It's the vinegar in the mustard that does it, so clean your blade with alcohol to remove oil, then soak the blade (not the handle) in a jar of vinegar for a while, an hour or two probably. Blade will darken, turn dark grey.
After this happens, dry it till it's smooth to the touch, put a light film of oil on it, and sharpen. Good to go. Keep using the knife. Don't leave it wet.
All my knives are carbon steel, I live on the gulf coast and don't have much problem with rust. If you use a carbon steel knife every day, it will develop a patina on it's own, but it takes a while. If small rust spots show up, just clean them off. If you don't use it every day, just wipe it down where it's dry and light film of any kind of oil. Doesn't take much oil. Just wiping with a slightly oily rag is enough.