There are two methods of testing:
Wet Sponge Technique - Suitable for measuring insulating coatings on metal less than 500µm (20mils) on conductive substrates. The wet sponge technique is ideal for powder coatings and any thin coating where the User does not wish damage to occur to the coating.
A low voltage is applied to a sponge, moistened with a wetting agent. When the sponge moves over a coating flaw, liquid penetrates to the substrate and completes an electrical circuit, setting off the alarm.
This technique will identify coating flaws where the substrate is uncovered, i.e. cissing, cratering, pinholes and some forms of over and under coating flaws.
High Voltage Technique Locates all flaws in insulating coatings on conductive substrates, the high voltage technique can be used to test coatings up to more than 7mm (275mils) thick. This method is ideal for inspecting pipelines and other protective coatings. Coatings on concrete can also be tested using this method.
A power supply generates a high DC voltage which is connected to a suitable probe with an earth return connected to the substrate. As the probe is passed over the coated substrate, a flaw is indicated by a spark at the contact point which sets off the alarm.
This technique is suitable for identifying all of the flaws described above, however care is required on thin coatings.