The basis for a good paint job on your vehicle starts with the prep: from the first time you sand the metal down to the last coat of primer before paint. If you want a quality finish with a nice, smooth paint job, sanding the primer is critical. You want to take your time and really pay attention to the details. Although it isn't as glamorous as spraying color, without proper sanding, your final results will be less than perfect.
Park the vehicle in an area where you can get the ground wet and dirty as there will be a big mess when you're done so make sure you're on concrete with a water source nearby.
Spray the entire primered surface with the black spray paint, dusting color over the surface of the primer but not putting on a heavy coat of color. This is a guide coat, designed to be sanded off; and although you can buy specialty guide coat products, black spray paint works just as well.
Fill the bucket with water from the hose and place the sandpaper in the bucket to get the sandpaper flexible and make it easier to work with.
Wrap the 120-grit sandpaper around the sandpaper block and wet down the surface where you want to start sanding. Begin sanding down the area using your hand on the block, moving around the area frequently and keeping the surface wet. Try not to focus on one area more than another so that the entire surface has a uniform finish. Sand down the entire vehicle, switching to a fresh sheet of 120-grit paper when it gets gunked up with primer. If you don't get all of the black paint off, that's fine for now.
Repeat the sanding process with the 240-grit sandpaper. Continued sanding will straighten out the primer, giving your final paint job a smooth, flat finish, void of waves and wrinkles that are associated with poor quality paint. You want to get out all of the scratches that you put in the primer with the 120-grit sandpaper as well as smooth out the primer a bit more. Make sure the surface is always wet as it helps keep the dust down as well as make the job easier.
Repeat this process, stepping up to the 300-, 400-, 500- and 600-grit sandpaper progressively. You will be ultimately sanding the vehicle down six times. This may seem excessive, but the more time you take doing the details, the better the final result will be. If you still have low spots indicated by black spray paint, then you may want to consider re-priming the vehicle.
