Coatings defects: how to avoid and solve them

At Verona Impianti 2002, experts in the industrial painting sector, it is essential to ensure a high-quality final result that preserves the integrity and aesthetic features of the painted product over time.

As a company specialized in the design and construction of industrial painting systems, our primary goal is to provide our customers advanced solutions that ensure excellent results in the coating of manufactured products. However, during the painting process, defects may occur that compromise the final quality. For us at Verona Impianti 2002, ensuring the excellence of the coated product is a top priority. It is important to highlight that the coating process achieves the best results when there is a close collaboration among the various professionals involved, from the system supplier to the operator who manages it, each playing a key role in ensuring a perfect outcome.

In this article, we aim to analyze the main imperfections that can arise during the finishing phase, explore the causes behind them, and offer practical solutions to prevent them.

Main causes of painting defects

Painting defects can arise from two main causes: human error and environmental conditions, or a combination of both.
Defects related to external environment include blistering, chalking, corrosion, discolouration, and whitening, while those caused by human error include cracking, pinholes, uneven colour or gloss, orange peel, cissing, dripping, cratering, and staining. The causes of these defects are varied and include inadequate surface preparation, incorrect timing, wrong application methods (such as the wrong distance between the spray system/gun and the piece), insufficient drying times, temperature variations between the environment, paint, and surface, and paint thickness that is either too thin or too thick.

Blistering

In coatings field, the term "blistering" refers to the appearance of bubbles of varying sizes and intensities caused by moisture presence. This condition leads to the loss of adhesion of the coating, often resulting in the detachment of the paint film from the underlying surface. Possible causes include moisture infiltration between the paint film and the substrate, or between various paint layers. This can be caused by inadequate surface preparation, such as insufficient rinsing after pre-treatment, poor adhesion between paint layers, or the use of a painting system unsuitable for high-humidity environments.

Orange Peel

This defect manifests as an uneven, pockmarked surface resembling the skin of an orange. The main causes include insufficient dilution or the use of an unsuitable thinner, which can result in too dry an application.

Additionally, using incompatible products or improper mixing of the painting products, incorrect distance between the spray gun and the piece, or incorrect gun settings can contribute to the problem. Other causes may include excessively hot environments, using a blower to speed up drying, incomplete drying, or incorrect film thickness.
To avoid this defect, it is crucial to pay attention to the mixing of painting product components and dilution, respect drying times, use the correct thinner according to climatic conditions, and properly adjust the gun. In general, it is essential to control all application conditions, viscosity, equipment, and the work environment.

Burn Marks

This occurs when burns or bubbles form during evaporation. The main causes include using too fast a thinner, overly dry spraying with high pressure, excessive film thickness, not respecting drying times between layers, too short a distance from IR lamps, or excessively high temperatures.
To prevent this defect, it is essential to choose the right thinner, avoid accelerating drying with compressed air, maintain proper ventilation in the oven, correctly control the temperature and distance of IR lamps, and be mindful of temperatures when using transparent, metallic, or pearlescent coatings.

Runs

Runs occur when coatings accumulate in a specific area, creating an uneven surface. Common causes include using unsuitable thinners or catalysts, inadequate surface cleaning, incorrect viscosity, excessively low temperatures in the environment or on the substrate. It also happens if the application is too heavy or the gun sprays drops, if the atomization is insufficient or the fan is too narrow. Poor lighting inside the booth can also lead to inaccurate management of the coating application phase.
To prevent this defect, it is important to check the characteristics of the thinner and spray gun, control ambient temperature/piece/coatings, adhere to the recommended film thickness, and avoid overloading edges and corners.

Cracking

Cracks similar to crow's feet or crazing, which resemble the bed of a dry river, are caused by errors in the mixing process, incorrect use of additives or hardeners, incompatibility between products, wet-on-wet application without the right flash-off times or excessively high thicknesses.

To avoid cracking, it is sufficient to carefully prepare the product and scrupulously follow the instructions on the technical data sheets for mixing and flash-off times, avoiding the use of unforeseen additives.

Slow Drying

Sometimes, coating takes too long to harden or feel dry to the touch. This can be caused by using incorrect activators, hardeners, or additives, too slow thinners, excessive application thickness, unsuitable environments, poor ventilation, or high humidity.
This problem can be solved by carefully checking the quality and quantity of thinners and additives, applying correct thicknesses, and ensuring proper ventilation. Using ovens or heated booths promotes optimal drying.

Pinholes

Pinholes are tiny holes in the coated surface, often found in powder coating due to moisture in the powder, porous substrates, or castings releasing air at high temperatures.

To prevent pinholes, it is important to manage paint in optimal temperature conditions, check the substrate characteristics, and ensure proper drying times are followed.

Scratches or Lack of Flow


Scratches under the paint layer, especially on metals, can result from overly aggressive sanding on the previous layer, using overly dry paints, incompatible thinners, incorrect spray gun settings, or too thin a paint layer.
To avoid this defect, properly adjust spray parameters and adapt the thinner to environmental conditions and the product.

Granularity and Roughness

A granular and rough finish is caused by excessive paint viscosity, the use of unsuitable thinners, incorrect coating techniques, dirty equipment, or excessive air pressure.
To prevent granularity, it is essential to use the correct thinner percentage, apply appropriate techniques, keep equipment clean, and properly adjust air pressure.

Yellowing

Yellowing, often seen in powder coatings, can result from paint with poor resistance to this phenomenon or from excessively long polymerization times.
The best approach to prevent yellowing is to use coats with high resistance to this effect, carefully monitor polymerization times and temperatures, and ensure good ventilation.

Stains or Scratches

Colour alterations such as shading, bleeding or bleaching are typical defects in metallic and pearlescent coatings. These defects occur because the paint components are often poorly mixed, base drying times are insufficient, or film thicknesses are too high.
To prevent these defects, carefully mix pigments and follow spraying instructions.

Contour mapping

It is a painting defect that affects the structure or gloss of the surface, caused by improper application and drying of the primer.
Solutions include applying the primer according to recommended thicknesses, respecting drying and curing times, and using oven curing when necessary.

Poor Coverage


Insufficient coverage can reveal the undercoat beneath the enamel, usually due to excessive dilution or too low a thickness.
To prevent this defect, properly mix the products, follow dilution instructions, and apply the correct thickness.

Cissing (Crater Formation)


Crater-like defects, or "fish eyes", form on the surface due to inadequate surface cleaning or contamination by oil or dust in the work environment.

To prevent this, it is important to thoroughly remove dust and avoid using silicone-based products near the painting area. In some cases, applying a second coat with an anti-silicone additive may help.

Chalking

Chalking is a whitish film that forms on the coating, usually caused by heat breaking the chemical bonds of the paint, especially under UV radiation.
To prevent this, avoid direct sun exposure and use recommended substances in the products.

Peeling

Peeling occurs when the paint lifts from the substrate like sunburned skin, forming flakes that detach easily. The main causes are inadequate surface cleaning or using unsuitable products.To prevent peeling, thorough pre-treatment, proper product mixing, and correct spray distance and paint quantity are essential.

Dirt, Granules, and Specks

These defects occur when contaminating particles like dust or fibers end up in the coating, creating an uneven surface. To avoid this, maintain a clean work area, avoid dust on clothes and rags, and ensure the spray booth is well-filtered.

Whitening

A whitish halo may form in the fresh paint film due to humidity, often resulting from unfavourable weather conditions or accelerated drying. To avoid whitening, it is recommended to apply paint under favourable climatic conditions, use appropriate thinners, and avoid forcing drying.

Conclusions

The industrial coating process is essential for ensuring high-quality finishes, but various defects can compromise the final result. These can result from environmental factors, application errors, or improper material choices. However, many of these problems can be avoided with proper surface preparation, the use of suitable products, and careful equipment management.
At Verona Impianti 2002, quality is a top priority. For this reason, we work closely with our customers, providing targeted support to ensure their coating cycles are always defect-free. We offer advanced coating systems and customized solutions. The collaboration between all the professionals involved is crucial to achieving optimal results, and we are ready to support every stage of the process, ensuring an excellent and long-lasting final result.

Does your coating cycle show some of the defects we've mentioned above? Do you need to install a system? Or do you need more information about the painting process? For further information, advice, assistance, or to request a quote, contact us. Our team will get back to you shortly.