According to CarsDirect, a new application of primer paint is necessary before applying a new topcoat to a car. The process involves cleaning and sanding the car in preparation for the primer. Once completed, a properly primed surface is capable of backing a no-blemish topcoat.
Clean the car
Clean the car thoroughly starting with a grease or wax remover and finish by washing the entire surface with car washing liquid and water. Make certain that all dirt and debris is removed from the vehicle, as even a bit of dust can result in a flaw in the primer.
Sand the car
Sand the car smooth in preparation for the primer. Use a tough-grit sandpaper to deal with any rough areas on the surface, then switch to a fine-grit sandpaper to smooth the entire surface out. Fill any small dips or holes in the surface with putty, allowing them to dry before sanding them level with the rest of the surface. Wipe away any residue with a clean, lint-free cloth.
Dry the car
Allow the car to dry completely before applying the primer. Ground the car by attaching a wire to the frame of the car and attaching the other end to a grounded object. This should prevent dust from falling onto the car as it dries.
Mask the car
Cover any section of the car that isn't supposed to be primed with masking paper and tape.
Prime the car
Put on an air mask and goggles and cover any exposed skin. Ventilate the room by opening windows and doors. A standing fan may also be helpful in adding ventilation. Apply the primer to the car using a spray painter for a nice even coating. Work in long strokes, overlapping each stroke slightly over the last to ensure coverage. Allow the primer paint to dry and then sand the car with fine sandpaper to create a smooth even surface. Wipe away sanding residue with a clean, lint-free cloth.