Blistering, bubbles and pinholes in coatings
Raised domes, bubbles and pinholes can look similar from above, but may come from moisture pressure, substrate outgassing, trapped air, solvent or cure conditions. The size, contents and timing of the defect guide the diagnosis.
Identify the symptom
| Appearance | Potential mechanism | Useful evidence |
|---|---|---|
| Large domes after weeks or months | Moisture movement, soluble salts, water ingress or loss of bond | Liquid under film, slab moisture, drainage, fracture plane |
| Tiny craters during installation | Air released from porous concrete or roller entrainment | Timing, temperature trend, primer coverage, porosity |
| Fine through-film holes | Unsealed pores, insufficient build, missed detail or entrapped bubbles | Magnification, continuity and thickness checks |
| Soft or tacky bubbles | Mixing, solvent retention, cure or incompatible layers | Batch ratio, film thickness, temperature and recoat records |
A crater is not necessarily a through-film holiday; a water-filled blister is not automatically an osmotic blister. Confirm with inspection and testing.
Why moisture matters
Moisture can enter from the slab, leaking services, drains, washdown or condensation. Vapour drive and soluble contaminants can produce pressure at an interface, while water on a cold substrate can disrupt primer bonding. Use specified concrete moisture tests and investigate source, gradient and timing. One dry surface reading cannot rule out deeper slab moisture. Sika’s flooring guidance notes that moisture and incorrect primer selection can contribute to blistering and delamination.
Outgassing, pinholes and application variables
Porous concrete contains air in voids. When substrate temperature rises, air can escape through a fresh film. A suitable primer, pore-filling layer, appropriate application timing and inspection after priming reduce risk. Rough or honeycombed substrate, missed edges, inadequate thickness and overly aggressive rolling can leave pinholes. Film thickness and application technique must match the product data; more material is not automatically safer because excessive thickness can trap solvent or increase stress.
For linings and containment, small discontinuities can be critical even if a decorative floor might tolerate minor surface texture. Specify the project’s holiday or continuity test and its method, voltage and acceptance criteria with the lining designer; do not apply an arbitrary field test to every floor.
Investigation and repair
- Map defects by area, slab pour, moisture source, day of application and sunlight exposure. Record when first seen.
- Open representative defects and photograph the film underside and substrate. If liquid is present, preserve a sample for suitable analysis.
- Review surface preparation, primer application, moisture and dew-point records, batch and mix ratio, film thickness, recoat and cure times.
- Test moisture, adhesion and film continuity as appropriate to the project. Compare affected and unaffected zones.
- Correct the underlying cause, remove unsound material, prepare to sound boundaries and install the approved repair build. Verify adhesion and continuity before service.
Do not use a generic sealer to hide active moisture ingress. If the source is ongoing, resolve it or redesign the system with technical support.
Common questions
Are pinholes just cosmetic?
It depends on exposure and system purpose. In chemical containment or waterproofing, a through-film defect can compromise protection and must meet the specified inspection criteria.
Why do bubbles appear the next day?
Possibilities include outgassing, entrapped air, residual solvent or cure conditions. Timing alone does not identify the mechanism; inspect cross-section, substrate and application record.
Technical references and further reading
- ACI 515.3R-20: Assessment and Surface Preparation for Concrete Protection Systems
- Sika: Concrete surface preparation for resin flooring
- Sherwin-Williams: Preventing premature coating failure
- Additional technical reference
These references describe general mechanisms. ALKA’s current product TDS, approved build and the project’s measured conditions govern the actual repair.
Need a project-specific build?
Send the substrate condition, exposure, photos and programme to ALKA for a technical review of the complete system.
Planning guidance only. Confirm the current technically approved ALKA TDS/SDS, compatibility, testing and application limits before specifying or installing a system.