Brittle Paint Failure: The “Tap” Test

April 5, 2013

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Over time paint can become brittle and begin to disbond from the steel substrate. The paint itself begins to crack and break away. This can be caused by several factors. Determining the current condition of a painted surface is not always easy, nor is disbondment always obvious. 

One common test used to determine the current state of a painted surface is the pull-off adhesion test. The test is just what it sounds – standardized adhesives are applied to the surface and then pulled off. This test is effective for many conditions, but is not as effective as the tap test when diagnosing brittle paint failure. The tap test, though not standardized, more closely mimics the forces that lead to brittle paint failure, especially on metal surfaces. The test is done by tapping on the surface with a 5-1 painter’s tool. Brittle paint, when tapped, will show a distinctive star pattern of cracking. The adhesion test can miss this type of paint failure because of the inherent nature of the test itself.

The adhesive test evaluates disbondment through the application of vertical forces to the painted surface (it is pulled straight off, with little or no side shearing). In addition, the adhesion test applies the force more slowly while the tap test is more sharply applied. Brittle paint failure shows up more clearly when exposed to rapidly applied force (like a rock hitting a window). This imitates the shear forces that come and go more quickly to metal surfaces. This tapping test can actually yield better and more consistent results in diagnosing brittle paint failure than the standard adhesion test. That’s because the tap test more closely replicates the shear forces that metals are often exposed to.

There are several causes of brittle paint failure. Poor application initially can lead to failure. Paint type and quality also may lead to brittle paint failure. Age also impacts the longevity of coatings. Quality industrial painting contractors can not only diagnose disbondment failures, they can restore the surfaces to pristine condition. But before they begin, they will need to know the cause and type of failure, as well as its extent.

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