Toveling cast iron

Question

BS"D I kashered my old cast iron via the self-cleaning cycle. This left the keilim very rusty. I scoured them with sandpaper until they are almost completely free of rust, however, I think it may be impossible to completely remove every last bit of rust, as the metal is snot completely smooth, which is necessary for it to hold seasoning. So now what? Any suggestions on getting into those little bitty places? Can these keilim even be toveled? My rav says I must remove the seasoning from even new pieces, as the seasoning process is not supervised, even though OU soybean oil is used. Whenever any cast iron piece is kashered via a self-cleaning cycle of an oven, there is going to be small amounts of rust that simply will not completely scour off. Does this mean cast-iron is not suitable for a kosher kitchen unless purchased new and unseasoned? (almost impossible these days) Thanks.