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It refers to the process of straightening and correcting a distorted whetstone. As the knife is sharpened using a whetstone, the surface of the whetstone becomes dented or distorted. If you use a whetstone in such a state, you will not be able to sharpen the knife as you wish, and the line of the blade will be distorted. Therefore, it is necessary to keep the surface of the whetstone level by using a correcting whetstone to correct the surface.

Related websites
Mastering Japanese Knife Sharpening: Techniques & Tips | KIREAJI