Common Causes Of Premature Coating Failure
Introduction
One of the most persistent misconceptions in the roof refurbishment industry is that any problem affecting a roof coating system must be a product failure.
In reality, coating failures are often caused by issues that occurred before the coating was applied, during installation, or as a result of unsuitable site conditions. This principle is recognised throughout the wider protective coatings industry and is reflected in the training provided by organisations such as the Institute of Corrosion, ICATS (Industrial Coating Applicator Scheme) and others.
Understanding the difference between a manufacturing defect and an installation defect is important for building owners, surveyors and contractors alike. It helps ensure that the correct remedial action is taken and avoids costly disputes where responsibility has been incorrectly assigned.
This guide explains some of the most common installation-related defects found on industrial metal roof coating projects and the signs that may indicate a problem.
Inadequate Surface Preparation
No coating system can perform properly if it is applied to an unsuitable surface.
Surface preparation is often the single most important factor affecting long-term performance. Rust, loose coatings, dirt, oils and other contaminants can all interfere with adhesion.
Common examples include:
- Rust left beneath the coating.
- Loose plastisol not fully removed.
- Surface contamination from oils or silicones.
- Inadequate drying following pressure washing.
The coating may initially appear satisfactory but can later lose adhesion or allow corrosion to continue beneath the film.
Typical signs
- Peeling coating.
- Localised loss of adhesion.
- Rust appearing beneath otherwise intact coating.
This is typical of when a coating has been applied to a wet substrate. The coating can be peeled away with no resistance.
Preparation is key to a successful roof coating installation. Here we see that the surface has not been prerpared or primed correctly.
This section of roof has not been cleaned and prepared properly and additionally, rainfall has occurred during application leaving a pitted finish.
Coating Over Active Corrosion
A coating can slow corrosion but cannot reverse significant corrosion that has already developed.
Where heavy corrosion remains after preparation, oxidation may continue beneath the coating film. The coating may still be performing exactly as intended while the underlying substrate continues to deteriorate.
This is particularly common where:
- Deep pitting remains.
- Corrosion products are trapped beneath coatings.
- Preparation standards have not been achieved.
Typical signs
- Staining beneath coating.
- Blistering.
- Continued expansion of corroded areas.
Insufficient Film Thickness
Protective coatings are designed to perform within specified dry film thickness ranges.
Where insufficient material is applied, the expected barrier properties may not be achieved and the coating will be more susceptible to mechanical damage and adhesion may be affected.
Causes include:
- Over-spreading material.
- Failure to monitor wet film thickness.
- Incorrect application techniques.
- Areas missed during installation.
Typical signs
- Peeling or lifting coating
- Texture of colour of previous coating or primer evident
- Uneven appearance.
This type of defect will probably be identified in isolatedareas rather than across a whole installation that is otherwise satisfactory.
Failure to Correct Defective Fixings
Roof coatings are not intended to compensate for defective fixings.
Corroded, loose or over-tightened fasteners can continue to create problems regardless of the coating system applied.
Common examples include:
- Missing weathering caps.
- Corroded fixings.
- Over-tightened fasteners causing sheet deformation.
- Loose fasteners allowing movement.
Typical signs
- Localised leaks.
- Rust staining around fixings.
- Coating cracking around fastener heads.
It’s easy for coating applicators to miss loose or misaligned fixings. Sometimes they are not visible until somebody walks over the roof and the flex in the sheet, unwinds the fixing.
Where possible, professional specifiers should allow a contingency in their roof refurbishment budget to replace some of all of the fixings, particularly on older built-up roofs.
Incorrect Treatment of Cut Edge Corrosion
Cut edge corrosion remains one of the most common defects affecting industrial metal roofs.
Where corrosion is not properly prepared or treated before coating, deterioration may continue beneath the repair.
Common issues include:
- Insufficient preparation.
- Failure to remove corrosion products.
- Inadequate primer application.
- Applying coating directly over corroded edges.
Typical signs
- Rust reappearing at sheet edges.
- Localised coating breakdown.
- Progressive edge deterioration.
- Rust staining from beneath
A cut edge corrosion treatment is always dependent upon the condition of the roof, whether it is a stand-alone treatment or carried out as part of a full roof refurbishment project.
Learn more about underside corrosion on metal roofs.
Understanding the difference beween mid-lap corrosion and end-lap corrosion and the way both are treated is crucial for the professional specifier.
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Rainfall during silicone roof coating application causes surface pitting to the new coating. Whilst this may not affect performance, it is unsightly and causes a soiling trap.
Application in Unsuitable Conditions
Environmental conditions during installation can significantly affect coating performance.
Examples include:
- Rain during application.
- Excessive surface moisture after cleaning.
- Condensation and dew
- Temperatures outside the manufacturer’s recommendations.
While some specialist coatings can tolerate limited moisture, a clean and dry substrate remains the preferred condition for most coating applications.
Typical signs
- Adhesion problems.
- Surface defects.
- Inconsistent curing.
Failure to Follow the Specification
Many coating specifications are developed to address known roof defects and project-specific conditions.
Problems can occur where:
- Steps are omitted.
- Alternative materials are substituted.
- Recommended repairs are not completed.
- Coatings are diluted.
The result may be a roof that appears compliant immediately after installation but experiences avoidable defects later.
This highlights the importance of manufacturer-led site audits. In the image opposite, a hole has been left in the steel substrate and the area around it has been coated.
Failure to Follow the Specification
Many coating specifications are developed to address known roof defects and project-specific conditions.
Problems can occur where:
- Steps are omitted.
- Alternative materials are substituted.
- Recommended repairs are not completed.
- Thickness requirements are reduced.
The result may be a roof that appears compliant immediately after installation but experiences avoidable defects later.
This highlights the importance of manufacturer-led site audits. In this image, a hole has been left in the steel substrate and the area around it has been coated.
What Constitutes A Coating Product Failure?
While installation-related defects are common, genuine manufacturing defects can occur with all coatings and paints.
Examples might include:
- Incorrectly manufactured material.
- Contaminated product.
- Product supplied outside specification.
- Incorrect colour
These cases are comparatively rare and can often be identified through inspection, testing and review of batch records, which all responsible manufacturers do.
Most investigations therefore begin by examining the scope of any failure.
The manufacturer will look to see if the failure is localised, widespread or catastrophic.
Most defects concern adhesion failure and if this is localised rather than widespread, it is a strong indicator that the product has been applied incorrectly, since it isn’t possile to manufacture a product that only adheres to part of a substrate.
Where a manufacturing defect is suspected, retained batch samples can be tested against the original production specification. This allows them to determine whether the product supplied was compliant at the time of manufacture.
If the failure cannot be replicated, the installation will be inspected to check that instructions have been followed and if necessary, further tests will be conducted to try and determine the cause of the failure.
Failures usually occur right at the start of a project and will probably be identified during manuafcturer’s audit visits, assuming they are being conducted.
Nothing is taken for granted. All responsible manufacturers are keen to understand why their product has been deemed to fail and naturally and, they will want to put procedures in place to rectify any failure if the product is at fault.
Coating Warranty Implications
Understanding the cause of a defect is important because different parties may have different responsibilities.
A manufacturer warranty typically relates to the coating material itself.
A workmanship warranty, where provided, normally relates to the installation work undertaken by the contractor.
Correct diagnosis helps ensure that any remedial action is directed to the appropriate party and avoids unnecessary disputes.
Conclusion
When a coating defect becomes visible, the coating itself is often the first thing blamed.
When a coating defect becomes visible, the coating itself is often the first thing blamed. However, the visible symptom is not always the underlying cause.
A systematic inspection that considers roof condition, preparation standards, environmental conditions, installation quality and coating performance will usually provide a more accurate diagnosis and help identify the most appropriate remedial solution.
Most manufacturers follow a forensic approach to identifying the cause of failures. This process cannot be rushed and depending upon the findings and need for further investigation, can takle anything from a few days to several weeks.
Roof Coating Maintenance
Even correctly specified and installed coating systems should be periodically inspected. Blocked gutters, mechanical damage, unauthorised roof traffic and later alterations can all affect long-term performance.
Best practice to ensure your coating warranty remains valid, is to conduct inspections in accordance with the coating manufacturer’s recommendations. Ideally, inspections should be completed by an independent auditor, with a photographic condition schedule maintained.
This provides the building owner or occupier with a traceable action log so that any defects can be notified to the installer or the manufacturer for investigation and resolution.
It is important that defects are notified as soon as they are identified, because most warranties will contain a clause to this effect.