In coating operations, surface defects are a critical factor affecting both a product's visual quality and its service life. Issues ranging from wear and tear of spray booth equipment to lax process control can lead to various coating problems. This article systematically outlines the causes of common coating defects and their corresponding solutions, serving as a reference for industry professionals.

Analysis of Major Coating Defect Types and Their Causes

I. Orange Peel
Description: The paint finish appears rough, with a texture resembling orange peel. Specifically, the coating surface is uneven, characterized by dense, minute undulations and irregularities.
Causes:
1. Excessive Coating Viscosity
Poor atomization produces coarse paint droplets. Upon reaching the substrate, these droplets fail to level out sufficiently, preventing the formation of a smooth film before curing and leaving an orange-peel texture.
2. Improper Spray Gun Settings
Inappropriate parameters—such as fluid output, atomization pressure, spray pattern width, and gun angle—lead to insufficient droplet atomization and uneven distribution, causing localized poor leveling and orange peel.
3. Improper Spraying Distance
If the distance is too close, the wet film becomes excessively thick, leaving insufficient time for leveling. If the distance is too far, the paint droplets partially dry before reaching the substrate, preventing them from coalescing and leveling smoothly. Both scenarios result in orange peel.

4.Solvent Evaporating Too Quickly
The coating surface dries and skins over rapidly while the underlying layer still retains solvent; the surface loses fluidity and cannot level out irregularities, ultimately resulting in an "orange peel" texture.

Solution: To address orange peel defects, adjust the coating viscosity to meet process specifications and set the correct spray pressure to ensure uniform atomization. Maintain an appropriate spray distance to prevent the wet film from becoming too thick or the spray mist from drying prematurely. Use the recommended thinner and properly control its evaporation rate. Additionally, regulate the temperature and humidity of the spray booth to ensure sufficient leveling time and a stable curing environment for the coating. Implementing these measures comprehensively can effectively improve coating leveling, reduce orange peel defects, and enhance the visual quality of the paint finish.

II. Sagging and Run Defects

Description: Excess coating flows downward, creating uneven streaks or runs. Localized sagging or accumulation occurs; in severe cases, this manifests as teardrops or curtain-like drips, compromising the smoothness and visual quality of the paint finish.

Root Causes:

1.Excessive Coating Thickness
Applying too thick a layer in a single pass or excessive recoating causes the wet film to exceed its sag resistance limit; gravity pulls the coating downward, forming runs.

2.Low Coating Viscosity
When coating viscosity is too low, fluidity is excessive, preventing the wet film from maintaining a stable thickness and making it prone to sagging on vertical or inclined surfaces.

3.Improper Spraying Angle
An incorrect angle between the spray gun and the workpiece, or spraying too close to a vertical surface, causes localized coating accumulation that exceeds the sag resistance limit.

4.Excessive Spray Gun Movement Speed
If the spray gun moves too quickly, operators often compensate by increasing paint output or applying multiple coats, resulting in localized wet film thickness that is excessive, thereby triggering sagging.

Solution: Control the coating thickness to ensure it is appropriate and avoid applying layers that are too thick in a single pass; maintain the correct paint viscosity and use the compatible thinner as specified by the process requirements; improve drying conditions to accelerate solvent evaporation and coating curing; employ correct spraying techniques, adhering to standard specifications for spray gun angle, distance, and speed; and reduce the paint flow rate to lower the volume of paint dispensed per unit of time. Implementing these measures effectively prevents sagging defects and ensures a smooth, uniform paint finish.

III. Blistering Defects

Description: Air or moisture bubbles appear beneath the paint layer. Localized raised areas form on the coating surface, appearing as round or irregular blisters that feel elastic or hollow when pressed; in severe cases, the blisters rupture, compromising both the appearance and the protective properties of the finish.

Root Causes:

1.Residual Moisture Within the Paint Film
Residual moisture on the substrate surface or between coating layers vaporizes and expands when heated during baking or curing, pushing up the paint film to form blisters.

2.Improper Surface Treatment
Contaminants such as oil, rust, dust, or old paint residue on the substrate surface impair coating adhesion. This creates gaps between the paint film and the substrate where air or moisture can accumulate, leading to blistering.

3.Excessive Baking Temperature
If the temperature rises too rapidly or becomes too high, the coating surface dries and forms a skin quickly. Solvents or moisture in the underlying layers cannot escape in time; as they heat up and expand, they push up the paint film, creating blisters.

Solution: Ensure the surface is dry before coating to prevent residual moisture on the substrate or in underlying layers; enhance surface cleaning to thoroughly remove oil, rust, dust, and old paint residue; control baking temperatures to avoid rapid heating or excessive heat; and allow for an appropriate leveling period to ensure solvents evaporate fully before baking begins. These measures effectively reduce blistering defects, ensuring strong coating adhesion and a smooth surface finish.

What is the best way to prevent coating defects from recurring?

This involves establishing and implementing a systematic standard system that covers the entire process, with strict controls from surface preparation to the application environment: ensuring workpiece surfaces are clean, dry, and free of oil or rust; selecting compatible coatings while precisely controlling viscosity and dilution ratios; adjusting spray gun parameters, distance, and speed according to standards; maintaining optimal temperature and humidity levels alongside a clean, dust-free environment in the spray booth; and strengthening professional process monitoring. These measures fundamentally reduce rework rates and operating costs, ensuring the long-term smooth operation of the workshop.

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