Why do pickguards sometimes curl up and cause cracks?
Back when today’s vintage flat tops were being constructed the major guitar manufacturers attached pickguards to their instruments by spreading solvents on the back of a pickguard and clamping it on the instrument’s unfinished top. They would then apply finish over the whole guitar. The solvents actually melted the plastic into the wood making a strong bond. So strong, in fact, that when the plastic shrank over the years (at a much faster rate than the spruce top of the guitar) the guard would pull the wood with it and crack the top around the pickguard. To worsen the problem, some vintage pickguards were made from an acetate based plastic sheeting. These pickguards in particular tend to damage tops and or curl up around the edges of the pickguard. If a pickguard is starting to curl it usually should be replaced.
How is this repaired?
In repairing a guitar with this problem the approach we typically take here at Finger Lakes Guitar Repair is to remove the original guard, repair the crack with inside reinforcement when necessary and apply finish on the bare wood beneath the pickguard. After the finish has cured the original guard may be reattached with a self adhesive sheet which, will allow the pickguard to further shrink without damaging the top. This repair will leave visible a small area of the newly finished top bordering the reglued shrunken pickguard. If the old pickguard is warped to the point that new adhesive will not hold down the guard a new pickguard may be installed in place of the original.