Q. How about a slightly tarnished silver kidush cup. Does the cup have to be totally polished and shiny before Pesach kashering?

A. Mishna Berura (451: 21) rules that rust stains that do not have any “mamoshus”or substance to them, and when you scrape them no particles of rust fall off them, are not considered rust and do not interfere with the kashering process, as they only create a visual effect.
Therefore Horav Shlomo Miller Shlit”a maintains that even black tarnish stains that meet the above criteria, do not interfere. However he recommends that if there is a kosher lePesach question on the polishing cream or liquid, one should first polish and then kasher.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a