One Tiny Opening, One Big Problem: What Is a Holiday in a Protective Coating?

A protective coating can be hundreds of feet long, several coats thick, and nearly perfect—yet one tiny opening may give moisture, oxygen, or chemicals a direct path to the substrate.

That opening is commonly called a holiday.

Some holidays are obvious missed areas. Others are microscopic pinholes that cannot be seen during an ordinary visual inspection. In immersion service, tank linings, pipelines, wastewater facilities, and severe corrosion environments, a small discontinuity can become the starting point for corrosion, underfilm attack, blistering, or leakage.

Holiday testing helps locate these openings before the coated equipment is placed into service.

What Is a Coating Holiday?

A holiday is a discontinuity in a protective coating that allows the underlying conductive substrate to be detected through the coating.

The discontinuity may be:

• A pinhole.
• A void.
• A crack.
• A missed area.
• A thin spot.
• A cut or puncture.
• Porosity extending through the film.
• Damage caused during handling or installation.

Not every visible coating defect is a holiday. A run, sag, color difference, surface blemish, or area of orange peel may affect appearance without creating a continuous opening to the substrate.

For holiday detection, the important question is:

Does the discontinuity provide an electrically detectable path through the coating to the substrate?

Why Is It Called a Holiday?

The exact origin of the coatings term is uncertain, but “holiday” has long been used to describe a skipped, missed, or inadequately coated area.

In practical terms, it means the coating took a holiday while the rest of the surface went to work.

The name may sound harmless. The defect is not.

Why Can One Small Holiday Matter?

A protective coating works by separating the substrate from its service environment. If the coating contains a through-film opening, the barrier is no longer continuous.

A holiday may allow:

• Water to reach the substrate.
• Oxygen to support corrosion.
• Chemicals to attack the substrate.
• Electrolytes to establish a corrosion cell.
• Corrosion to spread beneath the coating.
• Product to contaminate the substrate.
• Stored liquid to escape through a lining defect.

On exposed structural steel, a small holiday may produce localized rust staining. Inside a chemical tank or wastewater structure, the same-sized opening may expose the substrate to continuous aggressive service.

The importance of a holiday depends on the coating system, substrate, service environment, and consequence of failure.

Where Are Holidays Most Likely to Form?

Holidays are often found around difficult geometry and areas where coating application becomes less consistent.

Common locations include:

• Sharp edges.
• Welds.
• Weld spatter.
• Bolts and fasteners.
• Pits and surface irregularities.
• Inside corners.
• Recessed areas.
• Pipe joints.
• Flanges.
• Repairs and touch-up areas.
• Areas damaged during handling.
• Thin areas near complex shapes.

These areas may benefit from stripe coating, edge rounding, weld preparation, additional inspection, or special application techniques.

What Causes Coating Holidays?

Insufficient Film Thickness

A coating applied too thin may not completely cover the surface profile, sharp edges, or irregularities.

Air Entrapment

Air trapped during mixing or application can rise through the wet coating and leave a pinhole.

Outgassing

Air or vapor escaping from concrete, porous substrates, seams, or surface cavities can create bubbles that break through the film.

Surface Contamination

Oil, grease, dust, moisture, salts, silicone, and other contamination can interfere with wetting and cause craters, voids, or loss of coverage.

Excessive Atomizing Pressure

Excessive atomization can contribute to dry spray, air entrapment, poor wetting, or an uneven coating film.

Incorrect Spray Technique

Excessive gun distance, poor overlap, incorrect angle, fast travel speed, and an unsuitable spray tip can create thin or missed areas.

Surface Profile

A deep or irregular blast profile may be difficult to cover completely, particularly if the coating is applied near its minimum thickness.

Sharp Edges and Welds

Liquid coating tends to pull away from sharp edges as it flows and cures. Rough welds and weld spatter also create difficult-to-cover geometry.

Coating Viscosity and Temperature

A coating that is too cold, too viscous, improperly reduced, or outside its recommended application range may not flow and wet the surface as intended.

Physical Damage

A properly applied coating can be damaged by lifting equipment, chains, tools, welding, assembly, transportation, backfilling, or installation.

What Is Holiday Testing?

Holiday testing uses electrical principles to locate discontinuities in a nonconductive coating applied over a conductive substrate.

A properly coated surface electrically separates the test electrode from the substrate. When the electrode passes over a holiday, the electrical circuit is completed through the defect.

The instrument responds with an audible, visual, or both audible and visual alarm.

The primary general standard is ASTM D5162—Discontinuity Testing of Nonconductive Protective Coatings on Metallic Substrates .

Another widely recognized industry standard is AMPP SP0188—Discontinuity Testing of New Protective Coatings on Conductive Substrates .

Two Basic Holiday-Detection Methods

Holiday detectors generally fall into two categories:

• Low-voltage wet-sponge detectors.
• High-voltage holiday detectors.

The coating type, dry-film thickness, substrate, project specification, and governing standard determine which method is appropriate.

Low-Voltage Wet-Sponge Testing

A low-voltage wet-sponge detector uses a damp sponge as the search electrode. The sponge is moved across the coated surface while the instrument is electrically connected to the conductive substrate.

If the moisture in the sponge reaches the substrate through a discontinuity, the circuit is completed and the detector alarms.

When Is Wet-Sponge Testing Used?

Low-voltage testing is generally associated with relatively thin, nonconductive coating systems. The governing standard or specification must establish whether the method is suitable for the actual coating thickness and service.

Wet-sponge testing is commonly considered for:

• Thin protective coatings.
• Tank linings within the permitted thickness range.
• Coated steel surfaces.
• Areas where high-voltage testing could damage a thin film.

Advantages of Wet-Sponge Testing

• Relatively simple equipment.
• Low test voltage.
• Reduced risk of electrically damaging a thin coating.
• A damp sponge can conform to moderate surface irregularities.
• Useful for locating small through-film openings.

Limitations of Wet-Sponge Testing

• Moisture must reach the substrate through the defect.
• Thicker films may prevent reliable detection.
• Contaminants can produce misleading indications.
• Excess water may bridge across the surface.
• The test may be unsuitable for water-sensitive coatings.
• Conductive or semiconductive coatings may produce widespread alarms.

High-Voltage Holiday Testing

A high-voltage detector uses a charged electrode, such as a brush or rolling spring, that passes over the coated surface.

When the electrode encounters a holiday, electrical energy crosses the opening to the conductive substrate. The current returns to the instrument through the ground connection, causing an alarm.

When Is High-Voltage Testing Used?

High-voltage testing is generally associated with thicker nonconductive coatings and linings where a low-voltage wet-sponge method may not reliably detect a discontinuity.

Common applications include:

• Pipeline coatings.
• Tank linings.
• Thick-film epoxies.
• Buried piping.
• Immersion-service coatings.
• Wastewater structures.
• Secondary containment systems.
• Heavy-duty protective linings.

For pipeline coatings, see ASTM G62—Holiday Detection of Coatings Used to Protect Pipelines .

Advantages of High-Voltage Testing

• Suitable for many thicker coating systems.
• Can inspect large areas efficiently.
• Rolling-spring electrodes can test around pipe circumference.
• Can locate small discontinuities through substantial film build.
• Provides audible and visual indications on many instruments.

Limitations of High-Voltage Testing

• Incorrect voltage can damage the coating.
• The coating must be sufficiently cured and dry.
• A reliable electrical ground is essential.
• Sharp geometry may create corona effects or false indications.
• Conductive coatings may not be testable using this method.
• High voltage introduces additional safety concerns.

Low Voltage Versus High Voltage

How Is the Correct Test Voltage Selected?

The correct voltage must be high enough to detect a discontinuity but not so high that it punctures or damages sound coating.

Voltage selection may depend on:

• Coating type.
• Measured dry-film thickness.
• Number of coating layers.
• Dielectric strength.
• Substrate.
• Specified test method.
• Coating manufacturer’s instructions.
• Project requirements.

Do not select the voltage by guesswork or by simply turning the instrument to its highest setting.

Excessive voltage may create a new holiday in a coating that was acceptable before testing. Insufficient voltage may pass over an existing defect without producing an alarm.

A technical discussion of high-voltage equipment and voltage selection is available from DeFelsko: High-Voltage Holiday Detection .

Measure DFT Before High-Voltage Testing

The test voltage is often related to coating thickness. Do not assume that the coating was applied at its nominal or specified DFT.

Before selecting or confirming the voltage:

1. Identify the coating system.
2. Confirm that the coating has cured sufficiently.
3. Measure DFT in representative areas.
4. Identify the specified holiday-test method.
5. Determine the correct voltage from the governing requirements.
6. Verify the detector output when required.

If film thickness varies significantly, the test procedure must address how voltage will be selected for both thinner and thicker areas.

The Coating Must Be Nonconductive

Conventional holiday testing depends on an electrical difference between the nonconductive coating and conductive substrate.

Some coatings contain conductive pigments, carbon, graphite, metallic fillers, or other ingredients that allow current to pass through otherwise sound film. These materials may produce continuous or widespread indications.

Before testing, confirm that the coating is suitable for electrical discontinuity detection.

If the coating is conductive, another inspection method or manufacturer-approved procedure may be necessary.

Why Grounding Matters

The detector must have a reliable electrical connection to the conductive substrate so current can return to the instrument.

Poor grounding can cause:

• Failure to detect actual holidays.
• Intermittent alarms.
• Unstable instrument operation.
• False confidence in the coating.

The ground connection should contact clean, conductive material. Paint, rust, insulation, dirt, or loose connections may interfere with the circuit.

Verify instrument operation before beginning the inspection and periodically during testing.

Preparing for Holiday Testing

Before testing:

• Review the current standard and project specification.
• Identify the coating and substrate.
• Confirm that the coating is nonconductive.
• Confirm adequate cure.
• Measure and document DFT.
• Select the appropriate detector.
• Determine the required test voltage.
• Verify instrument calibration or operating accuracy.
• Select the correct electrode.
• Establish a reliable ground.
• Confirm that the test area is clean and accessible.
• Establish the marking, repair, and retest procedure.
• Review electrical and site-specific safety requirements.

How Is the Test Performed?

The exact procedure must come from the specified standard and instrument instructions. In general:

1. The coating is confirmed to be sufficiently cured and ready for testing.
2. The detector is connected to the conductive substrate.
3. The instrument’s operation and output are checked.
4. The electrode is placed in contact with the coating.
5. The electrode is moved across the surface at the required rate.
6. The operator maintains adequate contact and coverage.
7. Each alarm location is marked without damaging surrounding film.
8. The defect is repaired using the approved procedure.
9. The repaired area is allowed to cure.
10. The repair is retested.

Moving the electrode too quickly, lifting it from the surface, using an electrode that does not fit the geometry, or failing to cover the entire area can leave holidays undetected.

Electrode Selection

The electrode must maintain contact with the coated surface and fit the shape being inspected.

Common electrodes include:

• Wet sponges.
• Flat wire brushes.
• Fan brushes.
• Conductive rubber paddles.
• Rolling springs for pipe.
• Half-circle brushes.
• Custom electrodes for unusual geometry.

An electrode that is too narrow may require many passes and increase the chance of missed areas. One that is too wide or stiff may fail to contact recesses, welds, or curved surfaces.

What Can Cause False or Misleading Indications?

Surface Moisture

Water extending across the coating surface may allow current to travel beyond the actual defect and make the holiday difficult to pinpoint.

Soluble Salts or Conductive Contamination

Salts, metallic dust, carbon, and other conductive material can create misleading alarms.

Conductive Coating Formulation

A conductive coating may alarm even when the film is continuous.

Sharp Edges

High electrical stress can concentrate around sharp geometry and produce indications that require careful investigation.

Incorrect Voltage

Too little voltage may miss a holiday. Too much may damage the coating or produce excessive arcing.

Poor Grounding

An incomplete return circuit may prevent the detector from responding to real defects.

Incomplete Electrode Contact

If the brush, spring, sponge, or paddle does not touch the coating, the area has not been properly tested.

Moving Too Quickly

The instrument and operator need sufficient contact time to detect and recognize an indication.

Is Holiday Testing Nondestructive?

Holiday testing is generally described as nondestructive when:

• The correct method is selected.
• The proper voltage is used.
• The coating is sufficiently cured.
• The instrument is operating correctly.
• The procedure is performed by trained personnel.

Improper high-voltage testing can damage otherwise sound coating. The operator should never increase voltage merely to make the test appear more sensitive.

When Should Holiday Testing Be Performed?

The coating must be sufficiently cured and dry according to the manufacturer and governing procedure.

Testing may be performed:

• After completion of the coating system.
• Before placing a lining into service.
• Before backfilling coated pipe.
• After transportation or installation.
• After repairs.
• At specified production or inspection stages.

Testing too early can damage a soft coating. Testing after equipment is inaccessible may make repair difficult or impossible.

Does Every Protective Coating Require Holiday Testing?

No.

Holiday testing is most valuable when continuity of the coating is critical to its intended service.

It may be specified for:

• Immersion linings.
• Tank interiors.
• Buried pipelines.
• Chemical containment.
• Wastewater service.
• Marine or offshore service.
• High-performance barrier systems.

It may not be practical or necessary for decorative coatings, conductive coatings, highly porous systems, or applications where occasional microscopic discontinuities do not affect intended performance.

The project specification and coating manufacturer should determine whether holiday testing is required.

Finding a Holiday Is Not the End of the Test

Every detected holiday should be:

1. Located.
2. Marked.
3. Documented.
4. Evaluated.
5. Repaired.
6. Allowed to cure.
7. Retested.

The job is not complete when the detector stops alarming over the original coating. It is complete when the repaired location is retested and found to be free of detectable discontinuities according to the specified procedure.

How Are Holidays Repaired?

The repair procedure depends on the coating, substrate, service, and manufacturer’s instructions. A general repair may involve:

• Removing loose or damaged coating.
• Feathering the surrounding edges.
• Restoring the required surface preparation.
• Removing dust and contamination.
• Applying the approved repair material.
• Achieving the required thickness.
• Allowing adequate cure.
• Retesting the repair.

Smearing coating over a pinhole without proper preparation may temporarily hide the defect without creating a durable repair.

What Should Be Documented?

A useful holiday-test report may include:

• Project and test location
• Date and time
• Inspector or technician
• Coating manufacturer and product
• Batch or lot number
• Coating system and number of coats
• Substrate
• Measured DFT range
• Test method and edition
• Detector manufacturer, model, and serial number
• Calibration or verification status
• Low-voltage or high-voltage method
• Test voltage
• Electrode type and size
• Grounding method
• Number and location of holidays
• Photographs or drawings
• Repair material and procedure
• Repair cure time
• Retest results

Holiday-Testing Safety

High-voltage holiday detectors require trained operators and careful handling.

Before testing:

• Read the instrument instructions.
• Inspect cables, connectors, electrodes, and insulation.
• Keep personnel clear of the energized electrode.
• Follow lockout, confined-space, and site-safety requirements.
• Evaluate flammable vapors and hazardous-area restrictions.
• Confirm that testing is permitted in the work environment.
• Consider implanted medical devices and other electrical hazards.
• Use the required personal protective equipment.

Do not use electrical test equipment in a flammable or explosive atmosphere unless the equipment, procedure, and work area have been specifically approved for that use.

Common Holiday-Testing Mistakes

• Using the wrong detector for the film thickness.
• Guessing at the test voltage.
• Failing to measure DFT first.
• Testing before the coating has cured.
• Using excessive voltage.
• Failing to establish a reliable ground.
• Moving the electrode too quickly.
• Missing edges, welds, corners, and repairs.
• Using an electrode that does not fit the surface.
• Testing a conductive coating with an unsuitable method.
• Failing to verify instrument operation.
• Marking a defect in a way that damages additional coating.
• Repairing without proper surface preparation.
• Failing to retest the completed repair.

Primary Referenced Standards

• ASTM D5162—Discontinuity Testing of Nonconductive Protective Coatings on Metallic Substrates
• AMPP SP0188—Discontinuity Testing of New Protective Coatings on Conductive Substrates
• ASTM G62—Holiday Detection of Coatings Used to Protect Pipelines

The Bottom Line

A holiday is a break in the continuity of a protective coating. It may be large enough to see—or small enough to escape an ordinary visual inspection.

Low-voltage wet-sponge testing is generally associated with thinner films. High-voltage testing is generally associated with thicker coatings and linings. The correct method and test voltage must come from the coating system, measured DFT, project specification, governing standard, and manufacturer’s instructions.

A coating does not have to fail everywhere to become a problem. Sometimes corrosion needs only one open door.

The purpose of holiday testing is to find that door, close it, and verify the repair before the environment finds it first.

Technical Notice: This article provides general educational information and does not reproduce or replace the complete ASTM, AMPP, coating-manufacturer, instrument-manufacturer, contract, electrical-safety, or project requirements. Standards are revised periodically. Always obtain and follow the current edition of the specified test method. Holiday testing should be authorized and performed by appropriately trained personnel using suitable, verified equipment and an approved repair procedure.

---

Copyright © 2026 Azimuth Spray Systems, LLC. All rights reserved. AirSprayTech.com—The Finishing Authority®. No portion of this article may be reproduced, distributed, republished, or transmitted without prior written permission.