# ISO 29601

Source: https://coatmentor.com/standards/iso-29601/ · Updated 2026-10-14

Assessment of porosity (pinholes and holidays) in a dry paint film.

**Publisher**: ISO

**Area**: Adhesion, cure and holidays

ISO 29601 sets how to find porosity, meaning pinholes, holidays and other discontinuities, in protective paint systems on steel and other conductive metals. A second edition came out in 2026 and replaced the 2011 edition.

## Low or high voltage

The choice depends on the mean dry film thickness of the non-conductive layers. A low-voltage wet-sponge detector is the default for thinner films, and a high-voltage DC detector for thicker ones. The 2026 edition changed how the high-voltage test voltage is calculated, in line with the recent ASTM and AMPP holiday-testing standards.

## The trap

Only the non-conductive layers count. A zinc-rich primer, galvanizing or thermally sprayed metal under the paint conducts, so its thickness is left out of the thickness that sets the method and the voltage.

## 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/) · [ASTM D4787](https://coatmentor.com/standards/astm-d4787/)
- **Defects:** [Holidays and misses](https://coatmentor.com/defects/holidays/)

## About this page

This page describes the subject of ISO 29601 in our own words; it does not reproduce the standard. For the full text, buy the current edition from ISO. 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.