Never.
Well, that is to say, almost never. If the blade’s rivets have come loose or its handle has fallen off, then yes, you should buy a new knife—this time, one with a single-piece construction . But if you’re thinking about tossing out your old, trusty knife just because its blade has gone dull, you need to think again. Almost any blade can be sharpened. You just need the right tools.
Professional chefs know to treat their blades with a sharpening steel before each use. But what they don’t typically know is just how a sharpening steel works. You see, these tools don’t do any actual sharpening. All they’re doing is realigning the blade. That’s why knife experts (e.g., the narrator in that video) insist instead on calling these things “honing rods.” The difference between sharpening vs honing a knife is subtle, but it determines just how well your blade slices through any given stalk of celery.
The metal at the very end of a knife blade is a lot like the metal in a pipe cleaner—it’s thin, bendable, and prone to take a new shape. When you press a blade down on something solid—be it bone or tomato or cutting board—its utmost edge will bend at a microscopic level, folding in on itself. A knife becomes dull when its blade develops this sort of microscopic hook.
Honing rods work by catching these invisible hooks and smoothing them back into their original shape. In this way, a honing rod can keep your knife sharp for a few months at a time. After that, though, it needs to be run through an actual knife sharpener.
Hone by Meal, Sharpen by Season
Just like with a pipe cleaner, after you bend and unbend and rebend and straighten out a blade over and over again, it becomes permanently warped. This happens even with diligent daily honing. Once again, a honing rod is not a knife sharpener, it’s a knife straightener-outer. To keep your blades fresh, you must sharpen them every few months with something that’ll actually grind a new edge into the metal.
Knife sharpening machines use diamond encrusted steel to grind away a knife’s old edge and cut a new, stronger one. To return your old blade back to its original sharpness, get a good light-duty knife sharpener for your home or a heavy-duty knife sharpener for your restaurant. Or, if you’re still hell-bent on manually rubbing your knife against some rock, knife companies do make what’re called knife sharpening stones–just make sure you read up on how to use it correctly first or you’ll end up chipping your blade, making it even worse.
So what’s the point of the honing steel, then? Why not just use a knife sharpener exclusively? Easy: repeated use of a knife sharpener will grind a knife into a long, useless metal toothpick. A honing rod and knife sharpener, when used together, can sharpen any dull knife and keep it that way for a lifetime.
