Bubbles, blisters, craters, and pinholes are appearances—not root causes. Air mixed into resin can create one defect; gas leaving porous concrete can create another; pressure beneath a cured film can form a blister; and contamination or application error can mimic parts of each pattern.
Do not diagnose every bubble as moisture or every pinhole as outgassing. Record when the defect formed, open representative defects, identify the failure plane and contents, review application conditions, and test the slab where the evidence points before choosing a repair.
This is separate from peeling and delamination. A blister may include disbondment, but a surface pinhole can exist without loss of adhesion. The repair depends on which mechanism is present and how far it extends.
Start with a symptom map
| Visible evidence | Plausible mechanisms | What would strengthen or weaken the diagnosis |
|---|---|---|
| Fine bubbles throughout mixed/applied material | Entrained air from mixing, pumping, rolling, or application technique | Batch timing, mixer speed, bubbles in the pail, product viscosity, and manufacturer instructions |
| Circular crater or pinhole appearing over porous concrete | Substrate outgassing or air displaced from pores | Timing during rising substrate temperature, a pore/opening at the center, distribution over porous areas |
| Dome beneath a cured film | Moisture/osmotic pressure, trapped volatile, intercoat contamination, or another pressure mechanism | Liquid or residue under the dome, failure plane, slab tests, service exposure, layer sequence |
| Localized defect over patch, crack, or perimeter | Different porosity, trapped air, incompatible repair, or missed preparation | Exact alignment with substrate transition and condition of the interface |
| Soft or gummy bubbly coating | Mix-ratio, incomplete mixing, temperature, chemical exposure, or cure problem | Batch records, unmixed streaks, retained samples, exact product cure data |
| Fish-eye-like circular separation | Surface-tension contamination rather than a gas bubble | Oil, silicone, cleaning residue, or contamination pattern at the exposed surface |
These clues narrow the investigation; none proves a cause alone.
Entrained air: bubbles that begin in the material
Two-component coatings can hold air introduced by aggressive mixing, an unsuitable paddle, repeated lifting of the mixer, pumping, or excessive rolling. Higher viscosity and shortening working time can reduce the opportunity for air to release before the film sets.
Check the exact instructions for mixer type, speed, mix time, induction time if any, transfer to a clean container, application tools, and maximum film build. One Sika coating document, for example, tells installers to use a low-speed mixer and avoid overmixing to minimize air entrainment. That is a product-specific procedure—not a universal rpm for all epoxies.
Evidence to collect:
- whether bubbles were present in the mix container;
- which batches and floor areas are affected;
- elapsed time from mixing to placement;
- material and substrate temperature;
- mixer, paddle, roller, squeegee, and pump used;
- actual wet-film thickness and coverage;
- whether the defect follows roller paths or batch boundaries.
Concrete outgassing: gas moves out of the substrate
Concrete contains connected pores and voids. As air or vapor moves outward while a liquid coating is open, it can create a bubble, crater, or pinhole. Sherwin-Williams describes outgassing as a site condition in which air or gas escapes from the concrete and produces these surface anomalies.
Temperature direction matters because warming concrete can expand air in the slab and drive it toward the surface. Some system instructions therefore favor application while substrate temperature is falling. But do not turn that into a universal schedule: dew point, humidity, cure rate, direct sun, building pressure, and each product’s temperature limits still control.
Porous slabs, newly opened concrete after mechanical preparation, patch transitions, and unsealed bugholes can be more vulnerable. A compatible epoxy floor primer may wet and seal pores or expose outgassing before the build coat, but primer is not a guaranteed cure and an arbitrary extra coat can create compatibility or recoat problems.
Pinholes are a result, not a single diagnosis
A pinhole is a small discontinuity extending into or through a coating layer. It may remain after an outgassing bubble breaks, after air leaves an incompletely sealed pore, or because the film did not wet and close a void. Application thickness, porosity, surface tension, primer coverage, and material rheology can all contribute.
Do not assume that spike rolling or a heavier coat will fix every pinhole. A thicker film can retain more air or exceed the product’s specified application range. Some primer instructions call for an additional coat to achieve pore-free coverage; that remedy belongs only to the documented assembly and recoat window.
Moisture-related and osmotic blistering
Moisture-related blistering is a different mechanism from ordinary mixed air. Moisture vapor, dissolved compounds, pressure, coating permeability, slab construction, and the service environment may contribute to a dome or disbonded area. Some manufacturer-designed moisture-tolerant underlayments expressly claim to reduce osmotic blistering in compatible systems, illustrating that mitigation is a complete-system decision.
A blister alone does not prove excess slab moisture. If moisture is plausible, use the concrete moisture testing guide to distinguish screening from standardized acceptance testing. Review plumbing, groundwater, drainage, vapor retarder, seasonal conditions, cleaning operations, and active leaks. Hydrostatic water or leakage is not simply “a high meter reading.”
Solvents, water, and other volatiles
Some coatings contain solvent or water; concrete, patches, cleaners, and prior layers can also retain volatile material. Applying too thickly, recoating before the previous layer reaches its specified stage, or trapping cleaner/water can interfere with release and cure. Use the exact cure and recoat schedule while investigating the relevant product-specific questions:
- Is the layer within its permitted wet/dry film thickness?
- Was the previous coat inside its recoat window?
- Were ventilation and temperature within the instructions?
- Was the slab or patch dry/conditioned as required?
- Are the layers chemically compatible?
“100% solids” does not mean bubble-proof, non-toxic, or suitable for a damp slab. Low odor and low VOC are also not interchangeable with respiratory safety.
Diagnose before repairing
1. Preserve evidence
Photograph the distribution, mark batch boundaries, record installation and weather/HVAC data, and retain labels and lot numbers. Do not grind the entire floor before representative defects are examined.
2. Open representative defects
Look for a central pore, liquid, crystals/residue, tacky material, contamination, or a clean interface. Record whether separation is:
- at coating-to-concrete interface;
- within weak concrete;
- between coating layers;
- within the coating itself.
ASTM D7234’s accessible scope explains that pull-off testing exposes the weakest plane within the fixture/adhesive/coating/substrate system. The full standard is paid; professional testing and the specification should determine method and acceptance.
3. Correlate with the installation
Compare affected and unaffected areas for porosity, patches, sunlight, substrate temperature, coating thickness, batch, recoat timing, and preparation. Review how the concrete was prepared.
4. Test the relevant hypothesis
Moisture tests, adhesion tests, coating thickness, cure evaluation, and contamination investigation answer different questions. Do not use one instrument as a universal floor-failure detector.
5. Define the repair boundary
Isolated, well-bonded surface defects may permit a local manufacturer-approved repair. Widespread blisters, soft coating, intercoat failure, recurring moisture, or unknown contamination may require removal to a stable layer or to sound concrete. Recoating over unresolved pressure or incompatible material only hides the evidence temporarily.
Prevention controls by mechanism
| Mechanism | Prevention emphasis |
|---|---|
| Mixed air | Exact mixer/paddle/speed, controlled placement, wet-film range, working time |
| Outgassing | Slab/air temperature trend, pore-sealing primer where specified, porous-area inspection, manufacturer sequence |
| Pinholing | Complete preparation, pore-free approved primer, thickness verification, inspection before next coat |
| Moisture blistering | Slab assessment, appropriate test methods, system acceptance limits, designed mitigation where approved |
| Trapped volatiles | Drying/ventilation, recoat window, layer compatibility, specified film build |
| Contamination | Decontamination, final cleanliness verification, test patches where appropriate |
Equipment and product-selection points
Future commercial modules can help readers select low-speed mixers, the specified squeegee/roller, wet-film gauges, moisture-test equipment, shrouded removal tools, and compatible pore-filling primers. Recommendations must be tied to the exact mechanism and system instructions. No tool should be marketed as a universal “bubble remover.”
Return to the epoxy flooring hub for the wider sequence from slab assessment through preparation, primer, application, and failure diagnosis.
Sources and technical references
Accessed September 2026. Product documents can change; verify the current revision before specifying or installing a system.
- Why Does My Epoxy Floor Have Bubbles? — Sherwin-Williams High Performance Flooring
- Why Is My Epoxy Floor Bubbly or Gummy the Next Day? — Sherwin-Williams High Performance Flooring
- Preventing Concrete and Epoxy Floor Blistering — Sherwin-Williams High Performance Flooring
- Sikafloor-165 FS epoxy primer product information — Sika USA
- Sikafloor-81 EpoCem product data sheet — Sika USA
- ASTM D7234-22: Pull-Off Strength of Coatings on Concrete — ASTM International