Its purpose: First, the workpiece surface suitable for coating deposition, increase the bonding area; the second is to overcome the shrinkage of the coating stress. Some parts of the workpiece corresponding pre-processing to disperse the local stress coating to increase the shear resistance of the coating. Common methods are cutting corners and pre-coated troughs. The rough surface of the workpiece thread is also one of the commonly used methods, especially in the spraying of large parts commonly used thread turning to increase the combined area. Turning thread should pay attention to two problems, first of all, the thread cross-section should be suitable for spraying, rectangular cross-section or semicircular cross-section is not conducive to the combination of coating. In addition, the thread should not be too deep, otherwise the coating is too thick, the cost increases. It is also possible to “knurl” the coated surface or to combine the thread turning and knurling.