Wetting Agents in Waterborne Coatings: How to Eliminate Cratering and Fish-Eyes

Understanding the Root Cause

Cratering and fish-eyes are not the same defect, though they look similar. The distinction matters for choosing the right additive:

Fish-eyes form during application when the wet film encounters a low-surface-energy contamination (silicone, oil, wax) on the substrate. The coating dewets around the contamination point, leaving a circular void.
Cratering occurs during film formation when surface tension gradients within the drying film drive convective flow (Marangoni flow) that creates depressions. The surface tension difference can come from uneven solvent evaporation, local concentration gradients, or surfactant migration.

Both defects share a common factor: the coating's surface tension is higher than the substrate surface energy, or the surface tension is non-uniform during drying.

Two Critical Surface Tension Measurements

Most formulators focus on static surface tension (measured at equilibrium, typically by pendant drop or Wilhelmy plate). However, what governs wetting behavior during high-speed application is dynamic surface tension — the surface tension at very short timescales (milliseconds) as fresh surface is continuously generated.

A coating with excellent static surface tension (e.g., 28 mN/m at equilibrium) may still crawl on a substrate if its dynamic surface tension at 10 ms exceeds 40 mN/m. This is why some formulations work well with a brush but fail with a high-speed spray gun.

Practical target values:

Static surface tension: ≤ 30 mN/m (for most substrates including plastics)
Dynamic surface tension at 10 ms: ≤ 35 mN/m (for spray applications)

Wetting Agent Chemistries

Polyether Siloxanes (Silicone Wetting Agents)

Silicone-based wetting agents are the most effective at reducing both static and dynamic surface tension. They migrate rapidly to the air-water interface due to the low cohesive energy of silicone backbones.

Advantages:

Reduce static surface tension to 20–24 mN/m at 0.1–0.3% dosage
Excellent for preventing fish-eyes on contaminated or low-energy substrates (plastics, powder-coated metal)
Improve crater resistance significantly

Limitations:

Can cause foam if overdosed — always pair with a defoamer evaluation
Some grades reduce recoatability — inter-coat adhesion can be compromised
Not suitable for certain applications (pressure-sensitive adhesive primers)

Typical dosage: 0.05–0.3% on formulation weight. Start at 0.1% and increase only if needed.

Fluorosurfactants

Fluorinated wetting agents reduce surface tension to 15–20 mN/m — lower than any silicone grade. They are used in demanding applications: electronics coatings, stone sealers, and anywhere requiring wetting of ultra-low-energy surfaces (PTFE, PE, PP).

Cost is 5–10× higher than silicones; use is justified only when silicones are insufficient.

Non-Ionic Polyether Wetting Agents

Ethylene oxide/propylene oxide block copolymers and acetylenic diol derivatives (e.g., SURFYNOL series) are the workhorses of waterborne industrial formulations.

Advantages:

Excellent dynamic surface tension reduction — particularly effective for spray and roll application
Lower foam generation than silicones
Better recoatability
Compatible with most binder systems

Limitations:

Static surface tension reduction limited to ~28–32 mN/m
Less effective on heavily contaminated substrates vs. silicones

Typical dosage: 0.2–0.8% for polyether types; 0.1–0.5% for acetylenic diols.

Diagnosing Cratering vs. Fish-Eyes in Practice

Apply a drawdown over:

Clean glass — if cratering occurs here, the problem is internal (surface tension gradient within the film, often from silicone contamination in the binder or pigment)
Substrate in question — if problems appear only here, the issue is substrate wetting

For fish-eyes specifically, wipe the substrate with IPA first and retest. If the defect disappears, substrate contamination is confirmed.

Formulation Troubleshooting Table

Defect	Surface	Likely Cause	Recommended Fix
Fish-eye	Plastic	Mold release or oil	Silicone wetting agent 0.1–0.2%
Fish-eye	Metal	Stamping oil	Increase wetting agent + clean substrate
Crater	Any	High dynamic surface tension	Acetylenic diol 0.2–0.4%
Crater	Any	Silicone contamination in batch	Check all raw materials for silicone
Crawling	Low-energy plastic	Surface energy below 30 mN/m	Fluorosurfactant or flame/corona treatment

Compatibility Testing Protocol

Before finalizing a wetting agent:

Foam test: Mix 100 mL of diluted coating (1:10 in water) in a graduated cylinder, shake 10×, measure foam height after 1 min. Acceptable: < 20 mL foam.
Intercoat adhesion test: Apply two coats with 24h interval, crosshatch adhesion (ISO 2409). Silicone grades must pass 0/5 or better.
Stability test: Age at 50°C for 4 weeks, check for phase separation or viscosity drift.

Summary

For most waterborne industrial coatings, a combination of a non-ionic polyether wetting agent (for dynamic surface tension and spray application) with a low-dose silicone surfactant (for fish-eye prevention) delivers the best performance. Always validate foam, recoatability, and stability before production. Chemzip can supply both silicone and non-ionic wetting agent grades along with comparative technical data for your specific substrate and application conditions.