Plastic filler will always leave some pinholes and surface irregularities. To provide a surface that's uniform and has a fine grain, the filler is glazed. In the old days, glazing putty (basically the same as the spot putty) was nothing more than primer solids in a paste applied with a spreader. Regular glazing putty is tricky to apply smoothly over large areas, and because drying is by the evaporation of solvents, it tends to shrink and require long drying times if applied too thick. Modern, two-component polyester glazes are watered-down plastic fillers with fine grains. These go on smooth, dry by a catalyst, and sand easily-same rules as when working with the filler, only here a smooth application is even more critical.