How can I tell if a scratch is too deep to polish out?

If the scratch has penetrated through the clear coat into the color coat or primer, polishing cannot safely remove it. Signs include a white, gray or different-colored line at the bottom of the scratch. A scratch that strongly catches a fingernail is more likely to be deep, but the test is only a rough guide. Proper inspection and paint-thickness awareness are needed before aggressive correction; deep scratches may need touch-up paint or repainting by a body shop.