Industrial painting quality control: steps, objectives and tools

In powder coating, quality control focuses on substrate conductivity, film uniformity and pretreatment quality; in liquid coating, it focuses on wettability, surface tension and thermo-hygrometric stability.

Quality control in industrial painting is a key operation for:

• Ensure quality and regulatory compliance of products;
• Reduce variability between batches;
• Ensure the repeatability of the painting processes, i.e., the consistency of the result;
• Preventing paint defects;

Structured as a progressive verification system that runs through the entire processing cycle, quality control in industrial painting begins before the coating is applied and ends after it is cured or dried. It consists of visual inspections, instrumental measurements and spot tests aimed at assessing paint adhesion to the substrate, coating thickness, color uniformity, surface strength and durability. The entire process has three key moments:

• Pre-painting operations: preliminary inspection of surfaces and verification of pretreatments to ensure optimal film adhesion.
• In-process monitoring: control of paint application parameters and baking or drying parameters to maintain consistent quality throughout production.
• Final verification: visual inspection and specific tests performed according to ISO and ASTM standards to certify the regulatory compliance of the products.

Validation of the painting process should therefore not be understood as the final stage of production, but rather as an integrated system of theories and practices that accompany each step of the painting process, and whose effectiveness determines the stability and reliability of the entire process.

In this context, Eurotherm coating quality control is an integral part of the design, installation and final validation phases of plants in both powder and liquid coating processes. Our technicians adopt verification protocols developed in-house over more than 50 years of experience in the industry, performing functional tests, thermo-hygrometric checks and metrological verifications that ensure compliance with international standards. Each system is then validated through adhesion, thickness, uniformity and strength tests, ensuring consistent and traceable performance over time.

This step is not limited to checking the state of cleanliness of the substrate but also includes checking the activation of the surface, the process that promotes the formation of chemical-physical bonds between the substrate and the coating, i.e., the optimal condition for the paint to adhere to the part.

At this stage, paint quality control focuses on substrate preparation, because any criticality in pretreatment directly reflects on the performance of the final cycle.

Powder coating quality control: pretreatment and surface activation

In the case of powder coating, the objective of surface activation treatments is to promote the electrostatic distribution of powder across a conductive surface. Phosphating, passivation and sandblasting, are the main reference techniques.

With this in mind, powder coating quality control verifies that the conversion layer and surface conductivity are consistent with the defined process parameters.

Quality control in liquid coating: wettability and film defects

In liquid coating, on the other hand, activation is concerned with wettability and surface tension, which determine that the coating is evenly spread and defect-free. Brushing, plasma, ionization are the typical methods.

Quality control in liquid coating thus focuses on proper wettability of the substrate and the prevention of defects such as craters, runs or lack of coverage.

Application case: quality control of pretreatments in a Eurotherm plant

The recent case of the installation carried out by Eurotherm for an Italian company active in the production of products and components for the Energy and Oil&Gas sectors demonstrates the crucial importance of pretreatments in ensuring long-lasting functional coatings on performance surfaces and special materials.

In monitoring the steps prior to the actual application the paint, the parameters to be taken into account for quality control of pretreatments are:

• Cleaning and removal of contaminants: verification of the effectiveness of mechanical and chemical treatments that remove oils, processing residues, and release agents. The check covers both physical processes (brushing, sandblasting, micro-abrasion, ionization, blowing) and chemical baths and rinsing treatments in washing booths and tunnels.
• Surface tension: checking the ability of the surface to uniformly accommodate the paint. Surface tension is monitored to verify successful activation of metal surfaces or proper substrate preparation (plastics and composites), ensuring proper wettability for liquid coatings and good electrical reactivity in powder processes.
• Uniformity and quality of the conversion layer: evaluation of the continuity and homogeneity of the conversion film, which is essential to ensure paint adhesion and surface performance. The check focuses on the visual appearance of the substrate and compliance with the chemical and physical parameters of the process.

Popular verification tools and control methods include:

• pH-meters, conductimeters and chemical analyzers for bath stability.
• Surface tension meters for wettability of substrates.
• Optical microscopy for direct observation of the conversion film.
• Integrated PLC systems to record critical parameters and deviations from nominal values.

Depending on the material, type of liquid or powder line, and pretreatment technology, the main corrective actions possible at this stage include:

• Restoration of chemical conditions of wash and conversion baths: when concentration, pH or conductivity go out of the operating range.
• Adjustment of thermal parameters and contact times: for treatment effectiveness and to stabilize the quality of treated surfaces.
• Maintenance of filtration and separation systems: to prevent contamination and associated paint defects.
• Correction in mechanical treatment: such as replacing worn brushes, checking the abrasiveness of grit, and cleaning the filtration and dust recovery and overspray systems in the booth.

All data collected from sensors, measuring instruments and PLCs are recorded and tracked to enable statistical analysis, identify deviations and optimize process parameters in real time. This operational transparency strengthens paint quality control and production continuity.

Many of these practices are common to powder or liquid processes while changing methods and purposes, as adhesion, thickness, and corrosion resistance are closely related to the chemistry of the coatings and the method of application.

Fundamental tests include:

Other tests, mainly required in liquid lines, concern gloss level and color analysis. In industrial liquid coating, particular attention is also paid to environmental safety: the VOC Directive 1999/13/EC, implemented in Italy by Legislative Decree 152/2006, regulates the emissions of volatile organic compounds-almost absent in powder processes, where the environmental issue concerns instead the systems for recovering excess dust in the spray booth.

In general, with regard to safety and the environment, both powder and liquid coating plants and processes are subject to international standards (ISO 9001, ISO 14001, ISO 45001), as well as specific regulations on solvents and pigments (REACH). Many companies operating in the industry are also adopting environmental and process certification approaches that attest to their commitment to a lower-impact industry, such as ISO 50001 for energy management. From this perspective, quality control becomes an integral part of a broader business vision, where technical expertise and corporate values contribute to improved production.

The standards mentioned above are now the gold standard adopted by major European players in the Automotive, Rail, Energy and Aerospace sectors.

In Eurotherm’s design, maximum attention is paid to energy-efficient and environmentally friendly technologies for painting processes, with the implementation of advanced filtration systems and the recovery and reuse of heat produced during drying and curing-as in the case of the high-temperature oven custom-developed to improve the overall energy output of the production line or in the zero-emission painting plant installed in the UK for a well-known furniture company.