Factors affecting the roughness of the workpiece surface: the size of the steel shot

　　1) Increase the actual bonding area between the coating and the workpiece surface, which is beneficial to improving the bonding strength of the coating. The size of the steel shot and the ratio of the actual surface area to the theoretical surface area are statistically analyzed.

　　2) The coating will generate a lot of internal stress during the curing process. The presence of surface roughness can effectively eliminate the stress concentration in the layer and prevent the coating from cracking.

　　3) The presence of surface roughness can support the quality of part of the coating, which is beneficial to eliminate the sagging phenomenon. For vertically coated surfaces, the effect is particularly obvious. Surface roughness is caused by the combined effects of impact, cutting and scouring when the steel shot and steel grit particles contact the surface at a very high speed.

　　Therefore, the size, hardness, shape of the steel shot, hardness of the workpiece material itself, pressure of the compressed air, distance between the nozzle and the workpiece surface, and angle between the nozzle and the normal of the workpiece surface determine the size and contour of the surface roughness. Like surface cleanliness, surface roughness should also be determined by corrosion engineers according to coating requirements. The main basis for determining the size of the surface roughness value is the type of coating and the thickness of the final coating. In the absence of the conditions or technical data required to establish the surface roughness value, the surface roughness value can be temporarily determined to be between 25%-30% of the final coating thickness.

　　For example, if the final coating thickness requirement is 150µm, the surface roughness value can be determined within the range of 38-45µm. Another method is to determine the surface roughness value based on the thickness of the primer. The surface roughness value cannot be greater than the primer thickness, except when the second coat of paint is applied immediately after the primer is sprayed. If the surface roughness value is too low, it will be detrimental to improving the adhesion of the coating; if the surface roughness value is too high, the coating thickness specified in the coating design may not be able to cover all the peaks in the surface roughness, and pitting will occur after a short period of time, causing the coating to fail prematurely.