After this first round of block-sanding, the Mustang was back in the booth for two more coats of primer, followed by a round of wet-sanding. What is the reason for the extra coats of primer? The first round of block-sanding can expose spots of body filler or metal glaze, which is more porous than primer. If you just topcoat over this stuff, it will absorb the paint, leaving a puckered spot that is lower than the rest of the surface. Also, block-sanding is usually done with 220-grit paper, which will leave sand scratches of its own. Priming the car one more time will cover any exposed body filler or glaze and all the 220-grit sand scratches.