Summit says its 2K primer can be blocked after three to four hours, I actually left it two days before sanding it with 80-grit paper. As with the filler, I used flexible long-blocks for this. Note the crosshatch pattern visible on the rattle can matte black guidecoat (arrows). Always sand this way and you won’t get grooves in the primer (or filler) from continually sanding in straight lines. Even after all the laborious sanding of the filler, there were still high and low spots in the primer. That’s where high build primer comes into its own. Don’t worry if you sand through the primer, as you’ll be priming again, though if you hit bare metal (or ’glass) and still have low spots, you probably didn’t prep the area correctly before priming.