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Air Bubble Within Coating Film Pinhole After Bubble "Pops" COATINGSPRO JULY 2016 55 Illustration 1. Pinholes developing in a single coat. Pinholes are typically caused by trapped air or possibly other volatile components that first form a bubble within the coatings film. That bubble then "pops," leaving an open depression and void in the coatings film. otherwise) coating flm to provide the intended protection. Tis is an absolute requirement for coatings/ linings in immersion service if the expected protection and service life of the installed system is to be achieved. Nothing less should be accepted. If the installed coating system is to function in an atmospheric ser vice environment, the same requirement to be free of pinholes and other described defects should be a require- ment. Holiday testing, though, is not ty pically performed to verif y the continuous film in these instances; the inspection is ty pically just visual in nature. A nd since it's a visual evalua- tion, it can be much more challenging to execute that inspection and catch ever y pinhole. So the question could be: "How many visually obser ved pinholes are too many? " You could argue that if the surface was specified to be 100 percent pinhole free, then holiday testing would be required and also should be specified. It's ty pical — and realis- tic — that although the specification requirement notes free from defects, there may be isolated sags and other defects in the coatings film that are accepted, and pinholes would fit into this categor y as well. Te best way to avoid that contro- versy is to look at the circumstances that cause pinholes and how they can be avoided, as that is the best solution for all parties. Cosmetic or Cosmic Problem? Pinholes are typically caused by trapped air or possibly other volatile components that frst form a bubble within the coatings flm. Tat bubble then "pops," leaving an open depres- sion and void in the coatings flm. In the case of this pinholed coating, Illustration 1 shows this phenomenon. Tis pinhole happened at a stage in the curing process when the coating was no longer in a liquid enough state to fow and fll the created void, thereby eliminating it. Instead, what was left behind was a small break in the contin- uous flm. Te reality is you can probably fnd a few pinholes in most atmospheric applied coatings and, depending on the installed coating system, their inclusion could be problematic or have a minimal efect. Would you repair the single pinhole in Photo 1? Some repairs may be worse than the pinholes themselves. So the frst consideration would be the type of system that was installed. On a particular project, a common coating system was used: a three-coat system that includes an inorganic zinc primer, followed with an intermediate coat of epoxy, and completed with a coat of urethane. It's well recognized that this system type can manifest a signifcant level of visible pin-holing. In this instance, the proper spraying technique was not followed: A mist coat of epoxy should have been applied over the inorganic zinc to "plug" the porosity of the inorganic zinc material prior to a full wet coat. It seems, though, due to the signifcant and frequent pinholes shown, that a mist coat wasn't used. Te resulting pinholes are evident in the surface flm (see Illustration 2). In this case, it is the primary corrosion protection (aka the inorganic zinc primer) that is so prone to causing pinholes in the subsequently applied intermediate and topcoats. In reality, a few pinholes in the surface flm (due to atmospheric service) would not Pinholes