Atoma Wide Surface Repair Grindstone with handle #400 (Tsuranaoshi)

Why do we need Surface Repair Grindstone? (Tsuranaoshi-面直し)

What if you sharpen knives with uneven whetstone? Your sharpening result would be uneven too. The surface of whetstone will gradually get distorted as you use it. From time to time, you'd better check the condition of whetstone and flatten the surface with this item. Great for those who take extra care for sharpening.

Why surface flattening is so important?

The most important element of sharpening is not expensive stones nor skills, but flat surface on the whetstone. A concave stone will cause the edge to get distorted, and a distorted edge does not comfortably touch a whetstone, so your sharpening will be more time-consuming.

With a whetstone being consistently flat, it ensures the area to be sharpened is perfectly attached to the surface, resulting in quicker and more effective sharpening. If you have this good habit, it keeps the edge in an ideal condition because your whetstone is always flat. 

It is very difficult to flatten a deeply concave whetstone so please do flattening your whetstone as frequently as you can, ideally flatten while you are sharpening.

See the tutorial video: How to fix the broken knife tips with rough sharpening stone.