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Water and Wastewater Protective Coating Systems

Article 22 of 24

Defects, Failure Analysis, and Coating Repairs

Finding the cause, defining the repair boundary, and restoring the protective system without concealing the evidence

Repair the Cause—Not Only the Appearance

A blister, crack, pinhole, soft film, or delaminated area is visible evidence that something has happened within the coating system, substrate, environment, application process, or service exposure. Covering the visible defect without understanding its cause can trap contamination, moisture, corrosion, or uncured material beneath the repair.

A professional repair begins with investigation. The contractor must determine what failed, where the failure occurred, how far it extends, why it happened, and whether the proposed repair will be compatible with the surrounding system and future service conditions.

Defect, Damage, and Failure Are Not Identical

These terms should be used carefully because they describe different conditions:

• Defect: A condition that does not meet the specification or expected workmanship standard, such as a pinhole, run, thin area, or missed location.
• Damage: Physical harm caused after application, such as impact, abrasion, cutting, welding, equipment installation, or improper cleaning.
• Failure: Loss of the coating system's intended protective function, adhesion, continuity, chemical resistance, or structural integrity.
• Normal deterioration: Gradual loss of performance after an expected period of service, rather than premature failure.

A visible defect does not always mean that the entire system has failed. Conversely, a coating that appears acceptable at the surface may conceal loss of adhesion, underfilm corrosion, incomplete cure, or chemical attack.

Stop Before Destroying the Evidence

Do not immediately grind, scrape, wash, solvent-wipe, cut, or coat over a significant defect. Those actions may destroy evidence needed to identify the cause and may expand contamination into otherwise sound areas.

Photograph and document the condition first. When the cause, responsibility, or repair scope is disputed, notify the responsible parties and obtain agreement before destructive examination or repair begins.

A Systematic Failure-Investigation Process

1. Secure the area. Address confined-space, process, chemical, biological, electrical, fall, pressure, and structural hazards before entering or disturbing the coating.
2. Record the existing condition. Photograph overall areas, individual defects, boundaries, elevations, penetrations, and nearby process features.
3. Review project records. Examine specifications, product data, daily reports, environmental readings, batch records, thickness measurements, inspection reports, and repair history.
4. Map the defects. Identify whether the condition is isolated, random, repeated, concentrated at details, or associated with liquid levels, flow patterns, temperature zones, or chemical exposure.
5. Perform nondestructive testing. Use visual examination, sounding, thickness measurement, holiday detection, hardness checks, moisture assessment, or other approved methods.
6. Conduct controlled destructive testing. Make test cuts, remove samples, or perform pull-off tests only after representative locations have been selected and documented.
7. Identify the failure plane. Determine whether separation occurred at the substrate, between coats, within a coating layer, within a repair material, or within the concrete itself.
8. Develop and confirm the probable cause. Compare the physical evidence with project records, service conditions, and laboratory findings.
9. Prepare a written repair procedure. Define removal, preparation, materials, application, inspection, cure, and return-to-service requirements.

Map the Pattern Before Naming the Cause

Defect location and distribution often provide valuable clues. The investigation should consider whether the condition appears:

• Only below or above the normal liquid line.
• At construction joints, cracks, penetrations, welds, edges, or repairs.
• Near chemical-injection points, aeration equipment, influent zones, or turbulence.
• On cold walls, exterior walls, roof areas, or locations exposed to sunlight.
• Only in one day's production or within one material batch.
• At the beginning or end of spray passes.
• Where ventilation, heating, dehumidification, or access was limited.
• In high-pressure cleaning, abrasion, impact, or maintenance areas.

A repeated pattern may indicate a system-level problem. A single damaged location may indicate localized mechanical damage. Do not assume that every visually similar defect has the same cause.

Pinholes, Voids, and Holidays

Pinholes and holidays interrupt the continuity of a lining and can expose the substrate directly to water, chemicals, gases, or wastewater. They may result from:

• Outgassing from porous concrete.
• Air entrained during mixing or application.
• Poor wetting or surface contamination.
• Excessive coating viscosity or incorrect thinning.
• Insufficient film thickness or incomplete coverage.
• Application over bugholes, pits, welds, edges, or sharp surface irregularities.
• Incorrect spray technique, atomization, tip selection, or plural-component operation.

Repair may require opening the defect to sound material, removing contamination, preparing the exposed surface, filling the void, and applying compatible repair coats. Every repaired holiday must be allowed to cure and then retested.

Blistering

A blister is a raised area caused by loss of adhesion or separation within the coating system. ASTM D714 provides a standardized method for describing blister size and density, but the rating alone does not identify the cause.

Possible causes include:

• Moisture vapor moving through concrete.
• Osmotic pressure caused by soluble contaminants beneath the film.
• Entrapped solvent, air, water, or reaction gases.
• Application over condensation or a damp substrate.
• Chemical permeation or service temperatures beyond system limitations.
• Poor intercoat adhesion or application outside the recoat window.
• Contamination or insufficient surface preparation.

Before opening representative blisters, document their size, density, distribution, location, and service exposure. When a blister is opened, record the failure plane, condition of the substrate, presence and appearance of liquid, odor, corrosion, and condition of each coating layer.

Delamination and Loss of Adhesion

Delamination may occur between the substrate and primer, between coating layers, within a coating layer, within a surfacer or repair material, or within weak concrete. The location of separation is essential to the diagnosis.

Common contributing conditions include:

• Insufficient surface cleanliness or profile.
• Dust, laitance, oil, grease, biological residue, salts, or chemical contamination.
• Weak or deteriorated concrete.
• Condensation or excessive substrate moisture.
• Application beyond the permitted recoat interval.
• Incompatible products or unauthorized substitutions.
• Off-ratio mixing, incomplete mixing, or inadequate cure.
• Thermal movement, structural cracking, impact, or excessive service stress.

Sounding, test cuts, knife examination, pull-off testing, and controlled removal may help define the extent. A repair must extend beyond visibly loose material to a firmly bonded, properly prepared boundary.

Cracking, Checking, and Splitting

Cracking may be limited to the coating film, extend through multiple layers, follow movement in the substrate, or result from excessive coating thickness, internal stress, embrittlement, thermal cycling, chemical attack, or poor joint treatment.

Before repairing a crack, determine whether it is active or dormant and whether it is structural, shrinkage-related, thermal, chemical, or associated with a construction joint. Rigidly filling an active crack may simply move the failure to the edge of the repair.

ASTM D661 provides a visual method for rating the degree of cracking in exterior paint films. Project-specific evaluation is still required for thick-film linings and concrete structures.

Soft, Tacky, or Uncured Coating

A coating that remains soft, tacky, greasy, or easily displaced may have resulted from:

• Incorrect mix ratio.
• Incomplete mixing or failure to reach the sides and bottom of the container.
• Plural-component equipment malfunction or component restriction.
• Material, substrate, or air temperature below the approved range.
• Unauthorized thinner, water, solvent, or additive.
• Expired, contaminated, frozen, overheated, or otherwise damaged material.
• Chemical contamination or premature service exposure.

Applying another coat over uncured material does not correct the underlying problem. Unless the manufacturer provides a verified corrective procedure, uncured material normally must be completely removed before the surface is prepared and recoated.

Runs, Sags, Curtains, and Excessive Thickness

Excessive localized thickness can produce more than an appearance problem. Thick areas may retain solvent, generate excessive cure heat, crack, wrinkle, remain soft, or cure differently from the surrounding film.

Causes may include:

• Slow gun movement or excessive pass overlap.
• Incorrect spray-tip size or material pressure.
• Material applied above its recommended film build.
• Incorrect thinning, temperature, or viscosity.
• Poor lighting, difficult access, or inadequate spray planning.

Repair may require removal or leveling after sufficient cure, followed by surface preparation and recoating. Do not assume that a heavy film provides better protection.

Dry Spray, Overspray, and Rough Texture

Dry spray occurs when atomized coating partially dries or loses flow before reaching or joining the wet film. The resulting surface may be rough, porous, poorly bonded, difficult to clean, and unable to receive the next coat without additional preparation.

Contributing conditions include:

• Excessive gun distance or poor gun angle.
• High air movement, temperature, or ventilation velocity.
• Incorrect spray pressure, tip selection, or atomization.
• Material viscosity outside the approved range.
• Poor work sequence that directs overspray onto previously coated surfaces.

The affected surface may require sanding, grinding, abrasive preparation, cleaning, and recoating. Loose particles must not be trapped beneath the repair coat.

Underfilm Corrosion and Rusting

Corrosion beneath a coating may begin at holidays, damaged areas, welds, edges, fasteners, poorly prepared surfaces, or locations contaminated with soluble salts. Corrosion products can spread beneath the film and lift coating beyond the visibly rusted area.

ASTM D610 provides a standardized method for rating the degree of rusting on painted steel surfaces. The rating describes the visible extent; it does not determine the complete repair boundary or cause.

Remove coating until sound, firmly bonded material and clean substrate are reached. Corrosion products, salts, moisture, and contamination must be removed before rebuilding the system.

Chemical Attack, Erosion, and Abrasion

Water and wastewater facilities may expose coatings to acids, alkalis, disinfectants, solvents, fats, oils, elevated temperatures, hydrogen sulfide, sulfuric acid, grit, turbulence, pressure washing, and mechanical cleaning.

Evidence may include:

• Softening, swelling, discoloration, loss of gloss, or increased brittleness.
• Surface loss, thinning, exposed aggregate, or wear-through.
• Attack concentrated at chemical-injection points or liquid lines.
• Directional wear associated with flow, grit, brushes, or cleaning equipment.
• Repeated failure after maintenance cleaning or process upsets.

Before specifying a repair, verify the actual chemical concentrations, temperatures, exposure duration, cleaning methods, and process conditions. Reinstalling the same system under service conditions beyond its resistance may repeat the failure.

Investigate Changes in Facility Operation

A coating may perform successfully for years and then deteriorate after the process changes. Increased chemical concentration, higher temperature, longer immersion, new cleaning agents, added disinfection cycles, altered flow, or different maintenance equipment can move the exposure beyond the original design.

Compare current operating conditions with the conditions used to select the original coating system. Process records can be as important as coating records during failure analysis.

Sampling and Laboratory Analysis

Laboratory analysis may be necessary when visual inspection and field testing cannot determine the cause. Representative samples may include failed coating, sound coating, corrosion products, blister liquid, deposits, concrete, process residue, unused material, or retained application samples.

Each sample should be:

• Photographed before removal.
• Assigned a unique identification number.
• Linked to an exact location and observed condition.
• Collected using clean, appropriate tools.
• Stored in a compatible container that will not alter the sample.
• Protected against contamination, moisture loss, leakage, or physical damage.
• Transferred under documented chain-of-custody procedures when responsibility may be disputed.

Laboratory testing should be selected to answer a specific question. Testing without a defined purpose can generate data without resolving the failure.

Determine the Repair Boundary

The visible defect is not always the full extent of the problem. The repair boundary must extend to material that is sound, clean, properly bonded, chemically compatible, and capable of supporting the repair system.

Boundary determination may involve:

• Visual examination under adequate lighting.
• Sounding or tapping to locate hollow areas.
• Adhesion or knife testing at progressively greater distances.
• Dry-film thickness measurements.
• Moisture, contamination, or soluble-salt testing.
• Holiday testing and controlled exploratory cuts.

When widespread failure is present, complete removal and replacement may be more reliable and economical than numerous isolated patches.

Prepare a Written Repair Procedure

The repair procedure should specify:

• The defect and confirmed or probable cause.
• The limits and depth of coating or substrate removal.
• Required steel or concrete surface preparation.
• Cleaning, moisture, profile, and environmental acceptance criteria.
• Edge treatment, feathering, termination, and overlap dimensions.
• Crack, joint, pit, bughole, and substrate-repair materials.
• Primer, lining, reinforcement, and topcoat materials.
• Mixing, application, thickness, cure, and recoat requirements.
• Inspection, testing, repair acceptance, and return-to-service requirements.

Obtain the coating manufacturer's written recommendation when repairing aged material, joining different systems, exceeding a recoat interval, or applying a new product over an existing lining.

General Repair Sequence

1. Isolate the structure and establish safe work conditions.
2. Document and mark the approved repair boundaries.
3. Remove loose, damaged, contaminated, uncured, or poorly bonded material.
4. Repair deteriorated concrete, corrosion damage, cracks, joints, welds, or other substrate deficiencies.
5. Feather or terminate existing coating edges as required by the repair procedure.
6. Prepare exposed substrate and retained coating to the specified cleanliness and profile.
7. Remove dust, debris, moisture, salts, and preparation residue.
8. Verify environmental and substrate conditions.
9. Apply approved repair materials at the required thickness and overlap.
10. Allow each material to cure within its required conditions.
11. Inspect, test, repair remaining deficiencies, and retest.
12. Document the completed repair and obtain acceptance before service begins.

Avoid Creating a Weak Repair Edge

Applying repair material over a glossy, contaminated, poorly bonded, or sharply terminated coating edge can create an immediate weak plane. Existing coating retained beneath an overlap must be firmly bonded, compatible, cleaned, and mechanically prepared as required.

Do not leave feathered coating edges so thin that preparation removes protection or exposes a weak transition. Follow the approved repair detail and manufacturer instructions for termination and overlap.

Retest the Completed Repair

A repair should be evaluated by the same type of inspection that identified the original deficiency, together with any additional testing required by the repair procedure.

• Visually inspect the entire repair and overlap.
• Verify wet-film or dry-film thickness where applicable.
• Holiday-test the repaired area after adequate cure.
• Repeat adhesion, hardness, cure, or moisture testing when specified.
• Confirm that termination edges are sealed and fully bonded.
• Record test settings, readings, defect locations, additional repairs, and final acceptance.

Document the Repair

The repair record should include:

• Structure, area, elevation, grid, or other exact location.
• Date discovered and date repaired.
• Description, size, and apparent extent of the defect.
• Photographs before, during, and after repair.
• Investigation methods, test results, samples, and probable cause.
• Removal and surface-preparation methods.
• Products, lot numbers, quantities, mixing records, and thicknesses.
• Environmental conditions and cure time.
• Final inspection, retesting, and acceptance results.
• Names of personnel performing and accepting the repair.

Contractor's Field Standard

Do not conceal a defect before it is documented and understood. Determine the failure plane, remove material to a sound boundary, correct the cause, rebuild the approved system, and retest the completed repair. A patch that only improves appearance is not a professional coating repair.

Knowledge Check

1. What is the difference between a coating defect, physical damage, and coating failure?
2. Why should a significant defect be documented before it is disturbed?
3. What information can the location and distribution of defects reveal?
4. Why is the failure plane important when investigating delamination?
5. Why should uncured coating generally not be covered with another coat?
6. How should the boundary of a coating repair be determined?
7. Why must completed coating repairs be retested?

Technical References

• ASTM D6577: Standard Guide for Testing Industrial Protective Coatings.
• ASTM D714-25: Standard Test Method for Evaluating Degree of Blistering of Paints.
• ASTM D610-25: Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces.
• ASTM D661: Standard Test Method for Evaluating Degree of Cracking of Exterior Paints.
• ASTM D772-24: Standard Test Method for Evaluating Degree of Flaking or Scaling of Exterior Paints.
• ASTM D4541-22: Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers on metal substrates.
• ASTM D7234-22: Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers.
• ASTM D5162-24: Standard Practice for Discontinuity Testing of Nonconductive Protective Coating on Metallic Substrates.
• AMPP Coatings Failures Series: Case studies addressing protective-lining, concrete, infrastructure, and other coating failures.
• OSHA 29 CFR 1910.146: Permit-Required Confined Spaces.
• Manufacturer requirements: Current product data sheets, safety data sheets, chemical-resistance information, repair procedures, cure requirements, and written technical recommendations.

Standards may be revised, withdrawn, or replaced. Confirm the current edition required by the contract and obtain the complete standard before developing inspection, testing, or repair procedures.

Professional responsibility: Failure analysis should be based on documented evidence rather than assumptions. Obtain manufacturer, engineering, laboratory, or independent coating-specialist assistance when the failure mechanism, system compatibility, structural condition, or repair scope is uncertain.

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