How to Flatten a Whetstone (and Why a Dished Stone Won't Sharpen)

If your knife stopped taking a good edge and you can't work out why, the stone itself is a prime suspect. Whetstones are made to wear — that's how fresh grit keeps coming to the surface — but they wear unevenly, sinking in the middle where you do most of your sharpening. Here's how to spot a dished stone and flatten it, from a Japanese stone maker who does this every day.

01 · WHY A DISHED STONE WON'T SHARPEN

Sharpening only works if the very edge of the knife meets the stone. On a stone that has gone hollow, the surface is a shallow valley — so when you lay the blade flat, it rides on the high ends of the stone and the edge floats over the low center. You grind and grind, but the apex barely touches the abrasive. This is one of the most common reasons a knife "won't get sharp" no matter how long you spend — see why won't my knife get sharp.

A dished stone also rounds the bevel and makes a consistent angle almost impossible. Flatness isn't a nice-to-have — it's the surface your whole edge is built on.

02 · HOW TO TELL IF YOUR STONE IS DISHED — the pencil test

You don't need special tools to check. There are two easy ways:

• The pencil grid. Scribble a grid of pencil lines across the whole surface of the wet stone. Rub the stone a few times on your flattening surface (below). Wherever the pencil survives, the stone is low (dished); wherever it disappears, the stone is high. When all the pencil is gone evenly, the stone is flat.
• The straightedge. Lay a metal ruler on edge across the dry stone and hold it up to the light. A gap in the middle means the stone has dished.

Most stones start to hollow faster than people expect — a soft, fast-cutting #1000 can show a visible dish within a few sharpening sessions.

03 · THREE WAYS TO FLATTEN A WHETSTONE

All three work on the same principle: rub the stone against something flatter and harder until the low spots are gone. Pick by budget and how much correction you need.

Method	Best for	Notes
Diamond flattening plate (or a coarse diamond plate)	Everyday flattening — the easiest, fastest, longest-lasting option	Stays flat itself; works wet; a bit of an investment. The tool most people settle on.
Dedicated flattening / truing stone	People who want a simple purpose-made block	Coarse and grippy; keep it flat too. Some are grooved to clear slurry.
Wet-dry sandpaper on glass/granite	The cheapest way to start, or a badly dished stone	Use waterproof (wet-dry) paper, ~#120–#400, taped to a truly flat surface. Replace paper as it wears.

A quick note on what not to use: a rough concrete block or a random tile is tempting, but it's rarely flat and can contaminate your stone with coarse debris. Use a surface you know is flat.

04 · STEP BY STEP (the pencil-grid method)

1. Wet the stone (a soaking stone can be soaked as usual; splash-and-go and finishing stones just need water on top).
2. Draw a pencil grid over the whole face.
3. Put your flattening plate (or sandpaper on glass) down and rub the stone over it in circles or figure-eights, with light, even pressure, rotating the stone often so you don't favor one side.
4. Check the pencil. Keep going until every line has disappeared — that's your proof the whole surface is now level.
5. Rinse off the slurry, knock off any sharp corners with a few strokes (this stops chips), and you're ready to sharpen.

Then go straight into sharpening — a freshly flattened stone bites beautifully. New to the sharpening motion itself? Start with the beginner's guide to sharpening on a whetstone.

05 · HOW OFTEN SHOULD YOU FLATTEN?

The best answer is little and often. A quick 20-second flatten before each sharpening session keeps the stone true and means you never have to grind out a deep hollow later. It feels like an extra step at first, but it's faster overall — and your edges get better immediately because the stone is always flat.

Softer, faster stones (many #1000 water stones) dish quicker and need it more often; hard ceramic stones hold flat longer but still need it eventually. If you only flatten when the dish is obvious, you'll remove a lot more stone each time.

06 · A FEW THINGS TO GET RIGHT

• Flatten every stone you own — coarse, medium, and finishing. A dished finishing stone undoes the flat edge your #1000 just made.
• Keep it wet. Flattening dry creates dust and clogs the surface. Water floats the worn grit away.
• Go easy on soaking finishing stones. Fine stones (#3000 and up) shouldn't be left to soak — splash water on top instead. Flatten them the same way, just briefly.
• Don't chase a mirror. The flattening surface only needs to be flat, not fine. A coarse plate that leaves a slightly toothy surface is perfect for sharpening.

FAQ

Can I flatten a whetstone with sandpaper?
Yes. Tape wet-dry (waterproof) sandpaper around #120–#400 to a truly flat surface like glass or granite, keep it wet, and rub the stone over it. It's the cheapest method and works well; just replace the paper as it wears out.

How do I know when the stone is flat?
Draw a pencil grid on the surface and flatten until every line has disappeared evenly. If pencil is still sitting in the middle, that area is still low.

How often should I flatten my whetstone?
A quick pass before each sharpening session is ideal. Flattening little and often keeps the stone true and removes far less material than waiting until it's deeply dished.

Do I really need a special flattening stone?
No. A diamond flattening plate is the most convenient tool, but wet-dry sandpaper on glass does the same job for a fraction of the cost. What matters is that your flattening surface is genuinely flat and harder than the stone.

Does flattening wear out my whetstone faster?
Only slightly, and it's worth it. Flattening removes a thin layer, but a flat stone sharpens so much better that you spend less time (and remove less stone) overall. Waiting until it's badly dished wastes far more.

THE BOTTOM LINE

A whetstone can only make an edge as flat as its own surface. Check yours with a pencil grid, flatten it against a diamond plate or wet-dry sandpaper, and do it little and often. Keep your stone flat and half of the "why won't my knife get sharp" problems simply disappear.

KEEP YOUR STONE FLAT — AND WORTH FLATTENING

ALTSTONE FUKAMI #1000 Whetstone

A Japanese-made #1000 medium stone with great bite — the everyday sharpening stone worth keeping flat, and an easy first stone for any kitchen knife.
Shop the #1000 whetstone