You really need to monitor your heat buildup, you can melt the finish with a PC with the right pad. Remember that plastic, especially the soft vinyl (or whatever flexible material the bumper covers are made of) has much less thermal conductivity than a metal panel, which means the heat of polishing will not be dissipated well and the interface may become quite warm. Stop frequently and feel the surface and the pad, if it overheats it will happen too fast for you to do anything about it.
