Shake the aerosol paint can hard and long to mix the primer paint thoroughly - follow the instructions on the can. Apply a coat of paint to the damaged area, but do not go over the edge on to the surrounding paintwork at this stage. Let it dry, then put on another coat, moving the spray in longer, horizontal sweeps to cover the whole area.
Small chips and blemishes are best repaired with a can of touch-in paint and a brush. For larger...