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What causes paint to rust? Holiday detectors identify micro gaps in painted surfaces

Diagnose the early onset of surface corrosion and paint defects due to faulty coating, having cracks, insufficient or excessive thickness.

In the protective and industrial coatings industry, there are a number of flaws to be aware of. Problems with surfaces prior to coting include:

  1. Runny and soggy coating: gravity pulls coating creating uneven thickness
  2. Cissing: uneven surface caused by visible traces of air bubbles on the coating
  3. Cratering: If the sub-coating surface is humid or the coat is lacking quality, gaps appear in the coating.
  4. Pinholes: Generated by captured air or dust particles coming to the surface
  5. Over Coating: Excessive coating application, resulting in paint cracking during or after its curing
  6. Under Coating: Lack of coating in pointy or uneven areas such as corners, welding or rough surfaces.

1. The Low Voltage Pinhole method

Ideal for testing insulating coatings less than 500μm (20mils) thick, where you do not wish to damage the coating. However, this method only detects pinholes. Pinholes are important to detect as they allow moisture to access the steel through capillary action, accelerating the process of corrosion.

Wet Sponge method (Elcometer 270)

The wet sponge method also works through capillary action. When you pass a damp sponge steadily over the coated surface, the water from the sponge triggers a low voltage circuit over each pinhole. The unit alarms, letting you know its location.


2. The High Voltage or Holiday Detection method is where a high voltage is applied to a probe, which is passed over the coated surface

In this case, the voltage should be high enough to break through the weak area and give a signal. This allows you to detect flaws that don’t go all the way down to the substrate, as well as voids within the coating.

Elcometer 280 how to

However, the voltage isn’t set too high, as this could break down and damage the coating. Testing with bright blue sparks is a clear sign the voltage is too high.

The high voltage method comes in 2 versions – Continuous DC and Pulsed DC


where the voltage is constantly sent to the probe and is used to test insulation coatings on conductive substrates up to 7.5mm (300mils) thick.

Elcometer has two continuous DC detectors: the Elcometer 236, where the high voltage supply is generated within the instrument and sent to the probe handle via a high voltage cable; and the Elcometer 266, where the high voltage supply is generated within the handle, and connected to the instrument via a low voltage cable, resulting in a safer way to test for flaws.



where the energy is contained in very short pulses, typically at a speed of 30 pulses per second (30Hz). The break-in energy stops the probe from building up a charge on the surface, allowing you to safely test for flaws on damp, dirty, or slightly conductive coatings. Using state-of-the-art electronics, the Elcometer 280 Pulsed DC Holiday Detector can test coatings up to 25mm (1”).

With the Elcometer 280 you don’t have to connect to the substrate, which is perfect for quickly testing large surfaces and pipelines, without constantly having to clip and unclip a return lead every time the length runs out.

There’s also the Elcometer 280 grounding mats and grounding pin, which are ideal for testing on un-grounded pipes.

3. The UV Pinhole technique (Elcometer 260)

This method is used to monitor the coating quality all throughout its service life. We offer a variety of Pinhole detectors from the 

It makes use of a fluorescent coating (inexpensive UV Pinhole Detector Flashlight) which is applied to the surface as a base coat. Once the second coat is applied, any pinholes fluoresce when you shine a UV light on them revealing their location.

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