Coating defects: Identification, causes and prevention in industrial environments

Coating defects: Identification, causes and prevention in industrial environments

Correctly applied protective coatings limit coating defects and are a critical line of defence for industrial assets, protecting steelwork and infrastructure from corrosion and premature deterioration. Whether applying paint to steel structures, storage tanks, bridges or processing facilities, coatings are designed to prevent corrosion, withstand environmental exposure and extend service life. However, even when high-quality materials are specified, coating defects remain one of the most common challenges faced in industrial maintenance and asset protection.

Defects introduced during surface preparation, application or curing can compromise performance long before a coating reaches the end of its intended lifespan. In many cases, defects are present from day one, gradually allowing moisture, oxygen or chemicals to penetrate the coating system. Over time, these weaknesses can lead to corrosion, structural damage and costly remedial work.

This article examines the most common coating defects found in industrial environments, explains why they occur, and outlines how they can be prevented through proper planning, access, competence and inspection, supported by effective quality control and quality assurance.

What are coating defects?

Coating defects are imperfections or irregularities within a coating system that reduce its ability to perform as intended. Some defects are immediately visible after application, while others may only become apparent months or years later as the coating is exposed to service conditions.

Unlike general wear or ageing, coating defects are typically introduced during one of three stages:

• surface preparation
• coating application
• curing and early service life

Although many defects become visible during or after application, their underlying root cause often lies in inadequate surface preparation — a factor explored in more detail later in this article. Unlike application defects, preparation-related issues are not always immediately apparent as distinct visual flaws. Instead, insufficient cleaning, poor surface profile or residual contamination create the conditions in which defects such as blistering, pinholing and premature coating breakdown are far more likely to occur. This is why standards such as ISO 12944 (parts 1 to 9) and surface preparation guidance including SSPC-SP 1 place strong emphasis on removing contaminants prior to coating application.

While individual defects may appear minor, they often act as initiation points for corrosion and coating breakdown. Identifying and addressing defects early is therefore essential for maintaining asset integrity.

Common types of coating defects

Blistering

Blistering occurs when pockets of moisture, solvent or gas become trapped beneath the coating film, creating raised bubbles on the surface. These blisters may remain intact initially, but once they rupture, they expose the substrate and accelerate corrosion.

Blistering is commonly associated with:

• coating over damp or contaminated surfaces
• inadequate drying between coats
• poor environmental control during application

Pinholing and porosity

Pinholing refers to small holes or voids that penetrate the coating film. Although often difficult to see without close inspection, pinholes are one of the most significant coating defects because they provide a direct pathway to the substrate.

Pinholing may result from:

• incorrect spray technique
• entrapped air in the coating material
• excessive thinning or poor mixing

Sagging and runs

Sagging occurs when excess coating material flows downward under gravity before curing. This defect is typically cosmetic at first, but excessive sagging can lead to uneven film thickness and weak points within the coating system.

Common causes include:

• over-application
• incorrect viscosity
• inadequate control during work at height

Dry spray

Dry spray occurs when coating droplets partially dry before reaching the surface, resulting in a rough or powdery finish. This reduces adhesion between coats and can weaken overall performance.

Dry spray is often linked to:

• incorrect spray distance
• unsuitable environmental conditions
• poor access positioning

Cracking and crazing

Cracking occurs when the coating film fractures due to stress, movement or excessive thickness. Fine surface cracking, often referred to as crazing, may not immediately expose the substrate but can allow moisture ingress over time.

These coating defects are frequently caused by:

• thermal expansion and contraction
• substrate movement
• incompatible coating layers
  

Why coating defects occur

Inadequate surface preparation

Surface preparation is widely recognised as the most critical stage of any coating process. Contaminants such as oil, grease, salts and corrosion products prevent coatings from bonding effectively with the substrate.

Internationally recognised standards such as ISO 8501-1 define surface cleanliness, rust grades and preparation methods for steel substrates prior to coating. Failure to meet these preparation requirements significantly increases the likelihood of coating defects and premature breakdown. Even the most advanced coating system cannot compensate for poor preparation.

Environmental conditions during application

Temperature, humidity and dew point all influence coating performance. Applying coatings outside manufacturer-specified limits can lead to condensation, solvent entrapment and improper curing.

Many coating defects originate from:

• high humidity
• low substrate temperatures
• inadequate monitoring of environmental conditions

Application technique and competence

Industrial coatings rely heavily on the skill and judgement of those applying them. Incorrect spray angles, inconsistent overlap or rushed application can introduce defects that compromise long-term performance.

Training and competence are widely recognised as critical factors in reducing coating defects, with organisations such as ICorr and Corrodere placing strong emphasis on the knowledge and practical skills required in the UK and Northern Ireland to apply and inspect protective coating systems correctly.

Improving application quality relies on structured training that develops understanding of surface preparation, coating application and defect awareness. This is why programmes such as Train the Painter are increasingly recognised as an effective control against coating defects in industrial environments, supporting consistent standards across different sites and conditions.

Access limitations and work at height

Access is a frequently underestimated contributor to coating defects. When technicians are forced to work at extended reach, from unsuitable platforms or with limited visibility, surface preparation and application quality can suffer.

Restricted access can lead to:

• uneven coating thickness
• missed preparation defects
• reduced inspection capability

Where access is poorly planned, defects are more likely to be introduced and overlooked.

Why coating defects are more common in complex industrial environments

Industrial assets rarely exist in ideal conditions. Many structures involve height, confined spaces, complex geometry or continuous operation, all of which increase the likelihood of coating defects if not properly managed. Areas such as edges, welds, flanges and transitions are particularly vulnerable. These locations are harder to prepare and coat consistently, yet they are often where corrosion initiates first.

In complex environments, specialist access planning becomes essential. Rope access techniques allow rope access technicians to maintain close, controlled positioning across the entire surface, enabling consistent preparation, accurate application and thorough inspection — even in difficult-to-reach areas.

The distinction between professional rope access and informal abseiling methods is important. As discussed in our earlier article, Abseiling and Rappelling, safe and effective access relies on structured systems, training and planning rather than improvised techniques.

When access is integrated into the coating strategy from the outset, the risk of coating defects is significantly reduced.

The role of inspection in identifying coating defects

Inspection plays a vital role in detecting defects before they develop into larger problems. Effective inspection should take place:

• before coating application, to verify surface preparation
• during application, to monitor film thickness and conditions
• after curing, to confirm coating integrity

Early identification of coating defects allows corrective action to be taken while repairs are still straightforward and cost-effective.

Preventing coating defects through planning and standards

Correct specification

Selecting a coating system that matches the service environment is essential. Factors such as exposure, temperature, chemical resistance and maintenance intervals must be considered during specification.

Organisations such as AMPP (formerly NACE) provide guidance on coating selection and performance expectations for industrial environments.

Proper access planning

Access should be treated as an integral part of the coating process, not an afterthought. Early access planning ensures all areas can be prepared, coated and inspected to the same standard.

Competence and training

Reducing coating defects depends on trained personnel who understand not only how to apply coatings, but why defects occur and how they can be prevented. Competence in industrial coating work is not achieved in a single step, particularly in complex or high-risk environments.

Progressive training pathways that build from fundamental preparation and application principles through to defect identification and inspection awareness play a key role in improving long-term coating performance. Tiered frameworks such as Train the Painter Bronze, Train the Painter Silver and Train the Painter Gold reflect this progression, supporting consistent quality and understanding across different experience levels and project demands.

Inspection-led approach

Inspection should be embedded throughout the process rather than limited to final sign-off. This structured approach significantly reduces the likelihood of defects being built into the coating system.

Long-term implications of coating defects

When coating defects are left unaddressed, their impact extends far beyond cosmetic appearance. Minor imperfections introduced during preparation or application can allow moisture and contaminants to reach the substrate, accelerating corrosion beneath what may initially appear to be an intact coating system.

Over time, this hidden degradation can lead to premature coating failure, reduced asset lifespan and significantly higher maintenance costs. In safety-critical environments, coating defects can also contribute to structural deterioration, increasing risk to personnel and operations. Remedial works carried out after defects have progressed are typically more disruptive, more expensive and more complex than preventative measures taken at the outset.

For asset owners and duty holders, managing coating defects is therefore not simply a matter of visual quality, but one of long-term performance, safety and lifecycle value. By prioritising correct surface preparation, competent application, appropriate access planning and inspection at every stage, defects can be minimised and the protective function of industrial coatings preserved for their intended service life.