The bathtub at my new place isn't in horrible condition, but it does have a few patches on the bottom that look a bit grayer than the rest of the tub, and are rougher. In general the bottom is not as smooth as the sides. We think it is a cast iron bathtub (at least the bottom is all black when we look through an access panel), but we're not sure. There's no rust showing through.
Would it be a good idea to get it refinished now to protect the tub from further damage? Or is a new coat of glazing not significantly protective, in which case maybe we should just wait until we really don't like how it looks before spending the money (especially since the refinished surface won't last that many years either)?