Metal Finishing Guide Book


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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. 528 s shows the fabricator had not followed the Customer Specification. lo The galvanizing was not a uniform, T coherent layer of zinc, which may c have been due to poor processing h procedures. The organic coating p system started by applying the s epoxy primer and followed with e a zinc-rich coating. Such a zincrich coating was not specified and r did not provide significant corrod sion protection as it had not been s applied directly to the bare steel. a Here is the absurdity: galvanizH ing means coating a steel product with zinc, which is used to protect the steel surface from corrosion t by means of cathodic protection. b Similarly, a zinc-rich primer proS vides cathodic protection, provided v it is in direct electrical contact with the steel substrate. In other words, th hot dip galvanizing and zinc-rich h primers serve the same purpose! p In this case, the fabricator and/or painting contractor applied two p zinc coatings, when only one was z required. More importantly, in r order to provide maximum corroo sion protection, the zinc-rich primer must be applied directly to the steel and not over an epoxy primer. s Hence, the zinc-rich primer served H no purpose other than to increase n the thickness of the total coating t system. s 2. Roof coating peels off in sheets 2 A roof coating system was applied during the summer months to d the top of a commercial building. th Several weeks after completion, S the building's owner noticed large th sheets of roof coating peeling. A s site inspection revealed the gray s primer was pulling away from the p underlying polyurethane foam, and u in some instances, the top coat was pulling away from the gray primer p as well (Figure 4).

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