By Clifford K. Schoff, Schoff Associates
I have published many of the thoughts and recommendations in this article previously in CoatingsTech. However, since paint problems continue to occur and there may be new readers facing them, I thought that this was a good time to discuss problem solving yet again. Surface defects probably are the most common type of paint problem, but it is not always clear what the defect is. That must be established before the investigator can look for causes and consider possible solutions. That may seem obvious to the reader, but I have seen many cases where it was not done, and the wrong countermeasures were taken. Although this article is aimed primarily at surface defects and failures, the basic process presented applies to nearly all coatings problems.
The keys to problem solving are using your eyes (often augmented by a microscope) and your mind. Look and think! Careful observations of the problem by the investigator are critical. Be sure to see the defect or failure first-hand. Be skeptical of other people’s opinions until after you have seen the problem yourself. The observations begin with looking at the defect or failure with your own eyes, but a hand lens or microscope is always better. If you are dealing with surface defects on a paint application line or in the field, a 40–60X shop microscope allows a good look. The latter device has moved into the 21st Century with the availability of new handheld digital microscopes that are just as portable, but can interface with mini-recorders, computers, and monitors. The resultant images can be stored and shared as well as being viewed. If possible, bring an example of the defect back to the lab and examine it with a microscope, first at low power (2–10X), then with greater magnification (100–300X). Ideally the microscope should be connected to a camera and/or a computer, so that images can be saved, pictures printed or e-mailed, etc. You should be able to tell what the defect is, although it sometimes can be difficult to distinguish craters from pops. It may be necessary to cross-section the defect to accomplish this. If you are dealing with a flow problem, besides making viscosity measurements, look at the wet paint at higher powers to see the quality of the pigment dispersion, whether there is flocculation, whether a waterborne resin is showing phase separation, etc. [See my article in JCT CoatingsTech, 3 (2), February 2006, pp. 36–43 for more details on microscope use and applications.] For many problems, a scanning electron microscope is a very valuable tool for characterization.
Once that you are certain what the problem is, the next step is establishing its cause. This may be possible via further analysis of the defects themselves. Pick a piece of dirt out of a dirt defect, or cross-section it to make the identification. Examination of a cross-section of a pop in a coating on plastic with a microscope will show whether the volatiles came from the plastic or from the coating. The back of a paint chip may show a layer of wood fibers, indicating possible substrate degradation before painting. If the defects are dirt or craters, then look for possible sources where the paint is applied. Observe operations at the customer—substrate preparation (including the pretreatment line), paint application, work attitudes and practices, cleanliness, and possible sources of contamination such as oil in the compressed air used for spraying. If the paint is applied outdoors or in a small shop, (auto refinish, maintenance, or architectural), then observe everything that you can—defects, substrate preparation or the lack of it, signs of contamination, whether the correct paint is being applied, etc. You may have to check your own plant for pigment dispersion quality, housekeeping, effectiveness of cleaning of tanks and other equipment, filtration, and sources of contamination. Ask a lot of questions—of people working on the line or at the work site, service people, and formulators. Obtain as much background information as possible. Colleagues may have dealt with the same or a similar defect. Be a detective!
Looking in the paint literature can be useful. Journal of Coatings Technology, JCTR, and CoatingsTech have published papers and articles on defects and other problems over the years. Do not ignore older papers. Much excellent work was published in the 60s, 70s, and 80s as well as more recently. Keep files of problems, causes, and solutions and refer to them as needed. The internet also can be very useful. Search engines such as Google will turn up much material, but you need to be careful because there may be false or antiquated information as well as good papers and articles.
Try to avoid making assumptions. I still make that mistake after all these years. Do not assume that the paint was formulated correctly and tested properly. Do not assume that the batch with the flow problem really had the same viscosity as the control batch. Measure both their viscosities. Formulation mistakes can happen, and testing may have been inadequate. I have encountered problems where customer conditions and/or substrates were different from those used in lab testing. Do not take the customer’s word that the cleaning and pretreatment line is working well. I have been in many customer plants where it was not, and the operators were quite willing to tell me so and why. Running tests on paints applied to the customer’s substrate rather than standard panels often shows why there is a difference between lab results and performance online or in the field.
Good luck with your problem-solving efforts!