Blistering
Why coatings form liquid- or gas-filled domes, how to tell osmotic, solvent, cathodic and thermal-gradient blistering apart, and how to repair and prevent each.
Key takeaways
- Blisters are local losses of adhesion where liquid or gas collects beneath the film or between coats. They are a symptom; the mechanism must be identified before any repair.
- Osmotic blistering driven by water-soluble salts left on the substrate is the most common cause in immersion and high-humidity service.
- Opening representative blisters and recording what is inside (wet or dry, pH, odor) and what lies beneath is the fastest route to a diagnosis.
- Repairs that skip decontamination, or ignore service conditions such as cathodic protection or a cold wall, usually blister again.
Blistering is the formation of dome-shaped projections in a coating film caused by localized loss of adhesion, with liquid or gas accumulating under the film or between coats. It ranges from a scatter of pinhead blisters to dense blistering that precedes widespread corrosion or lining failure.
Different mechanisms produce near-identical blisters, so an investigation asks three questions: where in the coating system the blister sits, what it contains, and what the coating was exposed to during application and service.
What blistering looks like
Blisters vary from pinpoint size to several inches across. They may be intact or broken, filled with liquid or dry (gas-filled), and they can form at the substrate interface, between coats (intercoat), or within a single coat (cohesive). The distribution pattern is often the most useful early clue:
- Concentrated on the coldest areas of a heated tank or vessel — suggests a temperature-gradient (cold-wall) mechanism.
- Clustered around holidays or damage on a cathodically protected structure — suggests cathodic disbondment.
- Random, often following old pitting on blasted steel — suggests salts trapped at the interface.
- Appearing shortly after application or on warming — suggests retained solvent or, on concrete, outgassing.
Rating blister size and density
Two standards are widely used to record blistering consistently. ASTM D714 compares the surface with reference photographs, rating size on an even-numbered scale from 10 (no blistering) through 8 (smallest blisters easily seen by eye) down to 2 (large), and frequency as Few, Medium, Medium dense or Dense. ISO 4628-2 rates quantity and size on 0–5 scales. Dated, scaled photographs allow progression to be tracked.
Root causes of blistering
Osmotic blistering from soluble salts
All organic coatings are permeable to water vapor to some degree. When water-soluble contaminants such as chlorides, sulfates or nitrates remain on the substrate, water diffusing through the film dissolves them and forms a concentrated solution at the interface. Osmosis draws in more water until the pressure exceeds the coating’s adhesion. Typical sources include marine and de-icing salts, contaminated abrasive or rinse water, and salts trapped in corrosion pits that blasting does not remove. See soluble salt contamination for testing and removal methods.
Solvent entrapment
Excessive film thickness, recoating too soon, low temperatures and poor ventilation can all trap solvent in the film. When the coating is later heated or immersed, the retained solvent can vaporize to form dry blisters or leave a porous, water-sensitive film that blisters later.
Cathodic disbondment
On structures under cathodic protection (CP), the reduction reaction at exposed steel generates hydroxyl ions. The resulting alkaline environment attacks the coating bond at the edge of holidays and spreads beneath the film, producing blisters filled with strongly alkaline fluid. Excessively negative potentials (overprotection), which can also evolve hydrogen, accelerate the process.
Temperature gradients (cold-wall effect)
When the substrate is colder than the liquid in contact with the lining — for example an uninsulated tank holding warm contents — water permeates toward the colder steel and can condense at the interface. Blisters concentrate where the wall is coldest.
Moisture and alkalinity in concrete
On slabs-on-grade without an effective vapor retarder, moisture vapor moving upward through the slab can cause osmotic blisters filled with alkaline fluid, sometimes months after installation. Gas-filled blisters formed during cure are a different mechanism (outgassing).
Intercoat contamination and premature service
Blisters between coats often trace back to contamination of the lower coat — dust, oil, or water-soluble amine blush on epoxies. Placing an under-cured lining into immersion can also cause blistering.
How to diagnose blistering
- Map the pattern. Record location, extent and an ASTM D714 or ISO 4628-2 rating. Photograph with a scale and note any relationship to welds, edges, cold zones, waterlines, anodes or service temperature.
- Open representative blisters. Use a clean knife and appropriate PPE. Note whether the blister is wet or dry, the color of any fluid, and odor — a solvent smell points to retained solvent.
- Check the fluid pH. Touch pH indicator paper to the fluid. Strongly alkaline fluid on a cathodically protected structure points to cathodic disbondment (on concrete, alkaline fluid is normal for osmotic blisters). Near-neutral fluid with rust staining on steel fits osmotic or corrosion-driven blistering.
- Identify the plane of failure. Determine whether the substrate is exposed (failure at the substrate), primer remains (intercoat failure) or the coating has split (cohesive failure). Inspect the underside of the cap for residues or rust.
- Test the exposed surface. Sample for soluble salts — for example by the ISO 8502-6 patch method with conductivity measured per ISO 8502-9 — at the blister base and at intact control areas nearby.
- Measure film thickness. Survey DFT in blistered and unblistered areas following SSPC-PA 2. Unusually high DFT supports solvent entrapment; low DFT increases permeation.
- Review records. Check dew point logs, recoat intervals, cure time before service, CP surveys and operating temperatures.
- Escalate when warranted. FTIR of blister undersides, ion chromatography of fluid and cross-section microscopy can resolve disputed cases.
Always test blister-free control areas next to blistered ones. A contaminant found only under the blisters is far stronger evidence than a single high reading taken in isolation.
Causes, diagnostic clues and prevention
| Cause | Typical clues | Confirming check | Prevention |
|---|---|---|---|
| Osmotic (soluble salts) | Liquid-filled; immersion or wet service; may follow old pitting | Salt test at blister base vs. control areas | Test and remove salts to the specified limit before coating |
| Solvent entrapment | Dry or solvent-smelling blisters; high DFT; appears on heating | DFT survey; lab solvent analysis | Control wet film build; observe recoat and cure times |
| Cathodic disbondment | Around holidays on CP structures; alkaline fluid | pH paper; CP potential survey | CP-compatible coating; avoid overprotection |
| Cold-wall effect | Concentrated on coldest areas of heated tanks or vessels | Wall vs. contents temperature survey | Insulate exterior; select low-permeability lining |
| Concrete moisture | Slab-on-grade; alkaline fluid; appears months later | ASTM F2170 RH or ASTM F1869 MVER | Moisture testing; vapor retarder; moisture-tolerant primer |
| Intercoat contamination | Blisters between coats; residue on lower coat | Underside examination; water-break test | Clean between coats; remove blush; respect recoat window |
How to repair blistered coatings
The right repair depends on severity, service and, above all, the confirmed cause.
- Few small, intact blisters in atmospheric service: may be monitored rather than repaired if no corrosion is developing.
- Localized blistering: remove blistered and loosely adhered coating back to tightly bonded material, prepare the area (for example power-tool cleaning to bare metal per SSPC-SP 11, or spot blasting), decontaminate, feather the edges and reapply the full system with an overlap onto sound coating.
- Widespread blistering in immersion: full removal is usually needed — typically abrasive blasting to the specified grade, such as SSPC-SP 10/NACE No. 2 or better — followed by salt removal, retesting and recoating with a system suited to the service.
- Concrete: remove the affected coating, measure moisture, then dry the slab, use a moisture-mitigating primer, or choose a system rated for the conditions.
Abrasive blasting alone often does not remove salts from pitted steel and can drive them deeper into pits. A wash–blast–wash sequence with retesting until results meet the specified limit is common practice. Recoating over residual salts usually reproduces the failure.
How to prevent blistering
- Specify and measure salt limits. Immersion specifications often set chloride limits in the low single digits of µg/cm²; the project specification governs.
- Control ambient conditions. Keep the surface at least 5 °F (3 °C) above the dew point and within the product’s temperature range — see environmental conditions for coating.
- Control film build. Check wet film thickness during application (ASTM D4414) and verify DFT per SSPC-PA 2; stay within the manufacturer’s maximum per coat.
- Respect cure. Observe minimum recoat times and full cure before immersion, following the product data sheet.
- Match the system to service. Use low-permeability linings for immersion, coatings evaluated for cathodic disbondment (for example by ASTM G8 or ASTM G95) where CP is used, and products rated for the operating temperature.
- Test concrete moisture. Measure by ASTM F2170 or ASTM F1869 before coating slabs — see concrete moisture testing.
- Find holidays before service. Holiday testing of linings removes the starting points for cathodic disbondment and corrosion-driven blistering.
Frequently asked questions
Should I pop blisters?
Only to diagnose them, and then repair those spots. An intact blister still offers some barrier; a broken one exposes the substrate.
Can blisters disappear on their own?
Water-filled blisters can shrink when the coating dries out, but adhesion lost in the blistered area rarely recovers and the blisters typically return on re-wetting.
Why did blisters appear months after application?
Osmotic blistering depends on water permeating the film, which takes time. It often appears after the first immersion cycles, a wet season, or, on concrete, as moisture vapor accumulates beneath the coating.
Is blistering always caused by poor surface preparation?
No. CP overprotection, temperature gradients, unsuitable coating selection, excessive film build and premature service can all blister well-prepared surfaces.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.