Porcelain has a tendency to develop fine gray marks on the surface over time that can easily be mistaken for scratches. Porcelain is glazed to prevent scratching, so those gray lines are actually metal deposits from cutlery use that sit on the surface, rather than scratches that are etched into the porcelain. To remove the gray lines, make a paste with baking soda and water, and scrub your plate in a circular motion. It’s best to do this as soon as you see them start to develop, since they are easier to remove when faint rather than built up and darkened over time.