Applying Powder to Steel

Question:

I am contacting you in regards to applying powder coating. I am a small job shop in Michigan and am having trouble applying the metallic and sparkle colors to steel. I am not getting a uniform color and when applying a two-coat system, I am having trouble with the second coat adhering to the first coat. I recently purchased an external muzzle for my powder gun. The final outcome looks a lot better but I am missing something. Any help would be greatly appreciated. M. M.

Answer:

The type of powders you are using (metallics, two-coat powders, etc.) are the most difficult to apply consistently. Using an external electrode muzzle helps, I am sure. However, part grounding, environmental conditions, compressed air quality, operator technique, film thickness and so on will all influence how the final product looks, especially spraying the types of powders you are using.

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The easiest powders to apply are flat and low-gloss materials. The next easiest are high-gloss materials, followed by clears and textures. Metallic powders come next on the difficulty scale, followed by tinted clears, flamboyants and finally multi-coat systems. You are using some of the most difficult types of powder to apply consistently. However, that does not mean it is impossible to get the look you want.

In order to obtain consistent results, you must control all the coating variables within very tight tolerances. For instance, film thickness control can greatly influence the look of metallic powders, tints, textures and flamboyant powders. The same is true for part grounding, environmental conditions (temperature and humidity) and compressed air quality. However, the most significant variable that affects how a powder coating looks using these materials is operator technique. A well-trained operator using the same coating techniques (i.e. gun motion, coating sequence, gun speed, powder output, etc.) can have great success in producing the same results. However, if the operator does not take great care in what they are doing, the results will be very different from part-to-part.

Inter-coat adhesion between two coats of powder is mostly influenced by powder formulation, but process control can also have an impact. First, check with your powder formulator to see if the two powders are compatible for a two-coat process. Next, verify that you are controlling film thickness and oven temperature and oven dwell time within their specified limits to ensure that you obtain good inter-coat adhesion. This problem should be easily corrected with the powder manufacturer’s help.

Finally, understand that no matter what you do, your defect rate will be higher using these high-risk powder coatings. The defect rates using these materials may be 2–3 times higher than flat and high-gloss powder coatings. Just be sure that you take that into account when you price the job.