Tom Swan, November 19, 2015 (republished from Corrosionpedia.com)
I’m often asked questions about holiday testing or, as the Europeans call it, porosity testing. Of course, pinholes in a newly applied coating can leave the substrate exposed to the environment and vulnerable to corrosion. It is the coating inspector’s job to detect and document these defects.
There are two main categories of holiday detectors: low voltage (sponge) testers and high voltage (Jeep) testers. According to almost all of the coating standards, you may use a low voltage tester for coating thicknesses of up to 500 microns (500 μm). If thicker than that, go to a high voltage tester. High voltage testers can be used down to about 250 microns, but I generally do not recommend it if you can use the low voltage tester.