A ‘holiday’ in coating terminology is a pinhole, void, or discontinuity in a cured coating film — a spot where the coating has failed to cover the substrate completely. The word comes from the idea of a gap or break, like a holiday from coverage.
Holiday testing (also called discontinuity testing or pinhole testing) is the process of detecting these defects electrically, using the coating film’s insulating properties. Where the coating is intact, it insulates. Where there’s a holiday — even one invisible to the naked eye — current passes through.
It’s one of those inspection steps that gets skipped on cost-pressured projects. It shouldn’t be. A coating that passes visual inspection can still have dozens of pinholes that will become active corrosion sites within months of service.
Two Methods — Low Voltage and High Voltage
There are two fundamentally different approaches, and which one applies depends on the coating thickness.
Low-Voltage Wet Sponge Test
Used for coatings below 500 µm DFT. A damp sponge connected to a low-voltage source (typically 9V or 67.5V, depending on the specification) is wiped across the coated surface. When the sponge passes over a holiday, the circuit completes through the conductive substrate and an alarm sounds.
The advantage is simplicity — the equipment is cheap, the method is fast, and it works well on thin coatings. The limitation is that it only detects holidays that reach all the way through to the substrate. A thin spot that doesn’t quite break through won’t trigger the alarm, even if it’s close to the surface.
The standard reference is NACE SP0188 (formerly NACE RP0188), which defines the voltage levels, sponge moisture requirements, and pass/fail criteria. Some European specifications reference ISO 29601 instead — same principle, slightly different parameters.
High-Voltage Spark Test
Used for coatings above 500 µm DFT — typically tank linings, glass flake epoxy systems, and other high-build applications. A high-voltage DC current (typically 100–125V per 25 µm of coating thickness, calculated from the DFT) is applied via a brush or rolling electrode. At a holiday, a spark jumps to the substrate, triggering an alarm.
The high voltage is necessary because thicker coatings resist current more strongly. Too low a voltage and you miss holidays; too high and you risk damaging an otherwise intact coating film. The voltage is calculated from the coating DFT using the formula in NACE SP0188 — or the manufacturer may specify a test voltage in their application procedure.
One practical point: high-voltage testing requires that the substrate is earthed. On a tank or vessel, this means connecting the ground lead to the steel structure before testing. Miss this step and the test doesn’t work.
When Holiday Testing Is Required
Not every coating job requires holiday testing — it’s most commonly specified for:
- Tank linings and pipe linings — where a pinhole creates a direct path for the stored product to attack the substrate
- Immersion service coatings — submerged or buried steel where any breach allows aggressive electrolyte to contact the steel
- Pipeline coatings — particularly for buried and subsea applications where remediation is extremely expensive
- Offshore and splash zone coatings — where the aggressive environment makes any coating defect a serious corrosion risk
For standard atmospheric steel structure coating — a typical commercial building or industrial shed — holiday testing is rarely specified. The environment is less aggressive and the consequences of a pinhole, while not ideal, are manageable.
How the Test Is Actually Done
The sequence for a tank lining, which is probably the most common holiday testing scenario:
- Confirm coating is fully cured — testing an under-cured film can give false positives and damage the coating
- Calculate test voltage from measured DFT — for high-voltage testing, DFT must be confirmed before you can set the voltage correctly
- Earth the structure — connect the ground lead to the steel substrate
- Set and verify voltage on the instrument — use a calibrated voltmeter, not the instrument’s own display
- Sweep the electrode across the surface at a steady pace — typically 0.1–0.3 m/s; too fast and you miss holidays, too slow and you risk damaging the coating
- Mark any alarm points — spray chalk or a felt pen; don’t scrape the coating
- Document locations and count — required for the inspection record
- Repair marked holidays — the repair method depends on the coating type; typically spot-blasting and reapplication, followed by re-testing of the repaired area
The whole tank is tested. Not a sample area, not representative sections — the whole surface. That’s what ‘100% holiday testing’ means.
What Holiday Testing Doesn’t Tell You
Worth being clear about this, because holiday testing is sometimes treated as a final quality sign-off when it’s actually just one check.
Holiday testing confirms that the coating film has no through-thickness discontinuities. It doesn’t confirm that DFT is adequate. It doesn’t confirm that adhesion is sufficient. It doesn’t detect thin spots that haven’t quite broken through — a 40 µm coating in a system specified at 250 µm will pass a holiday test. It also doesn’t tell you anything about the surface preparation quality beneath the coating.
This is why holiday testing is one inspection step in a sequence — it works alongside DFT measurement, adhesion testing, and surface preparation records, not instead of them.
Common Mistakes
Testing at the wrong voltage. Under-voltage misses holidays; over-voltage damages intact coating. The voltage must be calculated from the actual measured DFT of the cured film.
Testing before the coating is fully cured. An under-cured epoxy can conduct slightly, which creates false positives — apparent holidays that aren’t real defects. Confirm cure (MEK rub test, hardness, or simply elapsed cure time) before testing.
Not earthing the structure. High-voltage testing requires a complete circuit. No earth connection, no reliable test.
Moving the electrode too fast. Small pinholes can be missed if the sweep speed is too high. NACE SP0188 gives guidance on acceptable sweep rates.
Practical Questions
What voltage should I use for a 500 µm glass flake epoxy tank lining?
Using the standard NACE SP0188 formula: test voltage = 525V (roughly 100–125V per 25 µm × 20 intervals). But always check the coating manufacturer’s application procedure — they may specify a different voltage for their product, particularly if it has different conductivity characteristics from standard epoxy.
Do I need holiday testing on external atmospheric steel coatings?
Generally no — unless the project specification explicitly requires it. Holiday testing is typically reserved for immersion, buried, and tank service applications. For atmospheric coatings on structural steel, DFT measurement per SSPC-PA 2 and visual inspection are the standard quality checks.
What counts as a ‘pass’ on holiday testing?
For most specifications, zero holidays on the final test — after all repairs have been made and re-tested. Some specifications allow a small number of holidays on intermediate tests, with the requirement that all are repaired before the final test. The specific acceptance criteria should be stated in the project’s coating specification or quality plan.
Related Reading
- What is DFT in coating — dry film thickness explained, including measurement protocols and what happens when DFT is wrong.
- Storage tank lining selection guide — lining types, chemical resistance, and system selection for oil, water, and chemical tanks.
- Epoxy tank lining guide — materials, types, and application requirements for epoxy-lined tanks.
- Steel structure coating inspection checklist — surface prep, DFT, recoat intervals, and touch-up procedures.
Send your project coating specification or quality plan requirements via the project inquiry form and our technical team will advise on inspection protocols and system selection.
