Detailed Explanation of the Causes and Prevention Methods of Orange Peel Defects in Coatings

Detailed Explanation of the Causes and Prevention Methods of Orange Peel Defects in Coatings

Table of content.

1. Definition and Impact of Orange Peel Defects

Orange peel is a common quality defect that occurs during the coating process. It appears as an uneven, wrinkled surface texture on the coating film that resembles the skin of oranges or grapefruits. The surface undulation of these wrinkles is approximately 3 micrometers. According to the wavelength of the ripple structure, orange peel defects can be classified into two categories:

  • Long wave (LW): >0.6 mm

  • Short wave (SW): <0.6 mm

Orange peel defects not only affect the appearance quality of the coating but also reduce the distinctness of image (DOI), fullness, and smoothness of the paint film, directly impacting the decorative appearance and market competitiveness of the product.

On high-gloss surfaces, orange peel defects produce a ripple structure with alternating bright and dark areas. Because each point on the ripple surface has a different inclination angle, incident light is reflected in different directions. This results in blurred reflected images and distorted edges, severely affecting the visual appearance of the coating.

 

 

2. Analysis of the Causes of Orange Peel Defects

▲ Coating Material Factors

The characteristics of the coating itself are the primary factor causing orange peel defects. Excessively high coating viscosity leads to poor leveling performance, preventing the coating from flowing sufficiently after spraying to form a smooth surface.

In addition, unreasonable coating formulation design—such as improper solvent selection or insufficient leveling agents—can significantly affect the coating’s leveling performance.

When low-boiling-point thinners are used, the solvent may evaporate rapidly before the paint mist reaches the substrate surface. This reduces coating fluidity and prevents proper leveling.

Furthermore, if the coating mixture is left standing for too long after preparation—especially after adding a curing agent—chemical reactions may occur over time, gradually changing the coating’s properties and affecting its leveling and drying performance.

 

 

▲ Improper Spraying Process Parameters

Improper spraying process parameters are another major cause of orange peel defects.

Insufficient spraying pressure or an oversized spray gun nozzle can lead to poor atomization, preventing the coating from being evenly distributed on the substrate surface.

Improper spraying distance (either too close or too far) also affects the uniformity of the coating film:

  • Too close: causes local accumulation of coating.

  • Too far: allows excessive solvent evaporation before the paint mist reaches the substrate.

Uneven spray gun movement speed can lead to uneven coating thickness, thereby affecting leveling performance.

In addition:

  • Excessively thick single spraying: causes rapid solvent evaporation on the surface while solvents remain trapped in the lower layer, leading to uneven shrinkage.

  • Insufficient coating thickness: prevents the coating from fully leveling.

 

 

▲ Environmental Conditions

Environmental conditions have a significant impact on coating leveling.

Excessively high temperature (either substrate temperature or ambient temperature) accelerates solvent evaporation, shortening the leveling time of the coating and causing premature film formation.

Studies show that a temperature difference greater than 2°C can significantly affect orange peel measurement data on the paint film surface.

Excessive airflow in the spray booth can also accelerate solvent evaporation and disrupt the leveling process.

High humidity can interfere with normal coating leveling. Typically, the relative humidity should be controlled below 60%–70%.

 

 

▲ Substrate and Equipment Factors

The surface roughness of the substrate directly affects the final appearance of the coating film.

The greater the substrate roughness, the more coating is required to achieve full coverage. Additionally, the coating thickness on the peaks of the surface becomes thinner, which negatively impacts coating quality.

Data shows that when the substrate roughness Ra increases from 0.94 μm to 1.31 μm, the short-wave (SW) value of the topcoat increases from 14.6 to 23.4, resulting in a significant decline in appearance quality.

Poor spraying equipment performance is also an important factor. Poor atomization, worn spray nozzles, or blocked air caps can cause uneven paint atomization.

Furthermore, contaminated coating equipment containing oil, water, or other impurities can also interfere with normal coating leveling.

 

 

3. Prevention and Control Methods for Orange Peel Defects

1. Optimization of Coating Formulation

Proper coating formulation is the foundation for preventing orange peel defects.

Coating viscosity should be adjusted strictly according to process requirements, following the principle:

  • Higher viscosity at lower temperatures

  • Lower viscosity at higher temperatures

Selecting appropriate solvents is crucial. In particular, adding high-boiling-point organic solvents or leveling agents with slower evaporation rates can effectively improve coating leveling.

Choose thinners according to the construction environment temperature:

  • Use slow-drying thinners in high-temperature environments.

  • Use fast-drying thinners in low-temperature environments.

After adding curing agents, the coating should be used within the specified time to prevent property changes.

 

 

2. Optimization of Spraying Process Parameters

Optimizing spraying process parameters is key to preventing orange peel.

Adjust spray gun parameters:

  • Compressed air pressure: 0.3–0.5 MPa

  • Spray nozzle size: appropriate selection

  • Spraying distance: 15–25 cm

These settings help achieve optimal atomization.

Adopt multi-layer spraying techniques, following the principle of “thin coats with multiple passes.”

Each layer should reach the required thickness (without causing sagging), and sufficient flash-off time should be allowed between layers.

Control coating thickness appropriately. Increasing the thickness of the clear coat layer can improve flow and leveling performance, but the risk of sagging must also be considered.

 

 

3. Environmental and Substrate Control

Controlling the working environment is an important measure to prevent orange peel.

Recommended conditions:

  • Spray booth temperature: around 20°C

  • Relative humidity: below 60%–70%

  • Airflow speed: within specified process limits

The temperature of the coated object should be cooled to below 50°C before spraying to avoid high-temperature operations.

During hot summer conditions, construction should preferably be carried out during cooler periods such as early morning or evening, or the ambient temperature can be controlled using air conditioning.

Substrate preparation must also be strengthened:

  • Ensure the surface is clean, smooth, and free of oil, dust, or contaminants

  • Control substrate roughness (Ra ≤ 1.0 μm recommended)

  • Ensure the electrophoretic coating layer is smooth

 

 

4. Equipment Maintenance and Process Monitoring

Regular maintenance of spraying equipment is essential:

  • Clean spray nozzles and air caps

  • Replace worn components

  • Ensure proper atomization performance

Establish a comprehensive process monitoring system.

During coating inspection, visual inspection methods or orange peel measuring instruments can be used to detect defects.

An orange peel meter can objectively measure long-wave and short-wave values of the coating film, providing quantitative data for quality control.

Drying processes should also be optimized.

For water-based topcoats, the temperature should be gradually increased during baking, with a recommended heating time of approximately 8 minutes, which helps improve coating leveling.

 

 

Orange peel is a common and relatively difficult leveling defect in coating processes. Its occurrence is influenced by multiple factors including coating properties, spraying techniques, environmental conditions, substrate conditions, and equipment performance.

Preventing and controlling orange peel defects requires systematic measures, including:

  • optimizing coating formulations and mixing processes

  • properly setting spraying parameters

  • strictly controlling the working environment

  • improving substrate preparation

  • strengthening equipment maintenance

By comprehensively applying these measures, orange peel defects can be effectively prevented and resolved, thereby improving coating quality and production efficiency.

As the appearance quality requirements of automobiles and other industrial products continue to increase, deeper research and effective control of orange peel defects will become increasingly important.

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