Recoating Over Powder

Q: We work hard to get a good coating on our parts, but we sometimes need to recoat a part due to minor defects. I am having a lot of trouble getting good attraction to the surface and getting a good-looking and consistent second coat. We get some dull-looking areas sometimes and occasionally a grainy look. We also sometimes have trouble with adhesion of the second coat. Can you give some advice on recoating and ways to make it work more reliably?

A: Successful recoating starts with understanding the powder you are using. Some powder materials will adhere to the first coating with no problem and no special preparation. Some materials do not have good intercoat adhesion properties and may require some surface preparation to promote intercoat adhesion. In all cases, it is important to ensure the surface is clean and dry. You can do that by rinsing the surface and running it through your dry-off oven. With some powder materials, it is necessary to scuff sand the surface to ensure good adhesion of the second coat. Some trials can tell you if a powder has to be sanded before a recoat.

As for the appearance, keep in mind that powder is attracted to the grounded part surface by electrostatic attraction. The powder particle is loaded with electrons seeking earth ground to bleed back into the atmosphere. When they come in contact with the bare metal, the electrical charge will begin to dissipate from the powder to ground. All of the charge does not immediately go to ground and the material will cling to the bare metal for a long time. As the layer builds, the surface becomes somewhat insulated and the attraction gradually diminishes due to a reduction in the pull of earth ground. The cured powder surface is insulated and the earth ground attraction is very low. The application efficiency during a recoat is lower due to resistance from the first coating. The surface of the part is covered and it will reach a point where the charged powder on the part is at, or near, the same potential as the powder being applied, thereby creating back ionization or rejection.

The gun settings for the application of a second coat should be modified to compensate for the resistance of the first coat. The electrostatic charge should be reduced (either by lowering voltage or limiting current). The gun-to-part distance should be increased slightly to soften the forward velocity of the pattern and further reduce the current draw to the part. It is also helpful to adjust the powder delivery settings to increase the output by 10-20%. Be aware that the application will require more time and transfer efficiency will decrease when doing re-coats but application is possible. The reduction in charge, combined with a slight increase in flow rate and keeping the gun farther from the target, should provide the coverage needed. In order to avoid the dull areas and grainy appearance, be sure to cover the entire part thoroughly.