# Outgassing

Source: https://coatmentor.com/defects/outgassing/ · Updated 2026-10-14

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

*Figure: 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](https://coatmentor.com/defects/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)](https://coatmentor.com/defects/bugholes/):** 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).

## Related

- **Concepts:** [Zinc-rich primers](https://coatmentor.com/learn/zinc-rich-primers/) · [Holiday (discontinuity) testing](https://coatmentor.com/learn/holiday-testing/)
- **Standards:** [ASTM D4787](https://coatmentor.com/standards/astm-d4787/)

## 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](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.