Paulownia soundboards expand and contract with humidity, so cracking is one of the most common structural problems on a guzheng. A crack does not mean the instrument is finished; what matters is which layer it is in and where it sits.
Three kinds. A finish crack sits in the lacquer: a hairline you can see but not catch a nail on, usually from dryness or an aged finish, and it does not affect sound. A shallow drying crack follows the grain, is fine and does not let light through — common in northern winters and air-conditioned rooms. A through-crack is visible, passes light, and usually brings a buzz when pressing or a suddenly thinner tone.
Confirming it. Hold a torch flat against the board and the crack shows up; scrape lightly with a fingernail — if the nail catches, it is more than finish; tap either side of the crack and listen for a change, which suggests a glue joint has opened. Note the position too: a crack at a bridge foot or under the nut carries load and is far more serious than one at the rim.
What to do first. Do not reach for glue, wax or tape. Sealing the surface traps moisture in the crack and makes a proper repair harder. Move the instrument to a room with stable humidity, slacken the few strings nearest the crack (not the whole set), and watch for two or three days to see whether it grows.
What can be repaired. Finish cracks and shallow drying cracks are usually fillable: clean the seam, glue or fill, re-humidify, then touch in the finish. A mark may remain, but structure and playing are unaffected — no reason to replace the instrument.
What needs caution. Through-cracks, cracks with a sunken board, cracks in loaded areas, or any crack that buzzes when you press: repair is expensive and the original tone is hard to recover. If the quote approaches the price of a comparable new instrument, repairing does not make sense (when to replace).
Prevention is much cheaper. Keep relative humidity between 40% and 60%, away from radiators, direct air conditioning and sunlight; slacken the strings for long storage; after a move, let the instrument settle indoors before tuning (humidity and storage, long-term storage).
A dent without a crack is a different problem — see dents in the soundboard. If you cannot tell which kind you have, send a photo taken in raking light and we will advise from the picture.