Why does the finish have cracks? - May 2001
Finish on acoustic instruments protects the wood from humidity & dirt. But finish that is too thick inhibits tone. So, instrument builders apply the thinnest, most flexible finish, & rely on players to properly care for it. Care includes avoiding impact, preventing contact with solvents & minimizing exposure to extreme or rapid temperature changes.
Temperature changes are important because all of the good quality instrument finishes (spirit & oil varnishes, nitrocellulose lacquers & catalyzed polymers) expand & contract more rapidly than wood when exposed to a change in temperature. So, finish expands faster than wood on instruments exposed to a quick rise in temperature. In the extreme, finish can expand so much relative to wood that it buckles outward & separates from the wood. Repeated exposure can crack the finish & allow patches of finish to break free from the instrument. This phenomenon, though possible, is less common than small cracks throughout the finish caused by a quick temperature drop.
Rapid, large, downward temperature changes are fairly common if you live or travel in temperate climates. With dropping temperature, finish shrinks faster than wood. In the extreme, finish contraction is so much greater than the wood's that the finish cracks usually parallel to the grain. Severe thermal shock, however, can cause finish to crack with & across grain.
The severity of the temperature drop determines the number of finish cracks more shock equals more cracks. Fortunately, the cracks are usually fine & often hard to see. Even if thermally induced cracking is visually notable, it does not generally degrade the sound or playability of the instrument (except that the wood will be a little more prone to moisture absorption in damp air & the slightly muddy sound that over-humidified instruments tend to have). Finish cracking can, however, significantly diminish the instrument's dollar-value.
What Can I Do?
To prevent finish cracks, case instruments during temperature changes. If you take a warm instrument into cold air without a case, the risk of finish cracking is very high. Cases make temperature changes more gradual & reduce the risk of thermal shock.
Use a good quality hard shell case. Padding in better cases slows temperature changes & reduces thermal shock more effectively than cases with limited or no padding.
If you can't get a hard shell case, use a padded gig bag. While gig bags afford less protection than hard shell cases, their padding does reduce risk of thermal shock.
If you must use a chipboard case, line it with towels. Chipboard cases are the least protective option other than no case at all. If you have to use one, use towels as liners.
Use a thermal cover on your instrument case. Several manufacturers make thermal covers for people who regularly travel with instruments & experience temperature changes.
Open instrument cases only if they are the same temperature as the room in which you open them. If your case is warmer or colder than the room where you're opening it, the instrument inside is even warmer or colder. Therefore, thermal shock is possible; you should wait until case & room are the same temperature.