I've used Duke's Mayonnaise, doesn't seem to be as harsh as mustard and might take several applications. Why use Duke's?, because it beats the heck out of Hellman's, has more of a lemony taste, which probably means it has more real lemon juice, and it is my favorite mayonnaise for sandwiches and burgers and I always have it on hand. I have also cut a Granny Smith's apple in half and rubbed it on the steel it will give you that medium gray to light black finish. You will have to just tinker around until you get a finish to your liking.