I too can vouch for Harpic Power Plus to remove what seemed like ceramic damage caused by a metal edge. I had used a plunger which had a metal disc on it to clear a blockage, and it looked as though the metal disc had scratched through the enamel. I tried scouring the damage marks, and also tried a fine blade to see if it was just surface damage, both to no avail. I was convinced I had damaged the ceramic beyond repair, and was thinking I would have to buy a new toilet. I tried the Harpic Power Plus, and thankfully within 10 minutes it had dissolved the marks completely, leaving a pristine toilet again! Thank you to the OP for the Harpic Power Plus advice!