It is always a good idea to clean plastic parts before you try to glue them. Also, wash your hands before you clean the parts to keep you skin oils off of the plastic. This is true for painting bodies, gluing tires, getting double sided tape to stick, velcro, and most anything else with self adhesive tape on it. Plastics right out of the bag will have this problem since the manufacturer has to use some kind of mold release lubricant to get the parts out of the molds when they are made. Kind of like using cooking spray when you bake something in the oven. This mold release substance doesn't usually get cleaned off until you do it. I prefer to use rubbing alcohol since it will dry fast. I wouldn't recommend using it on clear plastics like bodies. For bodies, use warm water and plain soap. Does anyone else do this??