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Home / News / Industry News / Floor Coating Looks Normal in Thin Film — Why Does Cratering Appear After Thick Application?
Industry News
Floor Coatings · Industrial Floors · Thick-Film Application
During floor coating application, thin-coat conditions look relatively normal — but when application thickness is increased to improve hiding power and protection, the film surface starts showing small depressions, circular defects, and even obvious local retraction. Many people assume the floor is dirty, or simply attribute the problem to the defoamer. But in fact, thick coating changes not only the film thickness — it also changes the flow, evaporation, and film formation process inside the coating. When these factors do not match, cratering is more easily amplified.
Why Does Cratering Appear More Easily After Thick Coating?
After floor coating is applied, the film needs to go through flowing, spreading, solvent evaporation, and curing. In thin coating, the film relatively quickly reaches a stable state, and some micro surface fluctuations may not develop into obvious defects. But in thick coating, the film maintains a flowing state for a longer time. As long as surface tension differences, local contamination, or additive compatibility problems exist inside the system, the coating may redistribute during the extended flow process. Originally minor surface defects may gradually form obvious depressions.
Micro-Contamination on the Floor May Also Become the Cratering Source
Floor coating is very sensitive to substrate condition. Concrete surfaces may have residual dust, oil, form release agent, curing agent, and other residues. If these substances are not fully treated, local surface energy differences may appear during coating spreading. The coating can spread uniformly in normal areas, but encountering a low-surface-tension contamination zone may develop local retraction — ultimately forming an obvious circular depression.
Why Do Thick Coats Allow Bubble and Cratering Problems to Interact?
When floor coating is applied thicker in one pass, the internal air and volatiles need longer to release outward. If the coating surface has already begun to form a relatively stable film, but internal gas is still migrating upward, micro-bubbles may form. After some micro-bubbles burst, depressions may remain on the surface. If the film's leveling capability is insufficient, these depressions cannot re-spread and easily form cratering-like surface defects. Therefore, thick-coat surface depressions cannot be immediately assumed to be typical cratering — it is also necessary to distinguish whether they are from true cratering, bubble rupture, or insufficient leveling.
Why Can Adding More Defoamer Make the Problem More Obvious?
Floor coating needs to balance both defoaming during application and final film appearance. Defoamers can help break foam, but different defoamers do not all have identical compatibility with resin systems. If an inappropriate choice is made, or addition exceeds what the system can stably support, local surface state differences may form on the film surface. When thick film maintains flow for a longer time, this difference is more easily amplified. Therefore, when cratering appears, it cannot be simply understood as "insufficient defoamer" — and the amount continuously increased. Sometimes what truly needs to be solved is the balance between defoaming efficiency and compatibility.
Leveling Agent Is Also Not Better in Larger Amounts
Thick-film systems typically hope the film can spread fully, so they focus on leveling agent selection. A well-chosen leveling system can help the film reduce surface tension gradients, enabling the coating to spread more uniformly. But if the leveling agent has insufficient compatibility with the resin system, or addition is too high, it may also change the dynamic state of the film surface. Therefore, when floor coating develops cratering, it cannot be simply assumed that "leveling is insufficient" and continuously increase the leveling agent amount. The resin, solvent, defoamer, and other additives must be judged together.
Why the Application Environment Is Also Important
Floor coating typically needs considerable time to complete film formation. The temperature, humidity, and ventilation conditions of the application site all affect the film flow and evaporation process. For example, at lower temperatures, coating viscosity rises and leveling speed decreases; excessive ventilation may accelerate surface evaporation, causing the film interior and surface to dry at different speeds. If the film formation speed difference between thick film interior and surface is too large, the probability of surface defects increases. Therefore, the same floor coating may perform completely differently under different application environments.
Summary
Floor coating cratering after thick application is not simply "too thick a film." Thick-film state extends the flow and film formation process, while increasing the influence of internal volatile release, bubble migration, and surface tension differences on the film. Therefore, solving this type of problem requires simultaneously considering: substrate cleanliness, application film thickness, defoaming capability, leveling performance, additive compatibility, and application environment. For floor coatings, a truly stable thick-film system is not just about being able to apply thicker — more important is that after thick application, the coating can still spread uniformly, degas successfully, and maintain stable film formation.
Thick coating extends flow time — amplifies surface defects
Micro-contamination on floor creates surface tension differences
Bubble-burst depressions differ from true cratering
Excess defoamer can cause local surface state problems
Leveling agent is not better when used in larger amounts
Temperature, humidity, and ventilation all affect thick-film result
Frequently Asked Questions
How can depressions from bubble rupture be distinguished from true cratering in a thick floor coat?
True cratering typically produces a circular depression with a raised rim (the coating that was pushed away from the contamination point), often with a visible spot of contamination or a distinctly different surface character at the centre. Bubble-rupture depressions typically have no raised rim and may have a small central point where the bubble wall collapsed. Insufficient leveling depressions are broader and shallower, without a distinct centre or rim. Examining the depression shape under good lighting before the coating fully cures helps identify which mechanism is responsible.
What maximum single-coat film thickness is typically recommended for solvent-free epoxy floor coatings?
For self-leveling solvent-free epoxy systems, single-coat application is typically limited to 2–4 mm depending on the formulation's viscosity and flow characteristics. Above this range, the internal exothermic reaction during cure becomes significant, and the progressive viscosity increase as cure proceeds competes with the flow and leveling process. Multiple-coat application — typically 1–2 mm per coat — gives each layer time to degas and level before the next coat is applied, producing better surface quality than a single thick application.
