Q: What is causing the finish on my guitar to fracture, check, or crack?
A: Without knowing for sure which specific guitar you have, it’s more likely to have a nitrocellulose lacquer finish than either polyurethane or polyester. As a “nitro” finish’s solvents evaporate (which can be rapidly or over the course of many decades), the finish actually shrinks, particularly if exposed to extreme temperature and humidity fluctuations over time. This can sometimes cause the problems you are seeing. Lacquer finishes applied during damp, cold weather or during a blistering summer heat wave will be much more susceptible to shrinkage and cracking. Thankfully, these types of problems occur far less often on newer guitars.
Remember, fast temperature changes are a big enemy of guitar finishes. If you’re taking your guitar from a cold winter car into a warm house, let it warm up thoroughly before you open the case!