The primer fills in sanding marks and slight variations in the height of the sanded surface, but it still needs to be block--sanded to make it dead flat. Black paint is misted over the primer as a guide coat while block--sanding with wet 180--grit paper. High spots in the sheetmetal are revealed by areas that become sanded to bare metal, and areas where the black guide coat is not removed are low spots. This is the most time--consuming portion of the job, and hence the most costly. The more you pay, the more block--sanding you’ll get. Gonzalez blocked the car three times.