The proper method for sanding filler is to use 80-grit sandpaper on a sanding block, following up with 150-grit. Ferre showed the students how to use the different sizes of sanding blocks to fit the various contours of the car. To avoid sanding grooves and flat spots into the repair, sand in an X-shaped pattern using the longest block that will fit in the area being sanded. It is also helpful to spray a guide coat-a very light misting of a contrasting paint color from an aerosol can. Then sand until the guide coat is gone.