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Humidity cracks are a very common issue, but not very well understood.
Your guitar is made to suit an enviroment of around 20 degrees celicus and approx 50 percent humidity.
Temperature swings are not to great of an issue, however, humidity swings can be quite detrimental.
Manufacturers recommend there guitars be stored between 45 and 55 percent humdiity. In reality most guitars live in a range of 40-60 percent relative humdiity.
So how do you protect your guitar from massive humidity swings.
First - do not store your guitar in the room you have a heater in, heaters dry the air right out and can plummet the humidity in your guitar to single digits.
Second - if you have your guitar sitting in a room, avoid sitting it near the window where the sun can have a negative affect on it, possibly store it in a corner where there is very little air flow around it, its about stability of the enviroment around it.
Thirdly - Avoid just throwing humidity correction devices in your guitar, if you do not know what the humidity reading is to start with, its incredibly easy to over humidify a guitar, which then leads into other structural issues.
If you travel, always keep it in its case, your case works as a great humidity controller.
Now all that being said, we need to fix the guitar if it does have issues, this process is rectifying the humidity in such a situation
Approx repair time is 3-4 days.
This guitar has some serious humidity issues, It has exposed frets, cracks in the top, sinking sections of wood, loose braces
The frets are so exposed, they catch a steel ruler
Tapping the top, we can hear loose bracing
Using a straight edge ruler, we can see the top sinks at locations
We have cracks running with the grain
More cracks on the side
Doing a pre work check, we have 37 percent humidity inside the guitar, its possibly gone lower than that, before we got it.
First back the tension of on the strings
The first and simplest way of restoring humidity and is very commonly used, is put a damp cloth into a plastic bag
Please note - This is not how we do it here, I am not a fan of this method, but mention it solely because others use it
The bag is placed inside the guitar, and the wood is allowed to draw the moisture from the cloth, the reason it goes into a bag, is so the damp rag does not contact the wood directly and leave stain marks.
Within 10 minutes, the humidity of the internal air has peaked at 67 percent, we do not want the wood to be 67 percent, so time to remove this method.
Do not get me wrong, this method works for rehydrating a guitar, however I personally believe you risk damage from the rapid Humidity change.
Most of your commercial humidity devices are equipped with a foam sponge, by-passing the commercial aspect, we are just using a normal sponge here, wet the sponge and then squeeze it dry.
Commercial packaging has a couple of advantages, the first is there is less chance of having some surface rust appear on your strings from the contact of a damp sponge, also, less chance of a drip mark happening inside your guitar from having not squeezed the sponge dry enough.