Well....IT SUCKS!!
Take the bumper and put it on a stand, get a bucket of water or a water hose, sponge if you use a bucket, 1500 wet/dry sand paper, wet the bumper, dip the paper, and LIGHTLY go over the bumper. If the runs are thick, work on them to get them to the level of the rest of the bumper, but BE CAREFUL NOT TO BREAK THROUGH!! Once you get the majority of the run cut down, sand the whole bumper, making sure you keep the paper & bumper wet. After you use the 1500, move to some 2000. Just remember, sand LIGHTLY or you'll break through and it will have to be repainted!!
Good luck!