Went to clean up a stem today and had a problem with the buffing wheel used for Tripoli. I got these white streaks show up on the stem. The streaks appeared to be a coating that I could scrape off with my fingernail. I did not add any Tripoli to the wheel when I started so I added some and got the same result. I have been using the same wheel for Tripoli for years and can't think of why this is happening. Any ideas?