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po wd ercoa tin g C L I N IC n AAMA SpecificAtionS for Steel Q. We manufacture steel and aluminum outdoor products. Do the AAMA powder coat specifications for extruded aluminum—AAMA 2603, AAMA 2604 and AAMA 2605—correlate to the application of steel materials? If so, are there any metrics for how similar powder coats will react differently on steel? A.J. A. The AAMA (American Aluminum Manufacturers Association) specifcations cover many different initial and long-term performance requirements, as well as pretreatment requirements for aluminum and test methods for the fnished product. One key element is outdoor weathering measured by gloss and color retention. Those items could apply to a fnish applied over steel. Dry adhesion, hardness, initial gloss, impact, nitric acid, window cleaner and a few other tests that relate to the endurance of the coating on the outside surface also would be similar with steel. However, some of the performance criteria would not ft with a steel substrate, and the pretreatment would be different for steel. Corrosion requirements, for example, are 3,000 hrs of salt spray for AAMA 2604 and 4,000 hrs for AAMA 2605, and the numbers are similar for humidity. That will be hard to achieve on steel without a primer and may still fail, depending on the quality of the surface treatment. Film thickness with steel should be heavier in general. A good aluminum conversion coating will create an excellent bond with the powder coating, and as long as the flm is complete, the penetration of moisture will be limited and corrosion protection will be quite good. Steel is more vulnerable to moisture penetration and corrosion, and a thicker flm is typically needed to get the same level of protection from failure. In general, you could adapt the AAMA specifcations to ft steel. However, if you want to develop a specifcation for steel, you should visit the Powder Coating Institute website (powdercoating.org) and click on the small red button at the top right labeled "project SOS". It pfonline.com/experts will allow you to select the environment your product will be subjected to in the feld and the substrate, and then defne a performance level and the needed process to achieve it. n fire-retArdAnt coAtingS Q. One of my customers is concerned about the safety of powder-coated products in environments with strict fire safety codes. Do powder-coated products have any merit as fire retardants compared with liquid-painted products? Also, how do they compare to uncoated products? A.C. A. Any organic coating can ignite if the concentration of the fuel (powder or paint) is high enough and the heat is intense enough. Powder will be more resistant to fame than some liquid coatings that are not thermally cured. A typical powder might ignite at around 900°F. A product with no coating will depend on the substrate material. Metal substrates will need much higher temperatures to melt and become molten. Steel does not melt until it hits around 2,600°F, brass will melt at 1,650°F and aluminum melts at 1,220°F. While some coatings are more resistant to faming than others, I would not consider any typical organic coating as a fre retardant. However, a powder coating is not going to be very easy to burn without an intense and sustained fre source. n coAting thickneSS control Q. How can I control the thickness of the powder film on my parts? We have some parts that have complex shapes, and there is a lot of variance in film build. We have some rejects from light coating, and some are rejected because of heavy film and too much orange peel. How should we control the gun to get better consistency? J.B. A. Any good powder coating expert will start this discussion by mentioning ground. Good earth ground is essential to flm-build control. Powder is heavily infuenced by the voltage that is discharged and the resulting current that fows from the gun tip to the part. Resistance occurs as a result of the part shape, and electricity fows along the path of least resistance. This causes two things to occur that could impact flmbuild uniformity. First, the powder is likely to follow the voltage lines, so if they are stronger in one area than they are in another, that area will likely have greater flm build. Second, the electrical resistance in the inside corners (Faraday cage effect) will grow RODGER TALBERT, Technical Contributor, powdercoating@pfonline.com 36 JUNE 2013