Coating defect

Blistering

Raised domes in the film, from pinhead size to several centimetres, filled with liquid or gas. They can sit between coats or under the whole system.

Blistering: the surface as you see it, and a schematic cross-section.

Causes

Soluble salts left on the steel draw water through the film by osmosis.
Moisture or condensation on the surface when the coat was applied.
Solvent trapped in a thick film or under a coat applied too early.
Too much cathodic protection, or a coating not suited to it.
Permeation in immersion service by a system not designed for it.

Prevention

Test for salts after blasting. If they exceed the limit, wash the surface and re-blast it.
Keep the steel at least 3 °C above the dew point.
Keep DFT within the specified range and respect recoat windows.

How it is assessed

Rate density (0–5) and size (S1–S5) together under ISO 4628-2, for example 2(S3), or use ASTM D714. Open a few blisters to note what is inside and at which interface they formed. This check is destructive and is outside the CIP 1 exam.

Repair

Remove the blistered coating to sound, adherent coating, find and correct the cause (often salts), prepare and recoat as the specification says. The specification and the coating manufacturer have the final word on the repair procedure.

37 practice items in the members area mention blistering (CIP 1: 9 · CIP 2: 16 · FROSIO written: 12).

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

About this page

Written from coating practice and the published rating standards, in our own words; the drawing is schematic. Checked on 2026-10-04; errors go in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.