ideally yes if you want it done properly depends also where it is flaking if you want to spray only a small section . But i would say the paint peeling is a sign it has had a bad spray job on it done . To remedy that and stop it coming back you should sand down where all the peeling is and primer that but spray the whole bumper. use a scotchbrite pad to take shine off whole bumper before painting again . the paint will be peeling because it has not been properly prepared for spraying 1st time, i.e. rubbing previous shine from laquer before paint job
It is safe to use 120 grit or 80 grit or smoother when sanding paint on plastic panels 40 grit is a bit rough cuts in to the plastic a bit much.
When you have your area for priming all sanded and ready you would key the surrounding are with 320 or 400 grit so the primer sticks to the paint . you do not want to primer onto a shiny surface or primer will lift off or tear this is why the paint is peeling, it has been painted on top of a shiny surface