stable (if not invisible) without removing the bridge and "overspraying" the top. Overspraying (spraying over the existing finish) is an option if you just can't live with the cosmetic imperfection of a crack, but it should be avoided on any vintage guitar.
Don't try this at home, but with cracks caused by low humidity, we always start by putting a trash bag over the entire body and adding humidity inside the bag for a couple of weeks. This will close up the crack dramatically before we proceed with the repair. We use thin Super Glue for most of the crack-sealing operations.
The longer you wait to fix the crack, the less likely it can be done invisibly.
If the crack has been open for years, the humidity method with the trash bag won't work as well, and a splint may be needed to close the crack. Top cracks are tricky because that's where the most stress is applied, but also because the light color of spruces and cedars most often used for tops makes for a much harder, cosmetic repair.
Since the guitar sides are usually the thinnest woods on the body, and since they are in just the right position to get whacked on something (often while playing), they sometimes get cracked. Even if the sides of a body are braced, a crack can still run right through the brace. I've seldom seen a case where a brace stopped a crack on a guitar.
The sides are the most likely place for a crack to occur when shipping an instrument. Always make sure that your guitar fits in its case well (not loose) when shipping it or traveling by plane. Cracks on the sides are repaired much the same way as top cracks, but repairs will usually look better cosmetically. This is because of the darker color of most side woods and the grain and grain fillers have a bit more going on to catch the eye and mask a crack than a top does.
The backs of guitars probably finish third in terms of frequency of cracks. The backs are usually a little thicker than either the top or sides, therefore a bit tougher. Matters of improper humidity usually don't affect the thicker, harder woods on the back as much. The repair procedure is much the same as the top and sides, but the person doing the repair should be diligent on the interior cleanup of the guitar since the work could be visible through the soundhole.
When it comes down to it, I personally feel that you should enjoy and play your guitar, live with the little cracks and dings that inevitably happen, and not make yourself miserable about cosmetics. If you have a repair that needs to be made, just be sure to get a qualified person to do a good job. Until next time, keep on pickin'!