A lining in a tank, wet well or pipe protects only where it is continuous. A single pinhole lets water, acid or the stored product reach the substrate, and corrosion or chemical attack can then spread underneath the film. Many of these defects are too small to see, and some are hidden inside an otherwise smooth surface.
Holiday testing, also called discontinuity, spark or porosity testing, is the electrical inspection used to find them. This guide explains how the two main types of tester work, how test voltage is chosen, what changes on concrete, and how to run a test safely. It is general information. The test method, voltage and acceptance criteria for a real project come from the specification written by a qualified engineer or coatings specialist, the coating manufacturer’s guidance and the applicable standards.
Key takeaways
- Low-voltage wet sponge testers are used on thinner films and find holidays that go right through to the substrate.
- High-voltage spark testers are used on thicker films and can also detect thin spots and voids close to the surface.
- Too low a voltage misses defects; too high a voltage can break down sound coating and create new holidays.
- Concrete substrates need special attention because their conductivity depends on moisture, grounding and sometimes a conductive primer.
- Holiday testing complements thickness and adhesion checks; it does not replace them.
What counts as a holiday
The term covers any discontinuity that leaves the substrate unprotected or nearly so:
- Pinholes caused by outgassing from concrete, trapped air or solvent, or poor wetting.
- Voids and blisters that break through the film.
- Missed areas at edges, welds, corners, bolts and behind obstructions.
- Cracks from excessive thickness, impact or substrate movement.
- Inclusions such as dust, abrasive or fibers that bridge the film.
- Thin spots that are continuous but below the thickness the system needs. Only high-voltage testing can find some of these, and only if the voltage is set appropriately.
On concrete, pinholes from outgassing are among the most common findings. Our guide to concrete surface preparation and CSP profiles explains how primers, pore fillers and application timing reduce them.
How holiday testers work
Every holiday tester completes an electrical circuit. One side of the instrument is connected to the substrate with a ground lead. The other side is connected to an electrode that is passed over the coating surface. A sound coating is an insulator, so no current flows. Where the electrode passes over a holiday, current flows through the gap to the substrate, and the instrument signals with an audible alarm, a light or both.
Low-voltage wet sponge testers
A low-voltage tester applies a small DC voltage, typically somewhere in the range of single digits to around 90 volts depending on the instrument and the standard being followed. The electrode is a cellulose sponge wetted with water. A small amount of non-sudsing wetting agent is often added so the water penetrates fine pinholes.
- Used for: films up to roughly 500 µm (about 20 mils) in many standard methods; check the method and the manufacturer’s guidance for the actual limit.
- Finds: holidays that go through to the substrate, because the water must reach it.
- Does not find: thin spots that are still continuous, or voids sealed at the surface.
- Advantages: does not damage the coating, low shock hazard, simple to use.
- Limitations: slow on large areas; the surface must be dried and any wetting agent residue rinsed off before further coats, because residues can affect intercoat adhesion.
High-voltage spark testers
A high-voltage tester applies a DC voltage, either continuous or pulsed, that can range from several hundred volts to tens of thousands of volts. The electrode is a conductive brush, a coil spring, a rolling spring for pipe or a conductive rubber pad, held close to or in contact with the surface. At a holiday, the voltage is high enough to break down the air in the gap and a spark jumps to the substrate.
- Used for: thicker linings and coatings, including many tank linings, pipe coatings and spray-applied elastomers such as those in what is polyurea.
- Finds: holidays, and also voids and thin areas where the remaining film cannot withstand the test voltage.
- Advantages: fast; finds defects that a sponge cannot.
- Limitations: can damage sound coating if the voltage is too high; not suitable on some coatings with conductive pigments; dangerous in flammable atmospheres.
Pulsed DC instruments are often preferred where the surface is slightly damp or contaminated, because they are less prone to false alarms from surface leakage. Continuous DC instruments respond to very small holidays. The specification or the manufacturer usually states which type to use.
Comparison
| Factor | Low-voltage wet sponge | High-voltage spark |
|---|---|---|
| Typical voltage | Low, usually under about 100 V | Hundreds to tens of thousands of volts |
| Typical film thickness | Thinner films, often up to about 500 µm (20 mils) | Thicker films and linings |
| What it detects | Through-holidays reached by water | Through-holidays, voids and thin spots |
| Risk to coating | Very low | Breakdown if voltage is too high |
| Speed | Slow | Fast |
| Surface condition | Leaves the surface wet; residues need rinsing | Surface must be dry and clean |
| Main safety concern | Low | Shock, ignition of flammable vapors |
Choosing the test voltage
For high-voltage testing, the voltage must be high enough to break down air across a holiday but low enough not to puncture coating that meets the specified thickness. It is therefore tied to the actual dry film thickness and the dielectric strength of the coating.
Standard methods give tables or formulas that relate voltage to thickness, and coating manufacturers often publish a recommended voltage or a maximum voltage for their system. Where the two differ, the specification should say which applies, and many specifiers defer to the manufacturer because it knows the dielectric properties of its material. In practice:
- Measure dry film thickness first, so the voltage matches the real film. See coating inspection basics for how thickness is measured.
- Set the voltage on the instrument and verify it with a suitable high-voltage meter at the electrode, because dial settings can drift.
- Use the same voltage across the area unless the specification allows different settings for different thicknesses.
- Do not raise the voltage to “make sure.” Excess voltage can create holidays that were not there.
Standards in brief
Several standards describe holiday testing, and the specification should name the one that applies:
- ASTM D5162 covers discontinuity testing of nonconductive coatings on metallic substrates, with both low- and high-voltage methods.
- ASTM D4787 covers continuity verification of liquid or sheet linings applied to concrete substrates.
- AMPP (which incorporated NACE) publishes a standard practice for discontinuity testing of new protective coatings on conductive substrates, plus separate documents for pipeline coatings.
- ISO has standards for assessing porosity in dry films of protective paint systems.
These documents describe equipment, procedure and how to set voltage. They do not replace the project specification, which sets the acceptance criteria.
Holiday testing on concrete
Steel is a reliable conductor. Concrete is not. It conducts through moisture in its pores, so the same lining may test well on damp concrete and give no response on dry concrete, even over an obvious hole. That creates two problems: holidays may be missed, and moisture that helps the test may be the same moisture that the lining manufacturer wants to limit.
Common approaches include:
- Grounding to reinforcement where it is accessible and continuous, or to a ground rod or anchor set into the concrete.
- A conductive primer or conductive layer under the lining, designed so the whole surface acts as the ground plane. This must be part of the manufacturer’s system.
- Verification on a test area: making a deliberate small holiday in a trial patch and confirming that the instrument, ground and voltage detect it before testing the main area.
- Testing after cure under the moisture conditions the method assumes.
Because conductivity varies across a structure, verification is worth repeating in different areas: walls near the water line, dry upper surfaces, repaired areas and new mortar. Linings for wastewater structures are a typical case, discussed in wastewater tank and wet well linings.
When to test
- After enough cure: coatings that still contain solvent or are not fully cured may conduct or be damaged by the test. The manufacturer states when testing can be done.
- Before topcoating, where specified: finding holidays in an intermediate coat can be easier than finding them through the full system.
- As a final acceptance test before the tank or structure returns to service.
- After repairs, to confirm each repaired holiday is now continuous.
- During maintenance inspections, for linings where the owner’s program includes it.
Running the test
- Confirm the coating type is suitable for the chosen method and the film is cured.
- Measure and record dry film thickness.
- Select the instrument and electrode for the geometry: flat areas, corners, welds, pipe.
- Connect the ground lead securely to the substrate or the designated ground point and check continuity.
- Set and verify the voltage, and verify detection on a known holiday where the method calls for it.
- Move the electrode over the surface at a steady speed that keeps full contact, without stopping on one spot with high voltage.
- Mark each holiday with a marker compatible with the repair coating.
- Repair according to the manufacturer’s procedure, usually by abrading, cleaning and recoating the area with overlap.
- Retest repaired areas after cure.
- Record locations, voltage, instrument, results and repairs.
Safety
- High voltage: the current is usually low, but shocks are painful and the reflex can cause falls from scaffolds or ladders. Keep the ground lead secure and other people clear of the electrode.
- Flammable atmospheres: sparks can ignite solvent vapors. Do not use high-voltage testers where flammable vapors may be present unless the atmosphere has been tested and the work is authorized under the site’s procedures.
- Confined spaces: tanks and wet wells need entry permits, gas monitoring, ventilation and rescue arrangements.
- Wet conditions and medical implants: follow the instrument manufacturer’s warnings, which commonly cover use in wet areas and by people with pacemakers or other implants.
Frequently asked questions
Does a pass mean the lining is good? It means no holidays were detected at the test conditions. Thickness, adhesion, cure and appearance are separate checks.
Can a tester damage a coating? A correctly set low-voltage tester will not. A high-voltage tester set too high, or held on one spot, can break down the film.
Can zinc-rich primers be tested? The topcoat over a zinc-rich primer is often tested, but coatings that are themselves conductive or contain conductive pigments may not be suitable for holiday testing. Ask the manufacturer.
Is holiday testing needed on every coating? It is usual for immersion linings, including drinking water tank linings, and for buried coatings, where a single defect matters. For atmospheric coatings on steel it is less common. The specification decides. Our guides to internal linings for steel storage tanks and secondary containment linings show where it fits in those projects.
Bottom line
Holiday testing finds the gaps that turn a lining into a leak path. Low-voltage wet sponge testers suit thinner films and find through-holidays; high-voltage spark testers suit thicker linings and also reveal thin spots and voids. Measure thickness first, set and verify voltage from the manufacturer’s guidance and the standard method, take extra care on concrete, and repair and retest every holiday. The method and acceptance criteria belong in a specification prepared by qualified engineers, using the manufacturer’s data and the applicable standards.