I parted the center out of a 12" brake rotor recently and then attempted to turn it flat attached to a face plate in an attempt to make a simple ring for use in tramming a mill. I ended up turning a warp into it, perhaps a clamping error or something....I have yet to troubleshoot it and try again.
I write all this because its clear to me from this experience and from watching rotors turned that there is a good reason to let someone with a brake lathe turn them :-) Yech.....what a mess. Turning cast iron in general is messy, but turing something out at the outside of 12" is a great way to end up with cast iron granules all over your good lathe. I tried to cover things up as much as possible, but its not all that easy when you fling particles around that much. That was a new, but scrapped rotor with no rust whastoever. An old rusty piece of junk would be *much* messier. The very-new brake lathe at our just-opened local OReilly auto parts has an enclosure that is a downdraft cabinet with a vaccuum. Pretty slick really....but I couldn't convince the (also brand new) employees that they could take more than about .003 per cut until they got close :-) Its hard to communicate that you can wear out carbide faster by taking a lot of light cuts than by using it to get the job done. It took them 1.5 hours to do two sides of two rotors.
Even when taking my last set in to be turned, I hit them with a wire cup brush to take off the majority of the rust after washing them with Castrol purple cleaner to remove the asbestos dust. Its nice to not have rust flaking off all over the place on your newly turned rotors.