Had an email last month from someone fairly cross about this. He'd bought a VG-10 gyuto, "stainless" right there on the box, used it for three weeks, and now there was a row of little orange freckles along the edge. Was it fake? Was it defective? Had the shop lied?

No, no, and no. Stainless means it stains less. Nobody ever promised never. And the harder a knife steel is, the more "less" starts to mean "sometimes, if you're careless." Which, in a kitchen, we all are.

The bit of chemistry you actually need

What keeps stainless from rusting is chromium. Get enough of it into the steel, somewhere around 12 percent or more, and it forms a thin oxide skin over the surface that you can't see and that patches itself when scratched. Water and air hit the skin, not the iron underneath. That's the whole trick.

Now the awkward part. A knife also needs carbon to get hard, and carbon and chromium are rather fond of each other. In something like VG-10, roughly 1 percent carbon and 15 chromium, a fair chunk of that chromium gets tied up with the carbon in carbides. Great for edge holding. Less great for the oxide skin, because the chromium stuck in carbides isn't out on the surface doing the protecting. So VG-10 is stainless, genuinely, but it's living a bit nearer the edge of stainless than a soft 13-chrome supermarket knife is.

Go harder still, ZDP-189, the top end of the powder steels, and you're into steels I'd honestly call "slow rusting" rather than stainless. Go the other way, to AUS-8 or the German X50CrMoV15, and you'd have to leave the thing in the sink overnight to see a mark.

A rough ranking

From most stain resistant to least, among steels you'll actually meet on a Japanese knife. This is from experience, not a lab, so take the ordering as approximate.

Most forgiving: AUS-6, AUS-8, the Misono and MAC moly-vanadium steels, Global's CROMOVA. These are the ones that genuinely don't rust in normal use. Ginsan (Silver 3) is up here too despite being a high-carbon steel, because it has very little else in it to tie up the chromium, and it's why it's the stainless of choice for traditional single-bevel knives.

Middle: VG-10, AUS-10, VG-XEOS, SG2. All properly stainless, all capable of spotting along the edge if neglected after a lemon. SG2 is a bit better than VG-10 here in my experience, AUS-10 about the same.

Least forgiving among "stainless": ZDP-189, HAP40, and the semi-stainless steels like SLD or SKD. These will discolour and spot with ordinary carelessness. Treat them like carbon that's slightly more patient. The steel guide has the chemistry for each if you want to see why.

Why it's always along the edge

The freckles are nearly always on the edge and nowhere else, which confuses people. Two things going on there.

First, the edge is the thinnest part, and if you've sharpened recently it's also brand new steel with no oxide skin yet. The skin rebuilds in minutes in open air. But if the knife went straight from the stone into a damp block, or got rolled up in a wet towel, the water won the race.

Second, the edge is where the food was. Lemon, tomato, brine, vinegar, all acid or salt or both, and both go straight after that oxide layer. I've seen a mark appear from a lemon slice left on a blade for an hour. On a soft stainless, too.

There's a third one that catches people out: the very edge is where the steel has been worked hardest. Sharpening leaves microscopic scratches and a tiny amount of stressed metal at the apex, and stressed, scratched steel corrodes faster than smooth steel. A polished edge from a 6000 stone resists spotting noticeably better than a toothy one from a 400. Another small argument for finishing on a fine stone.

What's usually to blame

The dishwasher, nine times out of ten. Hot water, chlorinated detergent, a long wet cycle, and then a slow dry with the blade leaning on a metal rack. Some steels will spot after one run. The cross gentleman from the first paragraph, it turned out, had a partner who'd put the knife through twice.

Wet storage is the other common one. Wiped "dry enough," slid into a wooden block, slot stays damp. You get spots on the flat of the blade where it touched wood.

Salt. A brined chicken, a board you've just salted, kosher salt dust near the rack. Chloride is the thing stainless hates most. It's why boat knives use a completely different alloy.

Other metal touching it. A stainless knife stored against a carbon one, or dropped in a wet sink next to a cast iron pan, picks up rust by contact. The carbon rusts, and the rust moves house. Steel wool is the worst offender: scrub a stainless knife with it and you leave tiny fragments of plain steel embedded in the surface, and every one of those will rust. Use a nylon pad or a rust eraser instead.

And on a Damascus or san-mai knife, the cladding. Core and outer layers are different steels. On our Premium it's an AUS-10 core in a softer stainless jacket, and the two don't react identically. Some makers' cladding is actually less stain resistant than the core, so the pattern is where you'll see spots first.

Tap water, in some places. Hard water with a lot of chloride, or heavily chlorinated municipal water, left to dry on a blade leaves mineral spots that look like rust and sometimes are the beginning of it. If your knives spot even when you're careful, try a final rinse under filtered water and see if it stops. Sounds precious. Works.

Rust, or just a stain?

Worth checking before you panic. Orange or brown and slightly raised, feels rough under a fingernail: rust. Grey, blue, or rainbow coloured, perfectly smooth: a stain, which is the oxide layer thickening unevenly, usually from acid. Stains are cosmetic. They'll polish off with Bar Keepers Friend or just fade with use, and some people like them. Rust needs dealing with, because it spreads and eventually pits.

Black spots on a carbon-clad knife are a different thing again; that's patina, and the kurouchi page and the rust vs patina article cover that. On a fully stainless knife you shouldn't see black at all.

Getting rid of it

Surface spots come off with a rust eraser, that rubbery abrasive block every Japanese knife shop sells, or Bar Keepers Friend on a damp cloth. Rub along the length of the blade, not across it, rinse, and dry it properly. The rust removal page has the full method and the things you shouldn't use.

Spots right on the edge go away when you sharpen. Three or four passes on a 1000 stone and it's fresh steel again.

Pits are different. If the rust has eaten an actual hole, the rust around it comes off but the hole's there for good. On the flat of the blade that's cosmetic. On the edge it'll chip out next time you sharpen and leave a tiny notch until you've ground past it. Annoying, not fatal.

Don't use vinegar or lemon juice to remove rust from a stainless knife, despite what the internet says. It works on carbon steel by dissolving the rust, but on stainless it also attacks the oxide skin you're trying to preserve, and you'll get a dull grey patch where you cleaned. Mild abrasive, not acid.

Not having it come back

Wash by hand, straight after you're done. Towel dry, don't air dry. Store it where air can reach the blade: a magnetic strip, a saya, an in-drawer rack with the edge up. If you're prepping for a while, wipe the blade between the tomatoes and the onions rather than leaving acid sitting on it. Keep it out of the dishwasher, out of the sink, away from the salt.

If you're doing all of that and still seeing spots, you've got a steel at the hard end of stainless, and the answer is to treat it like a semi-stainless: a thin wipe of camellia or mineral oil before any long stretch of not using it. The care basics page covers that.

And if what you really want is a knife that won't rust no matter what, buy a softer stainless and make peace with sharpening it a little more often. There isn't a steel that's 63 HRC and also doesn't care about a wet dish rack. Something has to give, and the steel makers have been trying to get around that for a century.

A routine that takes thirty seconds

This is what I actually do, and I own knives at every point on the scale above, including a couple that would rust in a humid room.

After cutting anything acidic, a quick wipe on the towel that lives next to the board. Not a wash, a wipe. Takes a second and it's the single habit that prevents most edge spotting.

When I'm done cooking, hot water, a drop of soap, a nylon pad along the blade, rinse, then straight onto a dry towel and wiped both sides and along the spine. The spine matters; water sits in the little gap where the handle meets the blade and that's where a wa handle starts to rot.

Onto the magnetic strip, edge up, nothing touching it. The strip's on a wall away from the sink and the kettle. Steam counts as water.

Once a month, more or less, the harder stainless knives get a drop of camellia oil on a cloth wiped down the blade. The soft stainless ones don't need it and don't get it.

That's it. Thirty seconds a day, and in several years of doing it I've had one rust spot, which was on a knife a houseguest washed and left in the rack.

Storing a knife for months

If a knife's going into a drawer for a long stretch, or you're moving house, or you're going away: wash it, dry it completely, oil it lightly, and put it in a saya or wrap it in paper, not plastic. Plastic traps moisture. Paper lets it out. Don't store it in a leather sheath either; leather holds damp and the tanning chemicals can mark the steel. A cardboard box in a dry cupboard is genuinely better than most fancy options.

What I told the cross gentleman

That the knife was fine, that the spots would come off in two minutes with a rust eraser, that the dishwasher was the culprit and the partner was forgiven, and that if he dried it after use he'd probably never see them again. He wrote back a week later to say the spots were gone and the knife was, in his words, "annoyingly sharp." Which is the right outcome.

Andrew Kuzmin · Editor-in-Chief, Okami Blades