How to Flatten a Whetstone (And When Yours Needs It)

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Updated August 2026 · 8 min read · UK Japanese knife specialists

If your knives have stopped coming off the stone as sharp as they used to, the problem usually isn't your technique. It's the stone. Every water stone wears hollow in the middle over time, and once it does, you're grinding your bevel against a curve instead of a flat plane.

Flattening a whetstone — sometimes called lapping or truing — takes about five minutes and needs nothing more exotic than a sheet of wet-and-dry paper and a genuinely flat surface. This guide covers how to tell your stone has gone out of true, the three methods that actually work, and how often you should bother.

If you're still choosing stones, our guide to what grade of whetstone to buy covers grits first, or browse the sharpening stones range.

Why whetstones go hollow in the first place

A water stone sharpens because it wears. The abrasive particles are held in a binder that breaks down under pressure, constantly exposing fresh, sharp grit. That self-renewing surface is exactly what makes a whetstone cut faster than a hard steel rod — and it's also why the stone can't stay flat forever.

The wear isn't even. Most people sharpen over the middle two-thirds of the stone and rarely work right to the ends, so the centre erodes faster than the edges. After enough sessions the face becomes gently concave, or "dished".

Coarse stones dish fastest. A 400-grit stone doing repair work on a chipped edge removes a lot of steel and sheds a lot of binder, so it can go visibly hollow in a handful of heavy sessions. A 8000-grit finishing stone, used lightly for a few passes at the end, might go months.

What a dished stone does to your edge

Three things go wrong, and they compound:

• You lose your angle. The whole point of holding a consistent sharpening angle is that the bevel meets the abrasive at the same pitch every stroke. On a concave surface the effective angle changes as the blade travels — steeper in the hollow, shallower at the ends.
• The bevel goes convex. Grinding a straight bevel against a curve rounds it. A rounded bevel feels blunt long before it looks blunt, because the very apex is no longer a clean intersection of two planes.
• The tip and heel get neglected. On a dished stone the middle of the blade sits proud in the hollow while the tip and heel skate over the raised ends, so those sections get less contact and stay dull.

None of this is dramatic in one session. It creeps. Which is why people usually blame themselves — or the knife — long before they think to check the stone.

How to tell your stone needs flattening

Two checks, thirty seconds, no special tools.

1. The straight-edge test

Dry the stone, lay a steel rule or the spine of a knife across the face, and hold it up to a window or lamp. If light shows through under the middle of the rule, the stone is dished. Rotate the rule and check diagonally as well as across the short axis — stones often wear more in one direction than the other.

A hair of light is normal and won't hurt anything. A gap you can clearly see, or one you can rock the rule in, needs correcting.

2. The pencil grid test

This is the one to use, because it doubles as your progress indicator when you start lapping.

Take an ordinary pencil and draw a grid across the whole face of the dry stone — a few lines lengthways, a few crossways, roughly 15–20mm apart. Now rub the stone briefly on your flattening surface. Wherever the stone is high, the pencil disappears immediately. Wherever it's hollow, the pencil survives.

If the marks vanish evenly from the whole face in a couple of passes, your stone is flat. If a band of pencil hangs on down the centre, that's your hollow, and you keep going until it's gone.

Three ways to flatten a whetstone

All three work. They differ in cost, speed and how much of the stone they consume.

Method 1 — Wet-and-dry sandpaper on glass (best for most people)

The cheapest reliable method, and the one we'd suggest if you sharpen a few times a year.

1. Find a truly flat surface. A thick glass pane, a granite or marble worktop offcut, or a floor tile laid on a level bench. A kitchen worktop or a chopping board is not flat enough — you'll simply transfer its shape onto the stone.
2. Tape down silicon carbide wet-and-dry paper. 120 to 400 grit is the useful range. Use the coarser end for a badly dished coarse stone and the finer end for finishing stones. Tape all four edges so the paper can't lift or shift.
3. Soak the stone if it's a soaking stone, then wet the paper generously. Water carries the swarf away and stops the paper clogging — dry lapping is slower and ruins the abrasive.
4. Draw your pencil grid on the stone face before you start.
5. Rub with light, even pressure in a figure-of-eight or a rotating circular motion, turning the stone 90° every twenty strokes or so. Varying the direction stops you introducing a new bias.
6. Stop when the pencil has gone. Rinse, dry, and re-draw the grid to double-check if you want to be sure.

Expect to change the paper once or twice on a badly dished coarse stone — silicon carbide paper wears out quickly against a hard stone.

Method 2 — A diamond lapping plate

A coarse diamond plate, sold specifically for flattening, is faster and lasts for years. You use it the same way: soak, pencil grid, water, figure-of-eight strokes, stop when the marks have gone.

It's the right buy if you sharpen weekly, run coarse stones hard, or maintain several stones. For a household with one or two stones and occasional use, sandpaper is honestly the better value.

Method 3 — Stone against stone

Rubbing two water stones face to face will flatten both, and it costs nothing. There's a real catch, though: it only works if one of them is already flat, and it wears down both stones at once. If both are dished, they'll happily wear into a matching curved pair — convex against concave — and you'll think you've fixed it when you haven't.

Treat it as a field fix, not a routine. Always verify with a pencil grid and a straight edge afterwards.

Finish the job: chamfer the edges

Whichever method you use, take thirty seconds afterwards to knock a small bevel — a couple of millimetres — onto the four top edges of the stone with the same sandpaper or plate. Freshly lapped edges are square and brittle, and they chip if a blade catches them. A chamfer also stops a sharp stone corner nicking your knife's edge on the return stroke.

How often should you flatten?

There's no universal interval, because it depends entirely on how much steel you're removing. Use the pencil grid rather than a calendar. That said, some rough guidance:

Stone	Typical use	Check for dishing
400 grit	Repairing chips, reprofiling	Every session
1000 grit	Your main sharpening stone	Every 2–3 sessions
3000 grit	Refining the edge	Every 5–6 sessions
8000 grit	Final polish, few light passes	Occasionally

Two habits reduce how often you need to do it at all. First, use the whole stone — deliberately work your strokes out towards both ends rather than parking in the middle. Second, ease off the pressure. Water stones cut on abrasive, not force; leaning on the knife wears the stone faster without sharpening the edge any quicker.

Common mistakes worth avoiding

• Lapping on the kitchen worktop. Almost no worktop is flat to the tolerance you need. Glass or stone, on a level bench.
• Flattening dry. Clogs the paper, glazes the stone and takes three times as long.
• Only checking across the short axis. Stones dish lengthways too. Check both.
• Skipping the chamfer. A square-edged freshly lapped stone chips easily.
• Flattening a diamond plate. Don't. Diamond plates are a rigid steel substrate — they don't dish, and abrading one destroys the diamond coating.

The kit we sell for this

We don't sell a dedicated lapping plate, so for the flattening itself you'll want wet-and-dry paper from any DIY shop — it's a couple of pounds. What we do sell is the stones themselves and the rod that keeps your edge going between sharpenings.

Frequently asked questions

How do I know if my whetstone needs flattening?

Lay a steel rule across the dry stone and hold it to the light. If you can see a gap under the middle, it is dished. The more reliable check is to draw a pencil grid over the whole face, rub the stone briefly on a flat abrasive surface, and see whether any pencil survives in the centre.

How often should you flatten a whetstone?

It depends on how much steel you remove rather than on time. A coarse 400 grit stone used for repair work is worth checking after every session, a 1000 grit stone every two or three sessions, and finishing stones far less often. Use the pencil grid test to decide instead of a fixed schedule.

Can you flatten a whetstone with sandpaper?

Yes, and for most home cooks it is the best method. Tape silicon carbide wet-and-dry paper to a genuinely flat surface such as a glass pane or a granite tile, wet it, and rub the stone over it in a figure of eight until the pencil grid has gone. The flat backing matters more than the paper.

What grit sandpaper do you use to flatten a whetstone?

Silicon carbide wet-and-dry paper between 120 and 400 grit covers almost everything. Use the coarser end for a badly dished coarse stone and the finer end for finishing stones, so you do not leave scratches deeper than the stone itself would cut.

Do you soak a whetstone before flattening it?

If it is a soaking stone, yes, prepare it exactly as you would for sharpening. Either way you want plenty of water on the abrasive surface while you lap, because water carries the slurry away and stops the paper or plate clogging.

Can you use two whetstones to flatten each other?

You can, but only if one of them is already known to be flat. If both stones are dished they will simply wear into a matching curved pair, and you will have consumed two stones without fixing either. Always confirm the result with a pencil grid and a straight edge.

Does a dished whetstone ruin your knife?

It will not damage the blade permanently, but it will round the bevel and make the edge harder to bring back. Because the effective sharpening angle changes across a concave face, the tip and heel also receive less contact than the middle of the blade. Flattening the stone and regrinding the bevel puts it right.

Do you need to flatten a diamond plate or a honing steel?

No. Both are rigid steel with an abrasive bonded to the surface, so they do not dish the way a water stone does. Abrading a diamond plate would strip the diamond coating, and a honing steel simply needs rinsing and drying after use.

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