Brake rotors are bare cast iron, so they rust - fast. A light film after rain is harmless, but heavy rust on a car that sits can cause noise, pulsation, and even bond the pads to the disc. Here is how to prevent and minimize rotor rust.
Why rotors rust so easily
The friction surface of a rotor is bare metal by necessity - a coating there would ruin braking. Exposed to moisture, it flash-rusts within hours - see why rotors rust overnight. On a daily-driven car the pads scrub the rust off in the first few stops, so it never builds up. The problem is cars that sit.
Driving is the best prevention
The simplest fix is use: a few brake applications each day keep the friction surface polished clean. If a car sits for weeks, the friction faces develop real rust. Where practical, drive the car periodically and make some moderate stops to scrub the rotors - especially important for low-mileage and EV/hybrid cars whose brakes rarely work; see do EVs wear brakes slower.
Coated rotors and storage tips
Quality rotors have coated hats and edges to resist the cosmetic rust that shows through wheels - see coated vs uncoated rotors - though the swept face is always bare. For storage, keep the car dry, avoid setting the parking brake long-term in damp conditions so shoes do not rust to the drum, and consider a dehumidified space. Do not paint or coat the friction surface.
When rotor rust matters
Surface rust that cleans up in a few stops is cosmetic. Deep pitting from long storage, or rust that causes lasting pulsation or noise, means the rotor may need resurfacing or replacement - see when to replace rotors. Fit new rotors with quality pads and hardware and bed them in.