Think of it this way. There is mechanical and chemical adhesion. Chemical is always stronger. After a certain period of cure time, chemical adhesion is lost. Epoxy takes much longer to lose chemical adhesion than 2K. Therefore, after something is fully cured, you are relying on mechanical adhesion only. Coarse, deeper scratches give better mechanical adhesion than fine, shallow scratches because there is more surface area for the product to grab on to. What this all boils down to is compromises. How much adhesion is needed to keep the paint from peeling, or chipping easily? The absolute best would be chemical adhesion start to finish, but that is not always practical. Second best would be mechanical with coarse scratches, again not always practical. You can't spray base over 180 scratches, for example, because of appearance, but the adhesion would be better than 600 scratches. My point in all of this is to shoot for as much chemical adhesion as practical, and use the coarsest sandpaper possible, that won't cause appearance issues.