I use a product called "Squeak Relief" by Lubegard. It's a compound of wax and aluminum powder in a spray form. You'll find it with the brake cleaner at your favorite parts store. There's a similar product from Wurth, can't recall the name. I was surprised that you could spray this all over the rotor to prevent rust on the hat and edges. Anything that hits the friction surface seems to burn off the first time you press the brakes, with no drama.

Squeak Relief Brake Finishing Treatment