As mentioned above, rust is a natural occurance for cast iron that is not a problem. If your issue with it is appearance then you could try a product/chemical called "ospho". I have used it many times in the past with great results. Brush or spray it on, (windex type spray bottle will do), let it sit over-night and the rust becomes a hard black paintable substance. I've even used it prior to painting a rusty engine block. The paint held up for as long as I kept the car. Search it on-line and it will explain the chemical reaction. Not all hardwares carry it, but some do.