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CLINIC PAINTING Another Source of Dirt in the Paint Film Q. The question from M.H. in the September issue regarding dirt in the paint film rang a bell. I ran into a similar situation at a large fixture manufacturer in Massachusetts. His white gloss paint finish was loaded with small particles. After filtering gallons of paint, we still could not get rid of the particles. When we looked for another source of the problem, however, we discovered that the particles were on the parts when they came out of his pretreatment washer. So the paint was not the problem; an imbalance of the chemicals in his power washer was. This imbalance caused larger-than-normal crystals of phosphate material to form on the substrate, giving the appearance of particles in the paint film. I suggest M.H. also consider this as a possible cause of his problem.—S.B. A. Thank you for writing to Painting Clinic, S.B. Since pretreatment washers are supposed to rinse off particulates in the final rinse, I wouldn't have expected that to be the source of the problem. But your experience indicates that M.H.'s contaminants could very well be larger-thannormal crystals of phosphate material produced by an out of balance pretreatment solution in his power washer. Paint Attacking ABS Parts Q. We paint ABS automotive parts, but they become brittle after application of a polyurethane enamel topcoat and lose the flexibility they had when they were unpainted. They crack on force application. Do you have any suggestions for solving the problem?—A.M. A. The solvents in the paint are probably attacking the ABS. One solution would be to apply a barrier coat to the ABS parts before painting with the polyurethane enamel. That would protect the substrate from solvent attack. You also could use a water-reducible primer or a primer with nonaggressive solvents. Another solution could be to have the supplier of your ABS parts use a solvent-resistant molding compound, but this solution is probably not viable because the ABS molding likely was formulated to meet certain performance parameters. To prove that the problem is, in fact, paint solvent attacking the ABS, you could soak a small part in the paint solvent for a time equal to the drying time of your paint. If the ABS cracks and crazes, the solvent is attacking it. An alternative would be to paint one section of an ABS part using your existing paint and paint the remaining section using a waterborne paint. If the solvent-thinned paint cracks and crazes, but the waterborne paint does not, the solvent is attacking it. Color Documentation Q. We document our color matches with a 60-degree spectrophotometer. Is a Delta E rating of less than 1.0 a good control, or is there a better way?— J.M. 28 DECEMBER 2013 — pfonline.com pfonline.com/experts A. In my past experience, I have run spectrophotometric analyses of paint samples, but I found it more convenient to use Color Drift Control cards (paint color chips) for color matching. In our paint lines at Westinghouse Electric Corp., we used the cards to check the color of each new batch of paint. Panels were spray-painted with the new paint, and the cured panels then were compared visually, instrumentally or both to the cards. The Pennsylvania Railroad and other companies also CARL IZZO / Consultant frequently used the cards paintingclinic@pfonline.com for color matching, and these cards still are used to provide paint color chips to suppliers of model railroad paints, manufacturers of model railroad equipment, prototype equipment restorers and model railroad enthusiasts. The company that produced our color cards also produces color-testing instruments that measure the color values of a sample and report them in Munsell; XYZ; and L*, a*, b* and other color notations using a computer interface. To me, these values are more convenient and meaningful than spectrophotometric curves. I suggest you investigate the use of these instruments. Suppliers of color testing instruments can be found on PFonline.com. Click on Suppliers, then Painting and then Testing Equipment, color. Controlling Paint Film Thickness Q. How can I achieve a paint film that is less than 1 mil (0.001") thick? Our design engineer wants to reduce the paint film thickness in certain areas to about ½ mil (0.0005"). Does the answer depend on the type of paint used and whether it is a water-based or solvent-based?—M.A. A. The amount and types of solvents added to the paint can control its viscosity and rheology. The type of solvents used to control these parameters of a particular paint depend of the type of application vehicle used (brushing, spraying, flow coating, roll coating or dipping) and whether the paint is solvent-thinned or waterborne (many waterborne paints contain organic solvents). In some cases of spray painting, the paint film thickness also can be controlled by reducing the wet film thickness. I suggest you ask your paint supplier to prescribe the solvent blend required to reduce the film thickness of his specific paint.