When 'scraping' for rubbing/polishing crug or rust off any surface, use a scraper made from something softer than the base metal or surface. For example, on steel, use a brass scraper, on other surfaces maybe even alum. or a credit card, etc. Always 'lube' or presoak the crud with even water; something to help soften the crud or scale before getting a bite at it with the scraper. This will minimize damaging the undersurface and even maintain patina.