Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 274 of 331

Figure 3. Various coating layers demonstrate whether the fabricator complied with the specification. 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 ap- plied 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 sub- strate. In other words, hot dip gal- vanizing and zinc-rich primers serve the same purpose! In this case, the fabricator and/or painting contrac- tor applied two zinc coatings, when only one was required. More im- portantly, in order to provide max- imum corrosion protection, the zinc-rich primer must be applied di- rectly 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. Figure 4. Solvent entrapment in the primer and de- lamination of the top coat from the primer. 2. Roof coating peels off in sheets A roof coating system was applied during the summer months to the top of a commercial building. Sev- eral weeks after completion, the building's owner noticed large sheets of roof coating peeling. A site in- spection revealed the gray primer was pulling away from the underly- ing polyurethane foam, and in some instances, the top coat was pulling away from the gray primer as well (Figure 4). Figure 5. Acrylic latex top coat peeling away from the drywall compound. An analysis by means of a stere- omicroscope 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 ex- 273

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