dd,
I assume your talking 'surface rust' correct? What area of the bumper are you seeing it? If it is on the 'chrome side' of the bumper then the plating was done incorrectly and you should seek a new bumper from your dealer as it is an issue that will never 'go away'. If it is on the underside of the bumper, surface rust can normally be found on all of these type of bumpers as it is an unpainted metal area.
Chris