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Home / News / Industry News / Spray Coverage Was Uniform — Why Did Cratering Appear on the Interior Surface After Curing?
Industry News
1
Uniform Spray Does Not Mean Defect-Free After Film Formation
When spraying has just been completed: film coverage is complete, surface state is uniform, wet film flow is normal. These characteristics only indicate that there were no obvious problems at the spray stage. As solvent evaporates and the film forms, hidden problems may gradually emerge. Therefore, uniform spraying does not mean the final film will definitely be cratering-free.
2
Substrate Contamination Easily Causes Localised Shrinkage
Automotive interior plastic parts during production and processing: may have residual mould release agent, surface oil contamination or dust, and fingerprint or other contamination effects. These low-surface-energy substances prevent the coating from wetting normally, causing the film to shrink outward in localised areas and form craters.
3
Surface Tension Differences Affect Spreading
During film formation: the coating needs to fully wet the substrate, components continuously migrate and adjust, and surface tension continuously changes. If local surface tension is uneven, the coating is prone to shrinkage, forming "fish-eye" type defects.
4
Additive Matching Affects Film Stability
The coating system typically needs: good wetting capability, appropriate leveling effect, and stable surface state. If additive matching is insufficient or system compatibility is poor, it may cause film surface tension imbalance, making cratering more likely to appear.
5
Spray Environment Can Also Increase Cratering
In actual production: oil mist contamination in the air, oil and water in compressed air, and insufficient spray booth cleanliness can all bring contaminants onto the film surface and affect normal coating spreading.
6
Multi-Layer Coating Systems More Prone to Problems
Automotive interiors typically involve primer, colour coat, and topcoat. If surface energy differences between different coatings are large, interlayer matching is insufficient, or drying time is unreasonable, subsequent coatings may also develop cratering.
7
Why Defects Are Not Visible Immediately After Spraying
Because when spraying has just been completed: the film is still in a mobile state, temporarily covering the surface, and defects have not yet fully formed. As solvent gradually evaporates, film viscosity continuously increases, and surface tension differences widen, localised shrinkage gradually becomes apparent.
Summary
Uniform spraying with cratering appearing on the interior surface is fundamentally the combined result of insufficient wetting, surface tension imbalance, substrate contamination, and system matching problems. When analysing interior cratering problems, in addition to focusing on spray technique, it is necessary to comprehensively evaluate substrate treatment, coating wettability, additive matching, and production environment.
Uniform spraying does not mean stable film formation
Substrate contamination disrupts coating spreading
Surface tension differences easily cause shrinkage
Additive matching affects film state
Spray environment increases defect risk
Multi-layer systems require good interlayer compatibility
Frequently Asked Questions
If the same coating craters on plastic but not on metal, what does that indicate?
This strongly points to a substrate surface energy issue rather than a coating system problem. Plastic substrates — particularly polypropylene, ABS, and others with low or variable surface energy — present a fundamentally different wetting challenge than metal. The fix starts with substrate pre-treatment or a leveling agent with sufficient surface tension reduction for the specific plastic.
Can increasing leveling agent dosage alone solve cratering from substrate contamination?
Leveling agents increase the coating's tolerance to surface energy variation — but where contamination is severe enough, no leveling agent can fully compensate. The most effective approach combines thorough substrate cleaning with a leveling agent that provides adequate surface tension reduction for the substrate type.
