Metal Finishing Guide Book

2012-2013

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Figure 3. Various coating layers demonstrate whether the fabricator complied with the specification. Figure 4. Solvent entrapment in the primer and delamination of the top coat from the primer. Figure 5. Acrylic latex top coat peeling away from the drywall compound. epoxy primer and followed with a zinc-rich coating. Such a zinc-rich coating was not specified and did not provide significant corrosion protection as it had not been applied directly to the bare steel. Here is the absurdity: galvanizing means coating a steel product with zinc, which is used to protect the steel surface from corrosion by means of cathodic protection. Similarly, a zinc-rich primer provides cathodic protection, provided it is in direct electrical contact with the steel substrate. In other words, hot dip galvanizing and zinc-rich primers serve the same purpose! In this case, the fabricator and/or painting contractor applied two zinc coatings, when only one was required. More importantly, in order to provide maximum corrosion protection, the zinc-rich primer must be applied directly to the steel and not over an epoxy primer. Hence, the zinc-rich primer served no purpose other than to increase the thickness of the total coating system. 2. Roof coating peels off in sheets A roof coating system was applied during the summer months to the top of a commercial building. Several weeks after completion, the building���s owner noticed large sheets of roof coating peeling. A site inspection revealed the gray primer was pulling away from the underlying polyurethane foam, and in some instances, the top coat was pulling away from the gray primer as well (Figure 4). An analysis by means of a stereomicroscope confirmed what the consultant anticipated. There was excessive solvent entrapped in the primer layer. This is seen by the large cavities in the gray coating and ex583

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