Generally, if bumpers are to be painted only, then bumpers are taken OFF the car, and you can take your car away.
No sane and sensible painter asks you to leave the car there, if bumpers have to be painted. And there is no fun in it too.
Why bound yourself, and your car, if bumpers have to be painted?
Take them OFF, give them to painter, Come on the time / day given by the painter, have them affixed on your car, and you are good to go.