Powder Coating Over Electrocoat

Q: We apply a black gloss powder over an electrocoated door guide that fits on the side of a mini-van. We sometimes have difficulty getting good coverage with powder, especially on the inside cavity of the u-shaped door guide. The coverage is light sometimes and we see some roughness and dry-looking orange peel on some parts. I was hoping you could give us some hints on ways to overcome the combination of the e-coat layer and the Faraday cage effect of the inside slot. 

A: Application of powder over any first coat presents a challenge because the first coat insulates the metal part and creates resistance for electrostatic application. The attraction of the powder is largely dependent on good grounding of the part to attract the charged powder. With the insulating layer of the electrocoat the attraction is weakened.

There are several simple steps to use in gun adjustment and spray technique to help overcome the resistance created by the first layer. First, it should go without saying that the parts should have clean contacts and good earth ground. It is always important but it becomes critical for the application of a second layer of powder. The normal recommendation for adjustment for a second coat is to reduce micro-amps, slightly increase flow rate and move the gun 1 to 2" farther from the target.

One other method that can help is taking advantage of the factory pre-set position on the gun control panel for recoat. Not all spray guns have this feature but the majority of newer guns will have it and it can help to get more uniform coverage of a second layer. The manufacturer has programmed the right settings for voltage and amperage to help overcome the resistance. If the gun you use does not have this feature you can reduce the micro-amps to around 20 to 40 and get similar results. Combined with a very slight increase in flow rate and a slight increase in gun-to-target distance you will have your best shot at getting uniform coverage.