Coating defect

Outgassing

Bubbles, craters and pinholes that appear minutes to hours after coating concrete or a porous zinc primer.

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

Causes

Air and vapour in the pores of concrete expand when its temperature rises.
Air trapped in a porous inorganic zinc primer or thermal spray coating escapes into the wet top coat.

Prevention

On concrete, prime or seal first and coat while the concrete temperature is steady or falling.
Over zinc primer or thermal spray, apply a thinned mist coat before the full coat.

How it is assessed

Visual; holiday testing finds pinholes that are hard to see.

Telling it apart

Pinholes: Pinholes from the spray technique are spread evenly. Pinholes from outgassing gather over porous areas and appear after application, as the substrate warms.
Bugholes (surface voids in concrete): Bugholes are the voids in the concrete; outgassing is air from such voids rising into the coating.

Repair

Abrade and fill or reseal the area. A new coat straight over pinholes lets the trapped air through again. The specification and the coating manufacturer have the final word on the repair procedure.

2 practice items in the members area mention outgassing (CIP 2: 2).

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 2026-10-14; 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.