Metal Finishing Guide Book


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VISCOSITY MEASUREMENTS OF THIN COATINGS Q: We currently measure the viscosity of our coatings using an S90 #2 Zahn Cup. Typical efflux times for various products range from 13–17 seconds. I understand this is lower than the range typically measured with a #2 Zahn cup. Should we be using a Zahn #1 cup instead? Note: The product is a solvent-borne coating. A: A viscosity of 13–17 seconds is so short that a small error in stopping the stop watch has a significant effect on the measurement. If you were to use a smaller diameter orifice, such as the #1 Zahn cup that you suggested, you would lessen the potential error. When you go to a smaller orifice, ensure that toward the end of the measurement the flow of the effluxing paint should not start, stop, start, stop, etc. When almost all the paint has drained from the cup, you should get a clean cut-off of the paint stream. POLYURETHANE VS. URETHANE Q: I am hoping that you will be able to help me with this problem. I am a guitar maker and use polyurethane/standard automotive lacquer for the finish. However, I find this a little soft, plus it is hard to build. Can you suggest an alternative that I can use, but not nitro-cellulose? Also, what is the difference between polyurethane and urethane? A: Polyurethane is probably the best coating I can suggest. I don't know why you find it soft, because it should be extremely mar resistant. Are you sure you are mixing it properly? Also, I don't understand what you mean by "hard to build." You can apply approximately 1 mil (0.001 inches) per application. It occurs to me that perhaps you are applying too many coats too soon, and not allowing the solvents to evaporate. You could consider a hard furniture coating, such as a catalyzed wood lacquer, but I don't know if that is any harder than the polyurethane. There is no difference between urethane and polyurethane. SETTING POWDER COATING CONTROLS Q: I am having trouble getting our process of powder coating dialed in. Specifically, on perforated panels for our products. These panels are typically made of sheet steel 16–18 gauge thickness. Regardless of the experience of our painters, we seem to continually get a drip that collects at the bottom of the panel. They are typically small but are considered as rejects by our customers. Our oven is running at 415°F on a typical day. I have asked our painters to isolate the panels in question to one rack. Parts with differing metal thickness are powder coated on other racks. We are using Tiger Drylac as our powder supplier. Any ideas would be greatly appreciated. A: I asked my colleague, Mike Cravens, to tackle this one, and here is his reply: "The dripping (or heavy edge coverage on the bottom surfaces) is likely caused by the powder material's gel time characteristics. Powder materials are heat activated. The powder material, once applied, must melt, flow, gel, and polymerize or cure. The typical gel time of a normal powder with a normal cure cycle is 20 seconds. Some materials are formulated with extended gel time to reduce orange peel and eliminate minor outgassing. I must also note that if you are applying the powder on a hot substrate (above the melt temperature of the powder) you may be forcing the powder through two flow stages." 560

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