If there is nothing wrong with the paint I wouldn't recommend stripping it. If so just scuff it and feather out all the chips and scratches with 320, then prime and paint. If the paint is checking, flaking, or just really bad you'll want to strip it. They make special paint strippers for bumpers or you could have it walnut blasted. If it is this bad you might want to locate a bumper refurbishing place. They have some around here that boils the cover and fixes any damage. When you get the bumper back its pre-primed and looks perfect. They are actually easier to repaint than the factory replacement bumpers nowadays.