Shopping for rotors, you will see some described as coated - often geomet or zinc coated - and others plain (uncoated). The difference is mostly about rust protection and appearance. Here is what it means for you.
What coated rotors are
A coated rotor has a protective finish - commonly a zinc or geomet coating - applied to the surfaces that are not swept by the pads: the hub, the edges, the cooling vanes, and the inner hat. That coating resists the surface rust that quickly forms on bare cast iron, especially in wet or salty climates. The friction surface itself still ends up bare where the pads ride, as it must.
The benefits of a coating
- Rust resistance on the non-friction areas, so the rotor stays cleaner and lasts better cosmetically.
- Better appearance through open-spoke wheels - no orange rust ring around the hat and edges.
- Cleaner install - coated rotors usually need less cleaning of shipping oil, though you should still wipe the friction surface with brake cleaner.
What uncoated rotors offer
Plain uncoated rotors cost less and stop exactly the same - the braking surface is identical. Their only real downside is that the exposed cast iron surfaces rust cosmetically over time, which is purely an appearance issue in most cases. For a hidden wheel or a budget job, they are perfectly fine.
Which should you choose?
If you live where roads are salted or wet, or you have open wheels that show the rotor, coated rotors are worth the modest extra cost for looks and longevity. For a tight budget or wheels that hide the rotor, uncoated is a sensible save. Either way, replace rotors in axle pairs and fit fresh pads and hardware - see do you need to replace both rotors.