Die cast zinc metal is a hard material used for casting the body, the surface of the casting is far from perfect and it requires a lot of workers to smoothen the surface bit by bit. To achieve a good paint finish of a metal body model car, every visible surface must be trimmed and polished by all kinds of files and sand paper until it is nice and smooth before applying the paint. The smoother the surface, the better the paint finish. All the parting lines, opening and edges must also be smoothened by files and buffed with polishing wheels. It is the most labour intensive process among hundreds of processes in manufacturing each of every piece of die cast model car.