Pitting: causes of contamination in industrial painting

In addition to affecting the aesthetic appearance of the artifact, this defect can weaken the continuity of the coating and, over time, affectadhesion and protective performance.

In the context of industrial painting, speckling should not be interpreted as simply a local anomaly of the application step. Its occurrence is almost always the result of contamination introduced along one or more process steps, from part preparation through deposition to drying or curing. For this reason, defect management requires integrated control of substrate, environment, equipment and plant parameters. Coating booths, air filtration, and temperature and humidity control play a decisive role in the final quality of the coating.

Technical focus on stippling

From a physical point of view, stippling is often attributable to non-wetting of the substrate and discontinuities in the surface tension of the film. When low surface energy contaminants (oils, silicones, organic residues) are present on the surface, the coating fails to spread properly, generating visible micro-craters or inclusions. This phenomenon is particularly critical in high-throughput industrial cycles where evaporation and leveling times are short.

When the coating encounters dust, oils, moisture or residues that are not removed, the result is a surface dotted with spots or small craters that interrupt the continuity of the film. Water- or solvent-based coating booths are designed precisely to limit the entry of impurities and to promote more stable application conditions.

Advanced filtration systems, suction walls, and dust recovery devices are intended to keep the application environment clean and preserve the quality of the recycled material. Below are the main sources of risk.

• Presence of environmental dust, fibers or solid debris in the cabin.
• Contamination of recovered powder during recycling and reuse stages.
• Residual moisture that alters smoothness and promotes agglomeration.
• Foreign particles in the vacuum system or filters not maintained properly.
• Uneven deposition due to suboptimal calibration of the electrostatic gun.
• Uneven firing, which can fix defects already present in the initial deposit.

In powder lines, stippling is often related to cross-contamination between different batches or colors and suboptimal recycling management. The presence of heterogeneous particles or agglomerates alters the particle size distribution of the powder, making the deposit uneven. In addition, high relative humidity values (typically >80%) promote lump formation, compromising fluidization and increasing the risk of inclusions in the film.

The stability of the electrostatic field is also crucial: improper grounding of guns, pipes or parts can generate parasitic charges that disrupt uniform powder deposition, increasing the likelihood of point defects.

Vertical flow spray booths offer more homogeneous and cleaner air conditions than other solutions, while in liquid booths temperature and humidity control is a key determinant of application quality. In powder coating, filtration and material recovery are also central to limiting contamination. Below are the most critical variables.

• Efficiency of inlet and recirculating air filtration.
• Thermohygrometric stability of the coating environment.
• Cleaning of surfaces, supports, and handling tools.
• Quality of compressed air, which should not introduce condensation or oily residue.
• Correct balance of air flows in the cabin.
• Maintenance of filters, suction walls, and recovery systems.

In more advanced systems, air filtration is done according to ISO 16890 classifications, with multiple stages up to high-efficiency filters (HEPA) to retain ultrafine particles. Proper sizing of the filters and their maintenance are essential to prevent the deposition of contaminants on the still-wet film.

Thermohygrometric control directly affects film rheology: high temperatures accelerate evaporation by reducing leveling time, while high humidity can cause surface condensation and adhesion defects. The use of air handling units (AHUs) helps maintain consistent and repeatable conditions.

The goal is not only to remove visible impurities, but to make the entire coating cycle stable, so as to reduce the variables that can generate surface defects. Eurotherm powder and liquid booths are set up precisely to limit contamination and ensure more consistent working conditions.

Substrate stabilization and pretreatment

In industrial cycles, substrate preparation should follow established standards (e.g., ISO 12944 for corrosion protection), including degreasing, washing and chemical conversion steps. Verification of surface wettability is a key indicator of pretreatment quality.

The first defense against speckling is a perfectly clean and dry surface. In fact, any residue on the substrate tends to show up in the final film and is therefore vital:

• Ensure effective washing and rinsing before painting.
• Ensurecomplete drying of the piece.
• Avoid handling after pretreatment without adequate safeguards.
• Check for dust, oils and condensates before entering the cabin.
• Limit the entry of impurities and maintain constant air quality.
• Use booths with filtration appropriate for the type of process.
• Keep air flows and interior cabin surfaces clean.
• Prefer, when required by the cycle, configurations that provide greater flow homogeneity.
• Continuously monitor temperature and humidity.

Spray booths must operate under slightly positive pressure relative to their surroundings. This prevents the entry of external dust and ensures a controlled, laminar airflow, drastically reducing the risk of airborne contamination.