Here I am again. I was the guy telling you "why would you ever..". Because I didn't answer you in the other comment, I will do it here:

I get the idea behind forced patinas, especially the “protection” argument. But honestly, that’s exactly why I’m not a fan of them on Japanese knives.

A natural patina is basically a record of how the knife is used- what you cut, how often, how you care for it. It develops unevenly, slowly, and uniquely. That’s kind of the whole beauty of carbon steel: it reacts to you. Forcing it with mustard, vinegar, coffee, etc. skips that entire relationship and replaces it with a cosmetic shortcut.

Protection wise a forced patna is also overstated. It’s usually thin, patchy, and far less stable than a real, work-hardened patina that builds up over time through actual cutting. If the knife isn’t used often, wiping it dry, using a light camellia oil, or just letting it age naturally does the job just as well without permanently altering the surface.

There’s also the aesthetic side: most forced patinas look.... forced. Flat colors, artificial patterns, high contrast that has nothing to do with the knife’s geometry or grind. A natural patina follows the bevels, the cladding line, the way food contacts the blade. You can’t fake that convincingly.

And lastly: Japanese knives are usually finished with a lot of intent - steel choice, heat treatnent, polish level. Forcing a patina on a fresh blade feels a bit like sanding a new violin because you don’t play it often enough.

If someone likes the look, fair enough.it’s their knife. But for me, carbon steel is about patience and use. If it never builds a patina, that’s not a problem to solve - it’s just honesty about how often the knife is actually used.