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Take your rotors off, sand off the visible rust, mask off the swept surfaces (as good as you can), and spray paint them with high temp exhaust paint. I painted mine black right after I got my car new, and a year later you still can't see any rust. Some people have painted theirs silver and have had great success.
It's not as good as cadmium plated rotors, but all I wanted is not to bring attention to the rust on my rotors, especially because I painted my calipers red.
here's a pic of my rears. it's basically the stock set up (minus the rims and tires of course) with the calipers powdercoated and stock rotors cadmium plated, slotted, and drilled.
PM sherwin (board name: docofmind) for price. he's the man.
Cad plating will not hold up with heavy use.. I have tried more than once and one good day at the track and the heat destroys all plating on the edges and vanes. Over time the plating on the hub portion will also be affected.
High temp paint actually holds up better in my experience. Although it is a lot of work as the best technique is to pull them off, clean and prep with rust treatment, paint, and then bake in the oven. The resulting finish looks good and will last longer than cad.
The other options are rotors made from a rust resistant alloy or a different type of coating. Neither are readily available.