Like Clint, my rasps never touch anything but wood. That's one of the first lessons I give new bowyers who come over. First time they stack them on top of one another, my best tools get put away until they go home.
I rarely use a farriers rasp, but when I do, it better cut like a new one. I was given a couple dozen used rasps by a farrier one time. I couldn't give those things away fast enough.