Use masking tape to limit the area where the putty will spread. Use a toothpick or spatula type tool to apply the putty evenly over the seam or depression. Remove as much excess putty as possible while it is still wet. Let the putty drive overnight, then sand the joint smooth with 400 grit sandpaper wrapped over a wooden Popsicle stick. The puttied in area will absorb paint more than the plastic, so be prepared to paint and sand it several times to remove any traces of the putty.