I bought a nice little copper sauce pan at an antique store today for $15.00. Inside, the pan was a lot of scale (or something). I soaked the inside of the pan with a 50/50 water, vinegar mix. Some stuff still remained. So, I used some very fine (600 grit) sandpaper to clean it out. Now, you can see a little bit of copper showing through on the inside of the pan. Did I ruin the pan, or can it still be used safely with some copper exposed.?