A bubble on a coating can be trapped solvent, an osmotic cell or a cathodic reaction. Cutting one open is the only way to know which.
An inspector finds small blisters on the inside of a water tank, six months after coating. The contractor says the product was defective. The supplier says the surface was contaminated. Both are guessing, because a blister is a symptom and there are at least four mechanisms that produce one.
Pinholes are not blisters
The two are often reported together and they are different defects with different causes.
- A pinhole is a channel through the film. It forms during application, it is open to the surface, and it reaches or nearly reaches the substrate.
- A blister is a dome with liquid or gas underneath. The film is intact on top; the problem is at the interface or within the film.
- The inspection consequence differs. A pinhole is found with a holiday detector. A blister is found visually and then investigated by cutting it open.
Four mechanisms behind a blister
- Solvent blistering. Solvent trapped in the film during cure expands and forms domes. It appears early, usually within days, and it is associated with thick coats, cold substrates, short recoat intervals and poor ventilation.
- Osmotic blistering. Water passes through the film, dissolves soluble material at the interface, and the resulting concentration difference pulls in more water. The blister grows slowly, appears in immersion service, and contains liquid with dissolved salts.
- Cathodic blistering. On a structure under cathodic protection, the reaction at a coating defect generates alkali and hydrogen. Blisters form around the defect, and the liquid underneath is strongly alkaline.
- Gas blistering. Air or solvent vapour expands under a warm surface, or a substrate that outgasses, such as concrete or galvanising, pushes the film up from below. The blister may disappear when the temperature drops.
The diagnostic that settles it
Cut the blister open and look at what is inside. This takes a knife and five minutes and replaces a week of argument.
- Smell it. A strong solvent odour means solvent blistering, and points at application rather than the product.
- Test the liquid with pH paper. Neutral or slightly acidic with a salty taste indicates osmotic. Strongly alkaline indicates cathodic and points to the protection system, not the coating.
- Look at the substrate. Rust or mill scale under the blister means the preparation was inadequate. Clean steel with a liquid film means the mechanism is in the coating or the water.
- Check the pattern. Blisters along a weld, an edge or a low point suggest a local preparation or geometry issue. Uniform blistering over a large area suggests a system or cure problem.
What pinholes actually come from
- Air trapped in the film during spraying, especially with airless equipment at high pressure and a viscosity that is too high.
- Solvent boiling off too fast in a thick coat under warm conditions, leaving channels as it escapes.
- Outgassing from the substrate. Concrete, galvanised steel and some primers release gas when heated or when solvent penetrates.
- Contamination on the steel. Oil, grease and dust cause the film to draw back locally, which opens a pinhole.
- Too thin a coat over a rough profile. The peaks of the profile are barely covered and the coating fails at the peak.
Repair that actually holds
- For pinholes, the repair is local: abrade the area, apply the specified coat, and re-test with the holiday detector. Do not simply add a thin coat over the top; a pinhole filled with a thin film is still a pinhole.
- For solvent blisters, the film is compromised across the affected area. Remove back to sound coating, check the thickness and cure of what remains, and reapply within the limits.
- For osmotic blisters, the source is at the interface. Removing the blister and repainting without removing the salt simply restarts the process.
- For cathodic blisters, the coating is being attacked by the protection system. Repainting without reviewing the current density or the coating's resistance to alkali will reproduce the failure.
Prevention, by mechanism
- Solvent: respect maximum film thickness per coat, extend recoat intervals in cold weather, ventilate without chilling the steel.
- Osmotic: test for soluble salts before coating, and specify a salt limit in the contract.
- Cathodic: specify a coating with documented resistance to alkaline environments, and coordinate the coating selection with the cathodic protection design.
- Pinholes: control viscosity and spraying distance, apply a mist coat on porous substrates, and holiday test the finished lining.
Blisters are the most misdiagnosed defect in protective coatings, because the visible result looks the same regardless of cause. The cut and the pH strip are the difference between fixing it and repainting it twice.