How to Sharpen Powder & Super Steel Knives (SG2, R2, HAP40, ZDP-189)

“I bought a lovely SG2 knife and now I can’t get it sharp” is one of the most common frustrations we hear. The good news: your knife is fine, your stone is fine, and you don’t need to buy a special kit. Powder (powder-metallurgy) steels behave a little differently on the stone, and once you know why, they’re not hard to sharpen at all — just a bit slower and less forgiving of a lazy finish. As a Japanese sharpening-stone maker, here’s the honest, practical version.

01 · What “powder steel” actually is (and why it holds an edge)

Powder-metallurgy (PM) steels are made by atomising molten steel into a fine powder and pressing it back into a billet. The result is a very even, fine-grained structure with lots of small, hard carbides (the wear-resistant particles in the steel) spread uniformly through it. That fine, hard structure is exactly why these knives take a keen edge and hold it far longer than ordinary stainless.

The names you’ll see on Japanese kitchen knives:

• SG2 / R2 — two trade names for essentially the same fine-carbide powder stainless (SG2 is Takefu’s designation), the most common on premium Japanese knives, usually around 63–64 HRC. Superb edge retention, still very sharpen-able.
• HAP40 (Hitachi Metals, now Proterial) — a powder high-speed steel, often 64–67 HRC. Harder and more wear-resistant than SG2; not stainless, so it needs a little rust care.
• ZDP-189 (Proterial) — an extreme high-carbon powder steel, often 64–67 HRC with a very high carbide volume. Technically stainless, but with so much chromium locked up in carbides it can still stain — treat it as semi-stainless. Holds an edge almost stubbornly, which is the same reason it resists the stone.

The trade-off is simple and honest: the properties that make these steels hold an edge (hardness + hard carbides) are the same properties that make them slower to sharpen. That’s not a defect — it’s the deal you signed up for, and it means you sharpen far less often.

02 · Why it feels “hard to sharpen” (it’s usually the stone, not the steel)

Here’s the part most guides skip. An ordinary waterstone cuts with aluminium-oxide abrasive (silicon carbide is actually harder, but it’s brittle and wears fast on steel, so alumina is the everyday standard — only diamond really escapes that trade-off). The hardest carbides in these steels — vanadium and some tungsten carbides — rival or exceed that abrasive in hardness. In SG2 and ZDP-189 most of the carbides are chromium-rich and actually a touch softer than the abrasive, but they’re packed in at high volume inside a very hard matrix. Either way, those carbides wear down and blunt the tips of the stone’s grit far faster than ordinary steel does. The surface goes smooth and shiny, loses its bite, and you end up rubbing for ages with little to show for it. Sharpeners call this glazing (in Japanese, me-tsubure — the grit “crushing”/dulling).

Two things follow from that:

• It’s mainly the grit going blunt, not just clogging. The slowdown is your abrasive dulling against those carbides (glazing) more than swarf filling the pores — though a quick dressing cures both, which is the fix below.
• It’s the wear resistance, not the HRC. A high hardness number (HRC) just means the steel resists denting; a stone cuts hardened steel fine. What actually slows you down is the carbides’ wear resistance — so even with a fresh surface it takes more strokes to raise a burr. People give up before the edge is formed, and a knife that never raised a burr never got sharp. See why won’t my knife get sharp if that sounds familiar.

03 · The fix: keep your stone biting

If the problem is a dulling stone surface, the answer is to keep exposing fresh grit. In order of everyday usefulness:

• Wake the surface up with a nagura. A few passes with a nagura rubs off the glazed layer and brings up fresh, sharp abrasive plus a little slurry that helps it cut. To restore bite, a nagura a touch coarser than the stone works best. Do it whenever the stone stops biting — on the hardest of these steels that can be every few minutes, and it makes a night-and-day difference. (Keeping the stone flat is a separate job: a nagura won’t flatten a stone — use a flattening plate for that.)
• Prefer a stone that renews itself. A softer, muddy soaking stone constantly sheds spent grit and presents new edges as you work, so it keeps cutting on hard steel. A very hard, glassy stone stays flat beautifully but glazes faster — it wants that nagura alongside it.
• Use light, patient pressure. Leaning harder doesn’t force a hard carbide to cut; it just rounds your edge and risks chipping a brittle steel. Let the stone work and give it time.

One honest caveat: a nagura restores the stone’s surface — it doesn’t make the grit itself tougher. If a stone is genuinely mismatched to the steel and the fresh grit dulls again almost immediately, re-dressing it only raises how often you stop, and the real fix is a better-suited stone (or a diamond plate). You rarely hit that wall with a decent waterstone on kitchen powder steel, but it’s worth knowing where the limit is.

None of this requires a diamond plate. A good waterstone that you keep dressed will sharpen SG2, R2, HAP40 and ZDP-189 perfectly well.

04 · Step by step (what actually changes)

The method is the same one in our beginner’s whetstone guide. Only three things change for powder steel: keep the stone fresh, be patient, and be thorough about the burr.

1. Soak and set up. A #1000 soaking stone wants about 5–10 minutes until the bubbles stop; splash-and-go and finishing stones don’t. Have a nagura or dressing stone within reach.
2. Sharpen at your usual angle. Most double-bevel Japanese knives like around 15° per side — the same angle you’d use anyway (see how to sharpen a Japanese knife). Keep it consistent and keep the pressure light.
3. Re-dress the stone the moment it stops biting. When the stone feels slick and stops throwing slurry, give it a few passes with the nagura. This is the single habit that makes hard steel feel easy.
4. Raise a burr along the whole edge. This is where hard steel punishes impatience. Keep working each section until you can feel a wire on the opposite side, heel to tip. On powder steel that burr is often small and hard to feel, so check by fingertip frequently — a faint burr along the entire edge is enough. Don’t keep grinding expecting a big one (that just wastes steel), but don’t stop before it’s there either. On ZDP-189 or HAP40 it simply takes longer to appear.
5. Flip, repeat, then deburr completely. Raise the burr on the second side, then remove it with a few very light alternating strokes. A clean apex on hard steel is fantastic; a leftover burr snaps off and feels dull fast. See how to remove a burr the right way.
6. Finish (optional). A #3000–#6000 pass refines the edge; it polishes rather than adds “sharpness.” A well-finished #1000 edge already bites beautifully.

05 · Do you need a special grit or stone?

Short answer: no. A single #1000 forms the edge and does about 80% of the work on powder steel exactly as it does on any knife (more on grit in our whetstone grit guide). What helps far more than chasing a fancy grit number is keeping that #1000 dressed and biting. If you want to add one thing, make it a nagura or dressing stone, not a #12000 polishing stone.

The one honest exception is at the very top of the hardness range: on the hardest, highest-carbide steels a diamond stone speeds things up — which brings us to the real question.

06 · When you actually need a diamond plate

Diamond (and CBN) sits well above every steel carbide on the hardness scale, so it shrugs off the carbides that glaze a normal stone. That makes a diamond plate genuinely faster on very high-wear steels. But “faster” isn’t the same as “required.” Here’s the honest split:

Steel	Typical use	Sharpens on a good (dressed) waterstone?	Diamond needed?
SG2 / R2	Premium Japanese kitchen knives	Yes — easily, kept dressed	No (optional)
HAP40	High-performance kitchen knives	Yes — a bit slower, dress often	No (nice for speed)
ZDP-189	Extreme edge-retention kitchen knives	Yes — slower, patience + nagura	Optional (speeds it up)
S90V / S110V / 10V	Western folders & EDC (very high vanadium)	Slow and frustrating	Recommended
M390 / 20CV / 204P	Western folders (high chromium, moderate vanadium)	Yes, slowly	Helpful (borderline)

A rough rule we use: aluminium-oxide stones start to struggle once vanadium climbs past about 3% (HAP40 sits around there — which is why it wants frequent dressing), and by the very-high-vanadium Western steels — S90V, S110V, 10V at roughly 9–10% — diamond becomes the sensible choice from the coarse work up. In other words: for the powder steels you’ll meet on Japanese kitchen knives, a good waterstone you keep dressed is all you need. Reach for diamond when you’re dealing with the ultra-high-vanadium Western “super steels,” or simply when you value speed over the feel of a waterstone. We cover which steels cross that line in whetstone or diamond?

07 · Common mistakes to avoid

• Giving up too early. Hard steel raises a burr slowly. If it isn’t biting, you usually need more strokes and a freshly dressed stone — not a different knife.
• Letting the stone glaze. A slick, shiny stone has stopped cutting. Re-dress it; don’t just push harder.
• Dropping to a coarse stone to “speed things up.” On wear-resistant steel a coarse stone’s large grit can skate and skid instead of digging in. You’re often faster staying on a well-dressed #1000 (with a nagura) than reaching for a coarser stone.
• Too much pressure. Hard powder steel is also more brittle. Heavy hands round the edge and can micro-chip it. Light and patient wins.
• Skipping the deburr. A burr left on hard steel breaks off and dulls the edge within a day. Finish clean.
• Honing with a coarse steel rod. On a hard, thin edge that invites chipping. Use a fine ceramic rod gently, or skip the rod — see honing vs sharpening.

Frequently asked questions

Can you sharpen SG2 / R2 on a normal whetstone?
Yes. A good #1000 waterstone sharpens SG2/R2 well — just keep the surface dressed so it keeps biting, and be patient raising the burr. You do not need a diamond stone for kitchen SG2/R2.

Why does my powder steel knife feel impossible to sharpen?
Almost always because the stone has glazed (its grit dulled against the steel’s hard carbides) and/or you stopped before raising a full burr. Refresh the stone with a nagura, use lighter pressure, and keep going until you feel a wire along the whole edge.

Do I need a diamond stone for ZDP-189 or HAP40?
No, but it helps. Both sharpen on a good waterstone you keep dressed — it’s just slower. A diamond plate speeds up the coarse work if you sharpen these often. For most home cooks, a dressed #1000 is enough.

What is a nagura and do I need one?
A nagura is a small stone you rub on the surface of your waterstone to raise fresh grit and slurry. On hard powder steel it’s the most useful accessory you can own — far more useful than an ultra-fine polishing stone. Note it’s for bite, not flatness: a nagura won’t flatten a dished stone — that’s a flattening plate’s job.

Does powder steel need a higher grit?
No. Grit choice is the same as any knife: #1000 to sharpen, an optional #3000–#6000 to polish. Keeping the stone biting matters more than the number on the box.

How often will I need to sharpen it?
Much less often than ordinary steel — that’s the whole point of powder steel. It takes a little longer when you do, but you do it far less frequently.

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The stone we’d reach for

For powder steel, the stone matters less than keeping it fresh — but you still want a #1000 that keeps biting on hard steel. Our ALTSTONE FUKAMI #1000 is the medium stone we reach for first: it has enough give to keep renewing its own grit, which is exactly what powder steel wants (a harder stone holds its flat longer but glazes sooner). Keep it dressed and flattened and it handles SG2, R2, HAP40 and ZDP-189 cleanly. Browse the full range in our stone collection.