What the bumper rechromers do is put the bumper in a press to get into the general original shape.
Then for the dents they do a process called Pick and Filing. 3/4" wide pick blade on a hammer that they use to whack the dents from the back side, then they grind the area smooth to shape on the outer side of the bumper. On the Rechrome bumpers you can see the hammer/pick marks on the back side from doing their prep work.
It's called metal finishing, a bumper rechrome shop has the skills to prep a bumper for you. Or a skilled body man may be able to help you out.
