paint right on to a rusty rotor, please help

[geddy lee]

2009 370. I just had new rotors and brake pads installed. 2 weeks later the rotors are all rusty and it looks like hell. I guess I got a cheaper grade steel ie: not OEM replacement.
Anyway , there is a solution without going back to the repair shop and hassling with them. When I had my 03 350Z, I found this paint on line called Rust Bullet. I simply wiped my rotors off then painted the rotors directly on to the rust. It dried to a silver color and completely covered the rust. Somehow it attaches to the rust chemically. They never rusted again. The only problem is you have to order it on line and it dries silver. I have a Magnetic Black Z and I would like to find a similar product that dries black. I have since heard that there are several paints that attack the rust and turn black but I have not found anything in less than a gallon ( for $ 80.00 ). Has anyone found something that will do what I wnat?

Thanks