When I first became a chef, I had no idea that whetstones needed to be flattened.
For nearly two years, I sharpened my knives on the same stone without ever checking if it was still flat.
The knives became sharp enough for work, so I assumed everything was fine.
Then one day, while visiting Japan, I took one of my knives back to the shop where I had bought it.
The craftsman looked at it and simply said,
“That’s no good!”
It was a little embarrassing, but it became one of the most valuable lessons I ever learned about sharpening.
What Is Whetstone Flattening?
Every time you sharpen a knife, your whetstone slowly wears down.
Because the centre is used the most, it gradually becomes lower than the edges.
Even if you can’t easily see it, that slight hollow can affect how your knife is sharpened.
Flattening simply restores the whetstone back to a perfectly flat surface.
Why Is It Important?
165mm Kiritsuke Thick Deba Aogami #2 Kurouchi Walnut Octagonal Handle
Japanese knives are designed with precise edge geometry.
If your whetstone isn’t flat:
- The edge won’t contact the stone evenly.
- It’s much harder to keep a consistent sharpening angle.
- The bevel can become uneven.
- Sharpening becomes less predictable.
Sometimes people think they’ve forgotten how to sharpen.
In reality, the problem may simply be the condition of the whetstone.
How to Flatten Your Whetstone
I simply use a standard flattening stone designed for whetstones.
Before I begin, I draw a light grid across the whetstone using a permanent marker.
There’s no need to colour the whole surface—just a simple grid is enough.
Wet both stones and rub them together using even pressure.
And the house was a rental.
As the surface becomes flat, the marker lines gradually disappear.
If some lines remain, those areas are still lower than the rest of the stone.
Once all the lines disappear evenly, rinse the stone thoroughly and it’s ready to use again.
It only takes a few minutes, but it makes a huge difference to your sharpening.
A Funny Story From One of My Former Chefs
One of the chefs I used to work with had a very different way of flattening his whetstones.
Instead of using a flattening stone, he rubbed them directly on the smooth concrete in his backyard.
To be fair…
It actually worked surprisingly well.
A smooth concrete surface allows you to apply good pressure and removes the high spots quite effectively.
The only problem was that it left grey sharpening marks and stains all over the concrete.
Even now, I sometimes wonder if everything went smoothly when he moved out!
So yes, it can work in an emergency.
But unless you’re prepared to explain mysterious grey stains to your wife, partner, or landlord, I probably wouldn’t recommend it.
If you’re renting, it’s definitely best to avoid using your driveway or patio.
If you really prefer this method, a better option would be to buy a smooth concrete paver or a flat tile and use that instead.
Just make sure it’s genuinely flat. If the surface isn’t flat, your whetstone won’t be either. Also, some tiles are so smooth that they don’t remove material very effectively.
For consistent results, a proper flattening stone is still the easiest and most reliable solution.
What I Learned
210mm Gyuto VG10 Polished Hammered 67 Layer Damascus Green Handleす
Back then, I thought that as long as my knife became sharp, I was doing everything correctly.
Now I know that good sharpening doesn’t begin with the knife.
It begins with a flat whetstone.
Sometimes the smallest maintenance habits make the biggest difference.
Looking After Your Japanese Knife
A quality Japanese knife doesn’t require complicated maintenance.
Keep it clean, dry it properly, sharpen it when needed, and remember to maintain your whetstone as well.
With proper care, a handcrafted Tosa knife can last for many years.
At Asaoya Yaichi, we’re proud to offer authentic handcrafted Tosa knives made by skilled craftsmen in Kochi, Japan.
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From Tosa, Japan, to your kitchen.
