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What pretty much all pros in the repair trade and many pros in the building trade know is that naphtha can be very useful in finding or determining if there is a crack in the top, back or sides.

We use an eye dropper and naphtha is so thin that it will migrate right through crack and come out the other side quite easily and very quickly.

So we drop a few drops on the suspected crack topside and then with mirrors or a camera look for the wet spot on the inside. No wet spot? No crack. See a wet spot and there is a crack.

It won't hurt wood or finish and flashes off in a few minutes.

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Ann Arbor Guitars

Wanted to add that you can also use naphtha to clean up the frets on Fender and other maple fretboards. It will clean the gunk off the board and frets and then flash off and of course will not harm the finish.

It is flammable as well so take precautions with wet paper towels. I let them dry completely before I pitch them.

There was my safety disclaimer as well.

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Ann Arbor Guitars