NAPA sells this de-squeak stuff that you spray directly on the rotors. The intended purpose is to spray on the rotor and fill in any imperfections in the metal…but it doesn’t quite work like it should there.
I figured it would work best as a rust inhibitor, so I tried it last winter and had great succsess!
You have to pull the wheel though. It is a graphite style paint that will keep the rust off, and it acts like a paint so be careful of overspray.