How blisters form
Organic coatings are not perfect barriers: water vapor slowly permeates them. What turns permeation into a blister is a driving force at or within the film.
| Type | Driving force | Typical clues |
|---|---|---|
| Osmotic | Soluble salts left on the substrate or between coats draw water through the film | Liquid-filled, often in immersion or condensing service |
| Cathodic | Cathodic reactions at the steel produce an alkaline solution that lifts the film | Alkaline liquid, near holidays or on cathodically protected steel |
| Solvent retention | Solvent trapped by thick films or premature recoating vaporizes later | Gas-filled, often after heating or sun exposure |
Osmotic blistering traces back to surface preparation. Soluble salts such as chlorides and sulfates, invisible after blasting, create a concentrated solution under the film, and water keeps moving in until the film lifts. Cathodic blistering is closely related to cathodic disbondment: the same alkali that undercuts the coating at a defect can push up blisters around it. In tank linings, a steel wall colder than the stored liquid — the cold wall effect — speeds permeation and makes blistering more likely.
How it is rated
ASTM D714 evaluates the degree of blistering of paints by blister size and frequency, using photographic reference standards. ISO 4628-2 does the same job in the ISO series, designating the quantity and size of blisters. Both give a common vocabulary for inspection reports and warranty discussions; the acceptance level for a given project comes from the specification, not from the standard itself.
What to do about it
Diagnosis comes first. Opening blisters, checking whether the contents are liquid or gas, measuring pH and testing the exposed substrate for salts usually point to the cause. Prevention follows from it: measure soluble salts after blasting and decontaminate to the specified limit, respect film thickness and the recoat window, allow full cure before immersion, and select linings rated for the service temperature and any cathodic protection. Localized blistering can sometimes be repaired by removing the blistered film and feathering the edges; widespread blistering usually means the system has to come off. See maintenance painting of steel structures for how repair scopes are set.