Premature corrosion of a substrate is usually due to a coating failure. A major cause is the presence of flaws in the finished coating, collectively referred to as porosity. One of the most obvious porosity issues would be pinholes - where the substrate is open to the elements through the pinhole. Pinholes are caused by air entrapment, which is then released from the surface, or by the entrapment of particulates (dust, sand etc.) which do not stay in place. Pinhole detection is therefore essential to monitor the integrity of the protective coating from its initial application, throughout its service life.

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Holiday test is used to detect holes, known as holidays or discontinuities, in a coating. Holiday testing allows the detection of even smallest coating flaws invisible to the naked eye. There are various techniques and equipment available for holiday detection.

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The voltage to be chosen depends on several factors: the thickness of the paint layer, the working specifications applicable to the project or dielectric strength of the coating.
The specification is often called a few volts per micron on which surfaces should be "sparked off." A rule of thumb is 3 to 5 volts per micron thickness. Example, a layer thickness of 500 microns should then be tested with a voltage of 1500 volts to 2500 (1.5 Kv to 2.5 Kv). However, there are applications where considerably higher voltages are required for pore test. Think of tanks and pipes that are underground, are placed on the seabed or the lining of water tanks.