Metal Finishing Guide Book


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section of the specification, which states: 4.3 … Eighty percent of measurements on primary exposed surfaces shall meet or exceed 30 microns (1.2 mil) total film thickness. Paint process capability may result in readings below 25 microns (1.0 mil). No more than 5% of the total readings, on primary exposed surfaces, shall be below 25 microns (1.0 mil) (or, 85% of film thickness specified), assuming appropriate color and hide. [Under lining added] Questions arose concerning the definition of "… exposed surfaces …" and "No more than 5% of the total readings, on primary exposed surfaces …" Is it adequate to assume that surfaces on the back side of the extrusion will not be exposed to the marine environment, especially fog? If the extrusion is part of a window opening, will it be exposed to fog? Most paint consultants are aware initial paint and corrosion failures do not occur on the face of an extrusion, but rather on the edges and corners. Once these have been breached, corrosion tends to creep under the coating and penetrate onto the face. Therefore, when the specification allows no more than 5% of the exposed surfaces to be below 1.2 mil, does that consider the edges? Frankly, the focus should be on the edges rather than the faces and ideally, the specification should mandate the minimum DFT along the edges and corners to be 1.2 mil. The micrographs in Figure 6 illustrate how microscopic analysis was valuable in assessing film thickness compliance with the specification. Also, notice how the coating thickFigure 7. Thick, corrosion-resistant coating applied ness varies significantly throughout to mildly shot-blasted steel surface with low blast profile. the aluminum part. 5. Surface preparation—abrasive blasting A 100% solids coating was applied to a steel surface cleaned by abrasive blasting. Massive delamination of the coating occurred after the product was placed into service, and the client wanted to know if the contractor had followed the correct Figure 8. Hard brittle wood cabinet coating compared with a softer and more ductile replacement product. 530

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