Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 276 of 331

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 open- ing, 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 pen- etrate 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 valu- able in assessing film thickness com- pliance with the specification. Also, notice how the coating thickness varies significantly throughout the alu- minum part. 5. Surface preparation—abrasive blasting Figure 7. Thick, corrosion-resistant coating ap- plied to mildly shot-blasted steel surface with low blast profile. A 100% solids coating was applied to a steel surface cleaned by abrasive blast- ing. Massive delamination of the coat- ing occurred after the product was placed into service, and the client want- ed to know if the contractor had fol- lowed the correct procedures when blast-cleaning the surface. By sectioning and polishing a sample of the coating and substrate, one could easily measure the blast profile even though the job had been conducted years before (Figure 7). Figure 8. Hard brittle wood cabinet coating compared with a softer and more ductile replacement product. 275

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