The degree of coating continuity applied on a substrate can be assessed using holiday test.
Discontinuities in a coating are typically very small and are not easy to detect with a naked eye, these include voids, cracks, thin spots, pinholes, foreign material inclusion or contamination. The holiday test allows detecting these discontinuities in new coating systems that are applied to conductive substrates using two types of test equipment: low-voltage wet sponge testers and high-voltage spark testers.
Inspection of coating which was previously exposed to an immersion condition is not advised as coating could be damaged in service or the result could be erroneous due to permeation or moisture absorption of the coating. Acceptable number of discontinuities varies depending on coating film thickness, design, and service conditions. During the test, low-voltage wet sponge saturated with a solution for exploring the coating surface or high-voltage spark brush are dragged across coated piece surface and with a ground connection provided, an audible and/or visual indicator signals a point of coating discontinuity.