A coating that is 99.9 percent perfect fails at the 0.1 percent: the pinholes, misses and thin spots the eye cannot find. Holiday testing exists to find them while they still cost a brush stroke, and it is mandatory territory on pipeline and tank lining work worth doing.
Two instruments, two jobs
Low voltage wet sponge detectors serve films below roughly 500 microns: a damp sponge sweeps the surface and completes a circuit through any discontinuity. High voltage spark testers cover thicker films and immersion critical linings, driving a test voltage through the film that discharges visibly and audibly at each holiday. Pipeline and tank work is overwhelmingly high voltage territory.
Setting the voltage honestly
Test voltage follows film thickness by standard, commonly on the order of a few volts per micron with formulas set in NACE and ISO documents and, decisively, the project specification. Too low finds nothing; too high punches new holidays into sound coating and then counts its own damage. The correct number comes from measured DFT, not from the dial position the last job used.
Running a test that means something
Cure the film to the testable state first; green epoxy reads false. Ground properly, sweep at the specified speed with full overlap, mark every indication, repair with the specified material, and retest the repairs. The record, area, voltage, findings, repairs, is what turns the exercise into an asset document the next inspection can build on.
The economics in one line
On a buried joint or a lined tank floor, a found holiday costs minutes; a missed one costs an excavation or an outage. Few QA steps buy more protection per dirham.
MENA Industrial Supplies supplies coatings with the DFT and testing guidance to pass first time, via the oil and gas coatings page.