22-09-24, 09:07 PM
Discounting chemical reaction, water marks etc, I'm leaning towards hairline cracks at the surface. Reason I say that is because the rotor looks to have its fair share of wear judging by the steps at the outer and inner edges, with the deepest scores across the affected holes. As rotors wear, the characteristics of the metal can subtly alter due to constant heating/cooling/chemicals on the road etc and this can show up on the surface as hairline cracks a few microns deep. I'd check the rotor thickness against the minimum spec first and if on the limit or below, then new rotors first up