On most paint colors, it's easy to spot a scratch that goes all the way through the base coat and into the primer, since the primer is white-ish. However... I'm looking at some scratches in Audi Ibis paint - flat white, not metallic - and I can't really tell if they went all the way to primer or it's just the clearcoat that's been scratched.

The scratches are deep enough for a fingernail to catch, so they will definitely need a touch-up and some wet sanding / compounding. Question is - do they need a base coat as well, or just the clear coat?