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Pinhole is a holiday or discontinuity in a coating film approximately the size of a pinpoint, and extending entirely through the applied film, normally caused by solvent bubbling, moisture, or foreign particles. It is the formation of small holes through the entire thickness of a coating. A fundamental reason for paint defects, such as pinholes and blowouts, are open surface pores on the composite substrate. Such defects can hurt the appearance of the coating. Metals, especially steel, are liable to corrode unless they are covered by a protective coating. Coatings such as paint, plastic and rubber protect metal against corrosion provided that the coating is free from pinholes and porosity.


Once the applied coating is dried and cured, it should be inspected to ensure that it meets the required standards. Typical inspection tests check for adequate film thickness, pinholes, and other conditions. Holidays or pinholes (i.e., discontinuities, voids, inclusions, areas of low film thickness, or breaks in the coating) should be detected and remedied to ensure an effective coating system.


Holiday testing is performed to find nicks, pinholes, and other defects or discontinuities in the film. The specifications should indicate the point in the job when holiday testing is done. The coating should be reasonably well cured (but not fully cured, for ease of repair) before testing is done. Coatings that are not cured may show false holidays. Holidays in coatings should be repaired. The coating should then be tested again, after the repair, to ensure that repairs were successful.


General types of holiday detectors used to detect holidays or pinholes in the coating include: -

Low-voltage DC

High-voltage DC

High-voltage AC  
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