How to Recoat a Part

Q. We have some blemishes in the metal or the coating that require rework and the application of a second coat. Is there a standard way to prepare the surface and apply a second coat of powder?

A. There are a few standard items to consider and maybe some variables depending on the coating and the nature of the defect. In general, make sure that the surface is smooth, clean and dry. Test a sample part for inner-coat adhesion. While many powders will have good adhesion with the second coat you should test it to confirm. If the adhesion is not good you may need to roughen the first coat with a light sand paper. Again, make sure that all dust is removed. If you use an in-line spray washer it is best if you do not run the already coated part through chemical treatment stages. A good rinse with good quality water is best. Residual chemistry can cause water spotting on the firs coat and potential failure of the second coat. Application of the second coat must cover the entire surface; you cannot spray a small area. Set the gun up with lower amperage than normal; say 10 to 20 micro-amps. Adjust the flow rate around 5 to 10 percent higher than the normal to deliver more powder but avoid excess velocity. The gun should be 1 to 2 inches farther from the part than normal. If your gun has a factory preset for recoat you should try it before manually adjusting amperage. This should get you a good part on the second coat but defects can still occur. I would recommend you spend time figuring out how to avoid them so that you do not have to recoat very often.