Learning "On The Job"
If you had to take a guess as to the source of my adhesion problems, would you say it is because of my pretreatment methods?
Q. I work for an automotive performance shop in the U.S. The company has been expanding its offered services, and we have been powder coating for close to a year now. I have been, more or less, put in charge of all things concerning powder coating, and I honestly don’t have the knowledge that I feel I should. My training was “on the job” and given to me by someone who I believe did not really know what he was doing.
All I have at the moment is a small Nordson manual gun (gravity fed, small funnel). I am lucky enough to have three ovens at my disposal, a small one the size of a mini-fridge, a medium oven sized like a refrigerator, and a large oven a good 8 ft tall and almost 20 ft deep, and pretty wide (we did a ‘67 Chevelle chassis recently, so it’s not small!).
However, I am personally very displeased with the results of my powder coating. The paint looks good—the coverage is very even and smooth and it looks lovely. However, the paint is not tough at all. It chips very easily and doesn’t seem to adhere to the metal very well. The vast majority of parts that I powder coat are steel, aluminum, or chrome-alloy. Smooth aluminum seems to cause the worst problems. I believe that my preparation work is the root of my issues. I try to have my customers bring me sandblasted parts, but this is not always the case. I only have acetone to clean parts with, but I read that zinc or iron phosphate should be used. However, I don’t have enough pull in the company to dictate what equipment I have at my disposal, so I am somewhat at the mercy of my superiors.
If you had to take a guess as to the source of my adhesion problems, would you say it is because of my pretreatment methods? I really would like to resolve this problem. I want to be able to present my customers with a quality product.
In addition, I also think I may be having problems with the powder itself. Storage is poor in terms of the temperature and surroundings, and I think that a lot of the powder is simply old. Some of the powder that I have to use is probably 6 months old, if not older. B. A.
A. You are a wise man to admit that you may not have all the knowledge you need to perform your job properly. Most people prefer to be “know-it-alls.” I know that I am learning something new all the time. This is what makes life interesting. I often tell my customers; “I am not smarter than you, I just did this before.”
First, let’s address your training issues. There are numerous classes offered by various trade groups and organizations across the U.S. The best and most comprehensive is the “Two-Day Hands-On Training” offered by the Powder Coating Institute. This course is commercial-free, and offers the most information and the best textbook—Powder Coating: The Complete Finisher’s Handbook.
As for the quality of the powder coating you are currently performing, I recommend the following procedure:
- All parts must be cleaned before coating. Usually media blasting or chemical cleaning agents work the best. Solvents like acetone will melt and dilute greases and oils but have no effect on other soils. Also, if you don’t change your cleaning rag often, you will just smear these oils all over the part. Without complete cleaning of the part surface before powder coating you cannot guarantee proper adhesion.
- The part surface needs to have some surface roughness (tooth) for the coating to bite onto. This can be accomplished by the mechanical methods (media blasting, sanding, wire brushing, etc.) used to clean the parts. You can also get this surface roughness chemically by using iron phosphate, etching, pickling, chromating, or other conversion coatings.
- Improvements to corrosion resistance can be accomplished by using iron phosphate or zinc phosphate on ferrous substrates, conversion coatings on aluminum substrates, or zirconium coatings on most all substrates. Zinc-rich epoxy primers also help improve corrosion resistance on ferrous substrates.
- Powder coating thickness control is key to coating performance, as well. Too thin, and the coating will not protect the surface as expected. Too thick, and it becomes brittle. Follow the coating supplier’s guidelines, and you won’t go wrong.
- The coating must be fully cured to ensure that all the coating properties are developed in the resultant film. If the powder is not fully cured, it will chip, flake, have adhesion failures, corrode more quickly, and soften when exposed to chemicals. An under-cured powder coating will often look the same as a fully-cured powder coating, so check the coating using MEK and a cotton swab to determine cure. Soak the swab in MEK and rub it on the cured powder coating using moderate pressure. Count the double rubs (a double rub is one back and forth motion of the cotton swab) to determine cure. Most cured powder coatings will withstand at least 50 double rubs without softening or coating removal. Check with the coating supplier for the number of double rubs to expect on a fully cured coating. By the way, perform this test in a hidden area that your customer will not see, as the test can change the gloss of the coating.
Finally, toss out any powder that is past its expiration date. Most powder coatings will have a one -year shelf life if stored in an area that controls temperature to under 70°F and relative humidity to less than 40%. If the powder is stored in an environment that is above these thresholds, then shelf life is dramatically reduced (to approximately three to six months). Low-temperature cure powder coatings are even more influenced by temperature, so these have even shorter shelf life. I always say: “When in doubt, throw it out.”
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