For all my CI grates, I keep a squirt or spray bottle of peanut oil on hand and as they cool after a cook (< 250*) I scrub them with a wire bripush then spray them down. Never seen a spot of rust and they are always ready for the next cook. based on comments above, I might switch to flaxseed oil, but my method works so I'm sticking with it.
Weber 26.75 OTG w/CI grates & SJ/WSM Mini
Home built 24"x72" reverse flow stick burner trailer with 18"x44" grill w/CI grates and upright SS gas oven/smoker.
Home built 48" fire pit with a 30"x30" Santa Maria style ranch grill.
Home built lump charcoal retort, iGrill & a bunch of other cooking toys.