Inspection Is a Process, Not a Final Walk-Through

Protective-lining inspection does not begin after the applicator puts down the spray gun. It begins before abrasive blasting, concrete preparation, mixing, or application starts. Many conditions that determine lining performance will be hidden after the next coat is applied.

A final visual inspection cannot prove that the surface was properly prepared, the environmental conditions were acceptable, the components were mixed in the correct ratio, or the recoat window was observed. Those facts must be inspected and documented while the work is taking place.

The inspector verifies compliance with the project requirements. The applicator performs the work and conducts quality-control checks. Although both parties may take similar readings, an inspector is not simply another painter carrying a gauge.

Know the Acceptance Criteria Before Work Begins

Every inspection result must be compared with an established requirement. A number by itself does not establish acceptance. Before the project begins, the contractor and inspector should identify the documents that govern the work.

• Project specification and lining schedule
• Approved product data sheets and application instructions
• Safety data sheets
• Approved surface-preparation standards
• Required environmental limits
• Specified wet- and dry-film thicknesses
• Maximum and minimum recoat intervals
• Required cure time before testing and service
• Holiday-test method, voltage, and acceptance criteria
• Required adhesion, hardness, or solvent-resistance tests
• Repair procedure and retesting requirements
• Inspection hold points and witness points

If the documents conflict, the conflict should be resolved in writing before the affected work proceeds. The inspector should not invent an acceptance criterion in the field.

Inspection Hold Points

A hold point is a stage at which work cannot continue until the required inspection has been completed and released. Hold points protect both the owner and the contractor by preventing important work from being permanently covered.

Inspecting the Substrate

The lining cannot compensate for an unacceptable substrate. Inspection should confirm that steel fabrication defects or concrete deficiencies have been corrected before the lining system is applied.

Steel Substrates

The inspector may need to verify:

• Specified degree of surface cleanliness
• Required surface-profile range
• Removal of oil, grease, dust, salts, and other contaminants
• Condition of welds, weld spatter, laminations, pits, and sharp projections
• Rounding or treatment of sharp edges where required
• Removal of spent abrasive and debris from difficult areas
• Absence of visible rust-back before coating

Concrete Substrates

The inspector may need to verify:

• Concrete age and cure requirements
• Surface soundness and tensile strength where specified
• Removal of laitance, curing compounds, sealers, oil, and contaminants
• Required concrete surface profile
• Repair of cracks, bugholes, voids, and deteriorated concrete
• Moisture test results and the approved test method
• Condition of transitions, joints, drains, penetrations, and terminations

Environmental Inspection

Environmental readings should be taken at the actual work area using suitable, calibrated instruments. Records commonly include air temperature, surface temperature, relative humidity, dew point, and the difference between surface temperature and dew point.

Readings should be taken before work begins, at the frequency required by the specification, whenever conditions change, and while the coating cures when required. Conditions inside a tank can differ significantly from those measured outside the tank.

The acceptable temperature and humidity ranges come from the project specification and the lining manufacturer's instructions. The commonly used three-degree-Celsius or five-degree-Fahrenheit separation above dew point must not automatically replace a more restrictive product requirement.

Material and Mixing Inspection

Inspection should follow the material from storage through application. This creates traceability and helps explain a problem if one later develops.

• Confirm the product name, component designation, color, batch number, and shelf life.
• Verify that storage and material temperatures meet the manufacturer's requirements.
• Observe component pre-mixing when required.
• Verify the correct mix ratio and use of complete kits unless proportioning is specifically approved.
• Record mixing time, induction time, thinning, and permitted additives.
• Record the time each batch was mixed and its calculated pot-life expiration.
• For plural-component equipment, verify ratio checks, temperatures, pressures, alarms, and spray quality.

Material that has exceeded its pot life, lost proper spray characteristics, or been mixed outside the approved procedure should be rejected. Adding unauthorized solvent to make expired material sprayable does not restore its performance.

Visual Inspection During and After Application

Good lighting is essential. The inspector should view the surface from different angles and inspect difficult areas closely. Mirrors, inspection lights, wet-film gauges, depth gauges, magnification, and photographic records may be useful.

Wet-Film Thickness

Wet-film-thickness readings provide immediate feedback during application. They allow the applicator to adjust technique before the material cures. A wet-film gauge is pressed into the freshly applied film, and the reading is taken between the highest wetted tooth and the next dry tooth.

Wet-film readings are most useful when the relationship between wet and dry thickness is understood. The product's volume-solids value can be used for estimating purposes, but surface texture, solvent addition, application loss, and reaction characteristics may affect the result.

Wet-film measurement procedures are addressed by ASTM D4414 .

Dry-Film Thickness

Dry-film thickness, or DFT, should be evaluated according to the specification, the approved lining system, and the required measurement procedure. The inspector must know the difference between a single gauge reading, a spot measurement, an area measurement, and the overall project acceptance criteria.

Before taking readings, verify that the instrument is suitable for the substrate and expected thickness range. Check its operation using certified standards or shims and adjust it on a representative prepared substrate when required by the procedure.

Rough blast profiles influence thickness readings. One unusually high or low gauge reading does not automatically describe the surrounding lining. Readings must be taken and evaluated using the sampling plan required by the governing specification.

Nondestructive DFT measurement on metallic substrates is addressed by ASTM D7091 . Project documents may also require an AMPP procedure for determining conformance to specified dry-film-thickness requirements.

Holiday Detection

A holiday is a discontinuity that allows the detection instrument to establish an electrical path to the conductive substrate. Holidays can be extremely small and may not be visible during ordinary inspection.

The lining must be sufficiently cured before testing. The test method, voltage, grounding, electrode, travel speed, and acceptance criteria should be established before the test begins. The lining manufacturer should be consulted because conductive pigments, retained solvent, moisture, and excessive test voltage can produce unreliable results or damage the film.

ASTM D5162 addresses low-voltage wet-sponge and high-voltage spark testing of nonconductive protective coatings on metallic substrates. It generally associates wet-sponge testing with systems 20 mils or less and high-voltage testing with systems greater than 20 mils, while also recognizing that system-specific approval and correct settings are necessary.

Adhesion Testing

Pull-off adhesion testing may be specified to evaluate the tensile strength of the installed system. The test is destructive: a loading fixture is bonded to the surface, the lining is cut when required by the method, and force is applied until separation occurs or the specified load is reached.

The reported pressure is only part of the result. The inspector should also record where the failure occurred:

• Within the adhesive used to attach the test fixture
• At the adhesive-to-lining interface
• Within a lining layer
• Between lining coats
• At the lining-to-substrate interface
• Within the concrete or other substrate

Pull-off testing of coatings is addressed by ASTM D4541 . Testing of coatings on concrete is addressed by ASTM D7234 . Test locations, frequency, acceptance values, equipment type, scoring procedure, and repair requirements must come from the project specification.

Checking Cure

A lining may feel dry while remaining insufficiently cured for testing or chemical service. Cure evaluation must follow the product and project requirements. Depending on the system, verification may involve elapsed time and temperature records, hardness, solvent resistance, chemical spot testing, or a manufacturer-approved procedure.

Solvent-rub testing is addressed by ASTM D5402 when that method is appropriate and specified. The solvent, number of rubs, pressure, cloth, evaluation method, and acceptance criteria must be defined. A field crew should not improvise a solvent test on an installed lining.

Inspecting Repairs

Repair inspection should be treated as part of the original lining work. Each defect should be marked, numbered when practical, and entered into the inspection record.

1. Identify and document the defect.
2. Determine its extent and likely cause.
3. Remove unsound or contaminated lining.
4. Prepare the exposed substrate and surrounding sound lining.
5. Apply the approved repair material within its application limits.
6. Allow the repair to cure for the required period.
7. Repeat the applicable visual, thickness, cure, or holiday tests.
8. Record the successful repair and close the item.

Simply covering a defect with more material may hide it without correcting its cause. Repairs must restore adhesion, continuity, thickness, and service suitability.

Inspection Records

A good inspection report allows another qualified person to understand what happened, where it happened, what was measured, and how acceptance was determined.

• Date, time, location, structure, and specific work area
• Contractor, crew, inspector, and responsible representatives
• Material manufacturer, product, color, batch numbers, and expiration dates
• Surface condition, preparation method, cleanliness, and profile
• Environmental readings and measurement times
• Mixing, induction, thinning, pot-life, and application records
• Wet- and dry-film-thickness readings with locations
• Holiday-test equipment, settings, calibration checks, and results
• Adhesion or cure-test procedures and results when required
• Defect and repair locations
• Photographs with useful identification and scale
• Nonconformance reports, corrective actions, and final disposition
• Final acceptance or release authorization

Instruments should be identified by make, model, serial number, range, and calibration status when required. A report stating only that the work “looked good” does not establish conformance.

Inspector Safety

The inspector is exposed to many of the same hazards as the application crew. Entering a tank to take readings is work inside the space, not passive observation.

• Follow the project respiratory-protection and personal-protective-equipment requirements.
• Observe confined-space permits, atmospheric testing, ventilation, communication, and rescue provisions.
• Use approved lighting and electrical equipment for the classified environment.
• Remain clear of pressurized spray lines, plural-component equipment, moving machinery, and active blasting.
• Control ignition sources during solvent use and holiday testing.
• Use safe access equipment and protect against falls.
• Do not enter a space solely to obtain a reading when entry conditions are unsafe.