Abstract: Broadly, the present invention is directed to a method for electropolishing elongate metal mandrels in an electrolytic cell, wherein the electropolished metal mandrels are ideally suited for growing CVD diamond thereon for making water jet nozzles and similar flow control devices. The method of the present invention comprises placing an elongate cylindrical mandrel in an electrolytic cell between a pair of centering caps. The cell comprises an elongate annular cylindrical electrode, which preferably is a cathode, and which has open ends in which said pair of centering caps are placed to center said mandrel within said cylindrical electrode. The cell further comprises an outlet and an inlet connected to a circulating source of electropolishing electrolyte. The mandrel and the cylindrical electrode are connected to a source of electrical power. This electrical power is applied to the mandrel and the cylindrical electrode to establish an electrolytic cell.