Hi. I have a Weber gas grill with cast-iron grates that will not stay seasoned. I use cast-iron cookware and know how to keep them seasoned. But it seems the high heat of the grill burns off the seasoning, on the grates, leaving them dry and rust-colored. One day I seasoned them five times in a row, thinking I would build up the seasoning, but the first time I used high heat afterward the seasoning was gone. Am I doing something wrong? Any advice would be appreciated.