We have used a Whirlpool Glass-top cooktop for several years. it is light bisque colored and now seems to have gotten permanent stains on one burner. My wife and I have tried using the white cooktop abrasive, a Mr. Clean magic eraser and a razor blade scraper to no avail. The grey stains won't come off. I am wondering if anyone has found a way to clean stuff that won't injure the glass top. I think it might be a soft aluminum popcorn popper that did this, so aluminum cleaners might work but we are afraid the abrasive will damage the surface.