Coating inspection: thickness, adhesion and holiday testing

An industrial coating is checked at every stage with four families of measurements: ambient conditions before and during application, dry film thickness to SSPC-PA 2, adhesion to ASTM D4541 or D3359, and film continuity by holiday detection to NACE SP0188. These measurements are recorded in a log and taken at the planned hold points, before the next coat hides the defect.

A film can look good and still hide low thickness, weak adhesion or holidays that will let water through as soon as it goes into service. Inspection finds those defects while they are still easy to fix. This guide is for maintenance managers, project managers and asset owners in Quebec and Ontario, alongside our coating inspection service.

Which checks should a coating job include?

Plan checks at every stage, not only at the end. The project's inspection and test plan sets what is measured, when, by whom and to which standard.

Cleanliness grade and surface profile are covered in our NACE 2 / SSPC-SP 10 surface preparation guide. This article focuses on the applied film.

How is dry film thickness measured to SSPC-PA 2?

Dry film thickness is measured with a non-destructive electronic gauge, verified on standards, following the SSPC-PA 2 sampling plan. The standard defines gauge readings, spot measurements, the areas to check and the restriction levels relative to the specified minimum and maximum thickness.

• Gauge verification. Accuracy check on standards and adjustment to ASTM D7091 and the gauge instructions, recorded before each series.
• Profile correction. On blasted steel, the gauge must be adjusted on the prepared surface or corrected by the prescribed method, otherwise the profile skews the reading.
• Sampling plan. The number of spots per area and the restriction level come from the standard and the specification; set them before work starts.
• Maximum thickness. Too thick is also a defect: the film can crack, retain solvent or cure poorly.

On a multi-coat system, measure after each coat to know the thickness of each one.

Readings only matter if someone acts on them. When a spot falls outside the range, the area around it is mapped to find the extent of the problem, then corrected: an extra coat where the film is thin, sanding or removal where it is too thick, always within the recoat windows of the data sheet. The corrected area is measured again and the result goes into the log next to the original reading.

How do you check adhesion with ASTM D4541 or D3359?

Adhesion is checked by pull-off with ASTM D4541, which gives a value and a failure mode, or by X-cut and cross-cut tape testing with ASTM D3359, which gives a qualitative rating. The specification sets the method, the number of tests and the acceptance criterion.

• ASTM D4541 (pull-off). A dolly is glued to the coating, then pulled off with a tester described in one of the standard's annexes. Record the stress and where the break happened: within the coating, between coats, at the substrate interface or in the glue.
• ASTM D3359 (tape). Method A uses an X-cut, method B a cross-cut, each suited to a thickness range defined by the standard. Simple and quick, useful as a field check.
• Panels or structure. These tests are destructive. Run them on test panels prepared under the same conditions, or on the structure at chosen spots, then repair.

A break at the substrate interface often points to a preparation problem; a break between coats, to an exceeded recoat window or contamination.

How do you detect holidays to NACE SP0188?

Holidays are found with a low voltage wet sponge detector for thin films, or a high voltage spark detector for thicker films, according to NACE SP0188. The method and test voltage depend on thickness and product: they are set from the standard and the manufacturer's data sheet.

• Low voltage (wet sponge). For thin films; a signal shows a holiday that lets moisture reach the substrate.
• High voltage (spark). For the heavy films of tank and pipe linings; excessive voltage can puncture a sound film, which is why the setting matters.
• Conditions. Film sufficiently cured, surface clean and dry, substrate grounding verified.
• On concrete. Holiday detection on concrete follows ASTM D4787, with its own precautions.

Each holiday is marked, repaired and retested. See the holiday test entry. The test is essential in immersion service, as our guide to industrial tank lining selection points out.

Why are ambient conditions checked before every coat?

Because a coat applied outside the planned conditions can fail even if everything else complies. Invisible condensation, a substrate that is too cold or excessive humidity compromise adhesion and cure.

• Measurements. Air and substrate temperature, relative humidity and dew point, with a psychrometer to ASTM E337 or an equivalent electronic instrument.
• Criteria. Substrate temperature above the dew point with the margin required by the specification, temperature and humidity ranges from the data sheet.
• Frequency. Before each coat starts and at regular intervals during application, more often when the weather changes.

In winter, these checks become decisive; our article on cold weather epoxy application covers the precautions.

What should the records contain?

Record everything that will later prove each area was prepared and coated to the specification. A complete log supports acceptance, warranty and diagnosis if a defect appears.

• Identification. Area, member, date, crew, product, batch numbers.
• Measurements. Ambient conditions, cleanliness, profile, thicknesses, adhesion, holidays, with locations.
• Nonconformances. Defect found, correction made, retest, acceptance.
• Instruments. Identification and verification of each instrument used.

When a coating fails, these records trace the cause; see our guide to coating failure modes.

Where should hold points go?

Put hold points where a defect would become hidden or costly to fix: after preparation, after the primer, after each coat and before return to service. At each hold point, work resumes only after acceptance by the designated inspector.

Hold points go into the inspection and test plan at the planning stage, along with witness points the owner can attend. During a plant shutdown, they are coordinated with the schedule, often through our project management service. To define the plan itself or audit an existing coating, our strategic advisory service can step in upstream. These checks accompany all our protective coatings.

Our crews and qualified subcontractors work from LaSalle across Quebec and in Ontario. Send the specification, photos and the type of structure through the quote request: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Can coating thickness be measured on concrete?

Not with a magnetic gauge. Use a suitable ultrasonic gauge, destructive measurements on a sample, or a check of product consumption per area, according to the specification.

Is adhesion testing always required?

No. It is required when the specification calls for it, for a procedure qualification or for a diagnosis. Since it is destructive, it is often done on test panels.

Which voltage should be used for holiday testing?

The one given by NACE SP0188 and the manufacturer's data sheet for the thickness and product involved. It is set before the test and recorded in the log.

Who should carry out the inspection?

An inspector designated by the owner or the contractor, depending on the contract, with the competencies required by the specification. Independent inspection is often preferred for critical assets.