# Pinholes

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

Tiny holes through the film, often in groups, that can reach the substrate.

*Figure: Pinholes: the surface as you see it, and a schematic cross-section.*

## Causes

- Air or solvent escaping from a porous surface such as inorganic zinc or thermal spray (and concrete for CIP 2).
- A full wet coat over a porous primer without a mist coat.
- Wrong thinner or spray technique; very hot steel.

## Prevention

- Apply a mist coat over porous primers, then the full coat.
- Use the specified thinner and keep the steel within the temperature limits.

## How it is assessed

Visual inspection with a light at a low angle; holiday testing where the specification asks for it.

## Telling it apart

- **[Outgassing](https://coatmentor.com/defects/outgassing/):** Outgassing pinholes gather over porous areas and form after application. Pinholes from spray technique are spread more evenly.
- **[Cratering (fish eyes)](https://coatmentor.com/defects/cratering/):** Craters are wider dimples with a rim, from contamination.
- **[Holidays and misses](https://coatmentor.com/defects/holidays/):** A pinhole that reaches the substrate is a holiday, and the holiday test finds it.

## Repair

Abrade and apply another coat, or fill the pinholes as the specification and data sheet say; re-test linings. The specification and the coating manufacturer have the final word on the repair procedure.

**29** practice items in the members area mention pinholes (CIP 1: 9 · CIP 2: 9 · FROSIO written: 11).

## Related

- **Concepts:** [Holiday (discontinuity) testing](https://coatmentor.com/learn/holiday-testing/) · [Zinc-rich primers](https://coatmentor.com/learn/zinc-rich-primers/)
- **Standards:** [NACE SP0188](https://coatmentor.com/standards/nace-sp0188/) · [ASTM D5162](https://coatmentor.com/standards/astm-d5162/)

## 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.