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You must use actual paste, not a gel toothpaste. The paste contains abrasive material that scrapes plaque off your teeth. This will sand down the scratched surface as well, hiding the scratch causing the laser to misread.  Baking soda toothpaste works well due to the extra grit. If you're lucky, your toothpaste will list an abrasion rating, called RDA. No one's tested how RDA affects disk cleaning, but a higher rating (120+) should work better. A small dab should be enough. As before, use a clean, lint-free cloth such as cotton or microfiber.
Choose the toothpaste. Apply a small amount of toothpaste to the polishing cloth.