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My son's Gibson acoustic electric jazz guitar developed some cracks in the finish last winter (Northern New Jersey). The repair man suggested a case humidifier, and the finish cracks closed up. Now, my son is not humidifying, which I expect is not needed in the middle of the summer, and the cracks opened again. So, if he humidifies in the summer, will he cause water damage? Is there a case humidifier that automatically stops adding excess water if the humidity is high? Any other advice about the finish cracks?

I am curious at your term "finish cracks", You had someone look at it and are they cracks in the finish or the wood?

Checking in the finish is caused by rapid temperature change, usually from cold to warm. It is not uncommon on guitars that are transported to and from places and not allowed to acclimate to the different temps. This is usually done by moving a guitar from a cold car into a gig or house and allowed to sit for a while without opening the case until the guitar has come up to temp. The problem is the wood expands and contracts at a different rate than the finish.

Wood cracks are a total different thing. If the guitar has cracks in the wood they should be repaired before they can become worse. Usually the minimal repair is to glue cleats inside along the cracks so it doesn't get worse.

How ever you choose to humidify it is a matter of choice but a good case humidifier is a very good idea. You should have a good case hygrometer and keep an eye on it to maintain 45% humidity all the time (winter and summer). If the humidity is higher you will need to get desiccant bags to put in your case (those little packets you find in different products).

Another way to fight humidity and dryness is to keep your guitar in a room with a humidifier and a dehumidifier or air conditioner (which removes humidity). Keeping a consistent 45% humidity is what you should strive for. A few % points up or down is OK but anything below 40% or above 50% you should give a serious thought about controlling in one way or another.

As for the cracks, without having the guitar in my hand I couldn't tell you what they really are but I can say if they are in the wood you should think about stabilizing them with cleats and humidity control.

Bob

I don't know if they are finish cracks or wood cracks. My son said the repairman told him they were in the finish. My son said the cracks had closed up last winter with the humidifier. I would hope that the repairman would offer to cleat them if they were wood cracks, but I cannot determine anything more. I am not near the guitar.

Finish cracks do not typically swell shut with humidity. Finish resins do not readily react with water, wood on the other hand does. Would need to examine hands on to tell what was going on but I ma suspicious they are just checks in the actual finish based on your description.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

The guitar is a Gibson 335. My son is in Morristown New Jersey (northern NJ). Any suggestions for a luthier in that area who would be really skilled with cracks, whether finish or wood?

I am about 2.5 hours away. if you cannot find anyone closer I would be happy to look at it for you.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Thanks