How do I prevent rust and care for a carbon steel knife?

Carbon steel reacts with water, acids, and food, so the single most important habit is drying the blade thoroughly the moment you finish washing it, and wiping it between tasks when cutting citrus, tomato, or onion. Never leave it soaking or in a wet sink.

A grey-blue patina that builds with use is normal and even protective; active orange rust is what to prevent. If a knife will sit unused, wipe on a very thin film of food-safe oil before storing it.

Our K&S take, naming only what we stock: use the medical-grade Kitchen Knife / Cutting Board Mineral Oil for routine blade protection, and lift light surface rust or refresh a finish with Koyo Sunlite Polishing Paste (Blue for Steel). We do not stock camellia or Tsubaki oil, so we recommend the mineral oil in its place.