Director of Paint Inspection Ian Patterson explains why assessing coating performance begins long before the first coat is applied
When a protective coating does not perform as expected, the coating itself is often the first thing to come under scrutiny. But premature deterioration is rarely down to one thing.
The performance of a coating system can be affected by decisions made before application starts, the condition of the substrate, the environment during the work and the checks carried out along the way. What happens after the project is complete, matters too.
Established corrosion protection guidance, including the ISO 12944 series, takes this wider view. It covers factors such as the environment, surface preparation, coating systems, application and maintenance rather than treating the coating as an isolated product. For asset owners, contractors and specifiers, that is an important distinction. A coating can be well suited to the job and still fail to deliver the expected service life if the conditions around it are not right.
SPECIFICATION SHOULD REFLECT THE ASSET
Coating selection should therefore start with the environment in which the asset will operate. Steel inside a relatively dry building faces a very different level of corrosion risk from infrastructure exposed to coastal weather, industrial pollutants, persistent moisture or immersion. Even within one structure, different areas can be exposed to diverse conditions.
The expected service life also matters. An accessible structure may be straightforward to inspect and maintain regularly, whereas a coating applied to a difficult-to-reach or costly-to-access asset may need to perform for much longer before major maintenance can be carried out. This is why specification should be based on the asset and its environment rather than simply choosing a familiar coating product. ISO 12944-2, for example, classifies different exposure environments because the level and type of corrosion stress is an important part of selecting a suitable protective system.
SURFACE PREPARATION
Surface preparation can sometimes be treated as a job that needs to be finished before the “real” work begins. In practice, it has a direct bearing on how the coating performs afterwards. The coating has to adhere to the surface it is being applied to. Rust, mill scale, grease, soluble salts and poorly prepared existing coatings can all interfere with adhesion or contribute to problems beneath a new system. Surface profile also needs to be appropriate for the coating being used. The surface needs enough profile to support adhesion, without creating peaks that are difficult to cover properly.
There is no universal preparation method that suits every project. The appropriate standard depends on the substrate, the existing condition, the coating system and the environment. ISO 12944-4 sets out surface preparation grades for different types of steel surfaces, reinforcing the importance of treating preparation as part of the specification rather than an afterthought. Two projects can use the same coating and still produce very different results if the surfaces underneath have been prepared differently.
APPLICATION CONDITIONS
Temperature, humidity and dew point can all affect the application and curing of a coating. If the steel temperature is too close to the dew point, condensation can form on the surface. That may not always be obvious to the person applying the coating, but moisture at the wrong stage can affect the finished system.
Film thickness matters too. Applying too little material can leave the substrate inadequately protected, while applying more than specified is not automatically better. Depending on the product, excessive film thickness can introduce its own problems. Mixing, application methods and recoating intervals also need to follow the coating manufacturer’s requirements. The coating needs to be applied in the conditions and at the thickness for which the system was designed.
THE IMPORTANCE OF INSPECTION
Inspection during surface preparation can establish whether the required level of cleanliness and surface profile has been achieved before the surface is coated. Environmental conditions can then be monitored during application, while dry film thickness measurements can confirm whether the specified coating build has been achieved.
This is where independent inspection can be particularly valuable. It creates a record of what actually happened during the work, rather than relying on assumptions once the finished coating is in place. It also means problems can be picked up when they are still possible to address. Finding an unsuitable surface before coating is applied is very different from discovering a failure several years later.
COATING FAILURES
When a coating begins to deteriorate, it is tempting to blame the product. The appearance of the failure alone, however, does not necessarily tell you what caused it.
Loss of adhesion may be associated with contamination, surface preparation or moisture. Localised corrosion may develop where the coating has been damaged or where the required film thickness has not been achieved. Problems between coats can also be linked to application conditions or recoating intervals. A proper failure investigation therefore needs to look beyond the coating itself.
Records of the original surface condition, preparation, environmental readings, coating thickness and application process can be extremely useful when trying to establish what happened. Without them, the cause can be much harder to determine.
MAINTENANCE MATTERS
A protective coating is intended to extend the working life of an asset. It does not remove the need for inspection and maintenance. Coatings can suffer wear, impact damage and localised corrosion even when the wider system remains in reasonable condition. Regular inspections can help identify these areas before a relatively small repair develops into a much larger project.
Future maintenance should therefore be considered when the coating system is first specified. Access, inspection requirements and the practicalities of carrying out repairs can all influence which system is most appropriate. Ultimately, coating performance comes down to the whole process rather than one stage in isolation.
The specification determines what the system needs to achieve. Preparation provides the surface it needs to adhere to. Application conditions affect how it cures and builds. Inspection provides evidence that the work has been carried out correctly, while maintenance helps protect the asset once it is back in service.
The coating itself remains important, but it is only one part of the equation. Looking at the full lifecycle gives asset owners and contractors a much better chance of achieving the level of protection and durability they expected when the system was specified.
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