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Home / News / Industry News / Coating Keeps Developing Cratering — How to Adjust the Substrate Wetting Agent?
Industry News
Coatings · Substrate Wetting · Cratering · Surface Tension
When coating develops cratering after spraying, the instinct is often to adjust the leveling agent. But leveling is only one stage in the film formation sequence. If the coating cannot spread uniformly across the substrate surface after application, localised retraction forms before any leveling process can correct it. Substrate wetting is therefore a distinct concern from film leveling — and when cratering appears, it deserves its own diagnostic attention.
Is Cratering Actually a Leveling Problem?
After spraying, coating goes through wetting, spreading, flowing, and film formation. If the coating cannot spread uniformly across the substrate immediately after contact, local zones may retract. Even if subsequent leveling state is good, it cannot correct an already-retracted zone. Therefore, substrate wetting is as important as leveling when diagnosing cratering.
How Does Insufficient Substrate Wetting Form a Crater?
After coating reaches the substrate surface, it needs to establish good contact first. If the interfacial relationship between coating and substrate is not appropriate, the paint liquid may have difficulty spreading. Partial zones retract toward the surroundings, ultimately forming visible depressions. This is more common on substrates with lower surface energy, more complex surface state, or special surface treatments — where the same coating on different substrates may produce completely different results.
What Does a Substrate Wetting Agent Actually Do?
A substrate wetting agent primarily acts at the interface between coating and substrate. An appropriate wetting agent can change the spreading state of the coating on the substrate, making the paint liquid more easily form continuous coverage. But wetting agent is not simply a "surface tension reducer." Pursuing ever-lower surface tension while neglecting matching with other additives and the resin system may introduce new defects. What truly matters is whether the wetting agent maintains a stable spreading state in the complete formulation.
Why Does Changing the Substrate Change the Cratering Pattern?
Different substrates differ markedly in surface properties. Metal, glass, plastic, and wood all have different surface energy, polarity, and surface treatment state. The same coating system naturally may show different wetting behaviour facing different substrates. Plastic substrates with lower inherent surface energy are more prone to insufficient coating spreading — where adjusting only the leveling agent without considering the coating-substrate interface relationship typically produces limited results.
Can Cratering Be Fixed by Simply Increasing the Wetting Agent?
This is not recommended as a first response. When wetting agent addition is insufficient, increasing it may improve spreading. But beyond the appropriate range, the system's surface state may shift further. Additionally, other additives with interfacial activity — leveling agent, defoamer — are also present and interact with each other. Gradient testing is more appropriate: at consistent resin, pigment and filler, solvent, and application conditions, gradually change the wetting agent amount and observe cratering count, spreading state, and overall film appearance.
Why Does Cratering Persist After Adding Wetting Agent?
If cratering continues after the wetting agent is added, consider that the cratering may not be purely from substrate wetting. If oil contamination, dust, or release agent is present on the substrate surface, local surface state abnormalities can cause cratering regardless of wetting agent. Defoamer incompatibility, leveling system changes, non-uniform application film thickness, and spray environment changes can also produce similar surface defects. When wetting agent testing does not achieve the expected result, avoid blindly continuing to increase dosage — instead re-evaluate which stage the cratering is actually occurring at.
How to Distinguish Substrate Wetting Agent from Leveling Agent?
Both may affect film surface state, but the stage of focus differs. Substrate wetting agent primarily addresses whether the coating can make adequate contact with and spread across the substrate. Leveling agent more influences the flow and final flatness of the film during formation. If the coating retracts from the substrate surface immediately after spraying, check substrate and wetting state first. If the coating already spreads uniformly but the surface is still uneven after drying or curing, then further investigate the leveling system.
Why Does the Defoamer Also Need to Be Included in the Investigation?
Some cratering does not originate from the substrate itself — it comes from the influence of other surface-active materials in the formulation. A defoamer, while addressing foam, also participates in the coating's interfacial state. If its matching with the current resin system is inappropriate, it may affect the film surface. Therefore, if cratering appears after changing or adjusting the defoamer, that change should also be included in the investigation scope.
How to Confirm Whether Cratering Is a Substrate Wetting Problem?
A controlled comparison narrows the range: keeping the basic coating formulation and application conditions unchanged, only change the wetting agent type or addition level, then observe film changes. Beyond counting craters, also observe: coating spreading state after spray; crater number and size; whether local retraction occurs in the film; performance on different substrates; and whether formulation state changes after storage. If a wetting agent can improve spreading on a specific substrate while film appearance remains stable, the match with the current system warrants further verification.
Systematic Adjustment Approach for Substrate Wetting Agent
When encountering cratering in practice, break the problem into stages: first confirm substrate condition — check whether oil contamination, release agent, or dust are affecting spreading. Then confirm formulation changes — check whether resin, solvent, leveling agent, or defoamer have recently been adjusted. Then screen wetting agents — compare different types under the same conditions with gradient addition. Then return to actual application — confirm that film thickness, spray method, and drying conditions are consistent with laboratory testing. Only by progressively separating these variables can blanket attribution of all cratering to wetting agent be avoided.
Summary
Coating cratering does involve substrate wetting as one important investigation direction — but not all cratering comes from insufficient wetting. Substrate surface state, resin system, wetting agent, leveling agent, defoamer, and application conditions may all affect the final film. The correct approach is to first judge whether the problem is at substrate contact, film flow, or formulation compatibility — then conduct comparative testing combined with the specific system. Attempting to reduce cratering simply by continuously increasing wetting agent dosage is less reliable than systematically isolating the stage where the problem originates.
Frequently Asked Questions
Is there a quick test to measure whether the coating is actually wetting the substrate before committing to a full spray trial?
Yes — measuring the static contact angle of the coating liquid (or the base resin) on the substrate surface gives a quantitative indication of wetting tendency. A contact angle below approximately 30° indicates good wetting; above 60° indicates marginal wetting that will likely produce spreading problems in practice. This measurement can be made on a laboratory bench with a microsyringe and camera in a few minutes, and gives useful guidance on whether the wetting problem is primarily at the liquid-substrate interface before the full spray trial is run.
Can the same substrate wetting agent be used in both waterborne and solventborne coating systems?
Most substrate wetting agents are system-specific. Waterborne wetting agents are designed for aqueous systems and may have limited compatibility in solventborne formulations; solventborne wetting agents may not disperse correctly in waterborne systems. Some products, including certain polysiloxane-polyether copolymers, are specified for use across both system types — but compatibility in the specific formulation should always be confirmed by trial rather than assumed from the general product category.
