I had the exact same problem in my Weber Genesis. I had upgraded from the grates that came with it to cast iron grates and they were not cheap. I could not keep them seasoned and I didn't like the idea of cooking on rusty grates. I finally just tossed them and purchased the porcelin coated cast iron grates and I love em.
18.5 WSM 18.5 Jack Daniels OTS Weber Genesis