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Industrial Coatings · Floor Coatings · Wood Coatings · Thick-Film Application
In industrial, floor, and wood coating applications, thick-film application is common — reducing application passes while achieving target film build. But as single-coat film thickness increases, air and volatiles trapped inside the film become harder to escape, manifesting after drying as bubbles and blistering surface defects. When this happens, the instinct is to adjust the defoamer. In practice, defoamer is only one of the relevant factors — thick-coat bubbles usually involve film thickness, application method, system viscosity, solvent evaporation, and drying conditions simultaneously.
1
Why Thin Coat Normal — Thick Coat Develops Bubbles?
In thin film, bubbles are close to the surface and have opportunity to escape after application. When film thickness increases, bubbles must travel a longer path before leaving the film. If the film surface has already begun to dry or cure, internal gas release is further restricted. Some formulations test normally at thin film but show bubbles as soon as film thickness is raised.
2
Air Introduced During Application Cannot Be Ignored
High-speed dispersion, stirring, recirculation, and spray application all introduce air into the coating. In low-viscosity systems, micro-bubbles may rise and escape after application. In high-solid-content, high-viscosity, or thixotropic formulations, bubble movement speed decreases. Thick coating enlarges the internal space, making trapped air more likely to remain — forming visible defects in the cured film.
3
Fast Solvent Evaporation Can Also Trap Bubbles Inside
In solventborne coatings, solvent evaporation rate must be matched with film formation. If surface drying is too fast, a relatively dense skin forms before internal solvent has finished migrating outward. Thick film amplifies this interior-surface drying speed difference — restricted outward migration of internal volatiles forms bubbles. When thick-coat blistering appears, the solvent system and drying conditions should also be checked, not only the defoamer.
4
High System Viscosity Makes Bubbles Harder to Escape
Coating viscosity affects bubble release. In high-viscosity systems, bubbles experience more resistance to movement — rising and breaking both become more difficult. If thick-film application is combined with high application viscosity and strong thixotropy, bubbles are even more likely to remain trapped inside. Thick-film systems therefore require simultaneous attention to application viscosity, thixotropic state, and the available flow window after application.
5
How to Adjust the Defoamer
The defoamer primarily acts on bubble rupture, foam suppression, and assisting micro-bubble release from the system. When thick coating produces large amounts of micro-bubbles, small-range gradient testing with different defoamer types and addition levels is recommended — not directly increasing addition significantly. Test simultaneously: internal bubble count; film surface state; gloss change; cratering; and leveling performance. Defoamer amount change affects not only foam state — it may also affect film surface appearance.
6
Why Do More Defoamer and Bubbles Still Remain?
If the problem source is film thickness, solvent evaporation, or application-introduced air, increasing defoamer cannot fundamentally change the entire film formation process. For example, if a large amount of air is already incorporated during application and the film surface dries quickly, there is insufficient time for internal gas to escape — even at higher defoamer addition. Defoamer adjustment is best treated as part of overall formulation optimisation, not as the sole solution to all thick-coat bubble problems.
7
The Substrate Can Also Be a Bubble Source
Some porous or highly absorptive substrates release internal air after application. Particularly during thick-film application or subsequent heating, air inside the substrate expands and migrates toward the coating. In such cases, even if the coating's own defoaming is functioning normally, blistering may still appear on the film surface. Diagnosing whether bubbles originate from inside the coating or from the substrate requires comparison experiments with different substrates, additional primer coats, or changed application methods.
8
Baking Coatings — Watch the Heating Ramp
For heat-cure coatings, heating ramp rate is also worth attention. If temperature rises too fast, solvent or air inside the film may release rapidly while the surface has already begun to cure — and internal gas cannot escape. Adjusting the heating curve, allowing appropriate flash-off and leveling time, and comparing different ramp profiles are all relevant adjustments.
9
How to Systematically Diagnose Thick-Coat Bubbles?
Step 1 — Film thickness: compare bubble differences between thin and thick application. Step 2 — Application method: check whether stirring, spray, and recirculation readily introduce air. Step 3 — System state: consider viscosity, thixotropy, and available flow time after application. Step 4 — Evaporation and cure: check solvent evaporation speed and baking ramp. Step 5 — Defoamer: gradient test at different types and levels, observing film appearance simultaneously. Step 6 — Substrate: if bubbles reduce significantly with a different substrate, investigate porosity and internal air release.
10
Different Coating Systems — Different Defoamer Requirements
Waterborne systems, solventborne systems, high-solid systems, and thick-film coatings differ in how bubbles form and release. Therefore, defoamer screening should not only observe surface foam right after the defoamer is added — testing at the actual application film thickness is also needed. For thick-coat formulations, verification using film thickness close to actual application conditions, combined with storage, drying, and baking condition comparison, gives results that more closely represent actual application.
Summary
Thick-coat bubbles require judgement across multiple factors. Increased film thickness makes internal gas harder to escape; application-introduced air, system viscosity, solvent evaporation, drying and cure conditions, and substrate state can all amplify this problem. Defoamer is an important formulation adjustment lever — but should be selected through gradient testing in the specific system. Identifying the bubble source and formation conditions first, then addressing formulation and application process together, is typically more effective than simply increasing defoamer dosage.
Thicker film = longer escape path for trapped air
Application-introduced air adds to film bubble load
Fast surface dry seals in volatiles before they escape
High viscosity slows bubble rise and rupture
Substrate porosity can supply additional bubble sources
Baking ramp rate affects gas escape timing
Frequently Asked Questions
How can the source of bubbles (application-introduced air vs solvent vapour vs substrate outgassing) be identified without expensive equipment?
Three controlled comparisons are usually sufficient: (1) Apply the coating by hand drawdown on a clean glass panel at the target film thickness — if bubbles appear, they are formulation-sourced (solvent evaporation or defoamer), not application-introduced. (2) Spray onto the same substrate used in production versus a non-porous reference panel (glass or metal) — if bubbles are substantially fewer on the reference panel, substrate outgassing is contributing. (3) Allow the coating to degas in a container for 30 minutes before application, then compare bubble count — if degassed coating shows fewer bubbles, application air entrainment is a significant source.
Does using a slower-evaporating solvent blend always help reduce thick-coat bubbling in solventborne systems?
Not always. A slower-evaporating solvent blend extends the time before the film skins over, giving more time for internally generated vapour and entrained air to escape — which helps when the problem is premature surface skin formation trapping interior volatiles. However, if the coating's viscosity at the point of application is already low (because slow solvents keep viscosity low longer), this can also reduce the viscosity gradient that helps push bubbles toward the surface. The optimal solvent balance for thick-coat bubble reduction is typically a blend with sufficient slow solvent to delay skinning without producing an excessively extended low-viscosity phase that allows other defects.
