Carbon steel takes a sharper edge than stainless and is easier to sharpen, but it rusts if you look at it wrong. Even stainless knives get spots in a damp pocket or pack. A thin film of oil is the cheapest insurance there is.
Pocket and folding knives
Folding knives collect lint and grit in the pivot, which makes them stiff. Blow or brush out the dirt, put one drop of Ballistol on the pivot and open and close the blade twenty times to work it in. Wipe off anything that seeps out. One drop really is enough; more just attracts dirt.
Hunting and bushcraft knives
Blood and moisture are hard on blades. Clean the knife as soon as you can after use, dry it fully and wipe on a thin film of oil before it goes back in the sheath. If the sheath is leather, don't store the knife in it long-term: leather holds moisture against the steel and the tannins can mark the blade. A light coat of Ballistol on the sheath itself keeps it supple.
Kitchen knives
Anything that touches food should get a food grade oil. Use Ballistol H1 Food Oil, which is NSF H1 registered for incidental food contact, on carbon steel kitchen knives and on wooden chopping boards. Wash and dry the knife, then wipe on a few drops with kitchen paper.
Wooden and horn handles
Untreated wood handles crack when they dry out. Every few months, rub in a little oil, leave it an hour and buff off the excess.
Removing light rust
For small orange spots, apply oil, leave it ten minutes and rub with a cork or a pad of fine (0000) wire wool. Deep pitting needs proper abrasive work or a professional, but catching rust early usually saves the blade.
Safety: always wipe away from the edge, and close folding knives before oiling the outside.
