Plastic injection molded model kit parts will sometimes have a rough edge or "mold line" on the part as a result of the manufacturing process and are not limited to Revell kits. These can be removed and the appearance of the part improved by simply sanding the edge off the part with a small file, emery board or even just sandpaper. Do this prior to painting your parts for a more realistic and professional appearance. Use a light touch as styrene plastic is soft and will easily sand.