Coating Porosity — ISO 29601 / NACE SP0188 / ASTM G62

What Is Coating Porosity?

Coating porosity (holiday/pinhole) test is an experiment in which pores and discontinuities in the insulating coating on the conductive sub-surface are determined with a low-voltage wet sponge or high-voltage spark detector.

Purpose of Coating Porosity

The purpose is to verify that there are no discontinuities in the coating that will lead to corrosion.

Which Products Is Coating Porosity Applied To?

External pipeline coatings
Tank and container inner coatings
Marine structures coatings
Protective coatings on concrete

In Which Sectors Is Coating Porosity Used?

AutomotiveMetalConstructionMachinery

Standards Related to Coating Porosity

ISO 29601
NACE SP0188
ASTM G62
ASTM D5162

Coating Porosity Conditions

Method: Low voltage wet sponge (typically on thin coatings) or high voltage spark detector (on thick coatings)
Gerilim: Defined in the standard according to coating thickness; excessive voltage may damage the coating
Scanning speed: Constant speed defined in the standard
Calibration: Verified by measuring device voltage

Equipment for Coating Porosity

Wet sponge holiday detector
High voltage spark detector
Coating-thickness gauge
Voltage verification scheme

Sample Requirements for Coating Porosity

Coating thickness must be measured; voltage is selected accordingly
The coating must be fully cured and dry.
Subsurface grounding must be provided
Any discontinuities found should be marked and repaired.

Duration of Coating Porosity

Hours to days depending on surface area.

Coating Porosity Procedure

1Coating thickness is measured.
2Method and voltage are selected; The device is verified.
3The lower surface is grounded.
4The surface is scanned at a defined speed.
5Discontinuities are marked and counted.
6After repair, the scan is repeated.

Coating Porosity Acceptance Criteria

The acceptance criterion is given in the specification: generally no discontinuities are allowed and those found are repaired.

Evaluation of Results

Excessive stress according to the thickness creates new damage to the coating; Voltage selection should be made according to the standard. The wet sponge method misses discontinuities in thick coatings, and the high voltage method is required in thick systems.

Coating Porosity Report

Standard and method applied
Coating thickness
Voltage used and its verification
Taranan alan
Number of discontinuities found and repair record

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Pipeline and tank coating specifications

Paint and Coating Testing — What Does This Testing Field Cover?

The section below describes the field that the Coating Porosity test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Paint and coating testing measures the adhesion of a coating to the surface, its mechanical durability and its resistance to environmental effects. Because a coating is a thin layer, the results depend both on the material and on the QUALITY OF APPLICATION; the same paint gives an entirely different result over a different surface preparation.

This dual dependence is the most important knowledge in the field. A coating failure usually arises not from the paint itself but from the surface preparation: oil residue, insufficient roughening, an oxide layer or moisture. Coating tests therefore assess not only the product but also the process, and the results are used for process control.

COATING THICKNESS is a recurring variable in the field. Many durability results relate directly to thickness; a thin area becomes the starting point of corrosion. Thickness measurement is therefore reported routinely alongside the other tests and is usually taken at several points — thickness falls at edges and corners, especially on complex geometries.

Three approaches are used on the adhesion side, each giving different information. The tape test is fast and suitable for field use but is a coarse scale. The cross-cut test shows behaviour at the intersections of layers. The pull-off method produces a numerical strength value and is the most suitable for comparison.

On the mechanical side, flexibility (mandrel bending), impact resistance, hardness, abrasion and stone chip resistance are measured. Stone chipping has a special place in automotive: gravel striking the coating under real road conditions is simulated, and the result is assessed according to whether the coating continues to protect the base metal.

On the appearance side, gloss and colour difference are monitored NUMERICALLY. Colour is assessed with a colour measuring instrument rather than by eye and the colour difference is expressed as a number; this makes it possible to compare different lots and different suppliers.

On the durability side, accelerated ageing (xenon arc, fluorescent UV), humidity, condensation and salt spray are applied. The result is assessed with scales for blistering, rusting, creep from a scribe and loss of adhesion; these scales are defined in separate standards and the report must state which was used.

In industrial processes such as powder coating and electrophoretic coating, the curing cycle (temperature and time) is what determines the result. An under-cured coating looks good but has poor chemical resistance and adhesion; oven profile measurement is therefore part of coating quality control.

Method groups in this field

Thickness
Dry film thickness by magnetic and non-magnetic methods; multi-point measurement and assessment of the distribution.
Adhesion
Tape test, cross-cut test and pull-off adhesion strength.
Mechanical strength
Mandrel bend flexibility, rapid deformation (impact), pencil and scratch hardness, Taber abrasion, stone chipping.
Appearance
Gloss measurement, colour difference calculation, orange peel and surface defect assessment.
Environmental durability
Salt spray, humidity, condensation, xenon arc and fluorescent UV ageing, chemical and stain resistance.
Assessment scales
Scales for blistering, degree of rusting, creep from a scribe and loss of adhesion.

Commonly confused in this field

A coating result depends on the surface preparation; even if the paint itself is good, a poor preparation gives a poor result.
A durability result without a reported thickness cannot be compared.
The tape test is a coarse scale; numerical comparison requires the pull-off method.
Colour cannot be assessed by eye; numerical colour difference measurement is required.
An ageing result without a stated assessment scale cannot be interpreted.
If the curing cycle is incomplete the coating looks good but its durability is poor; the oven profile is part of quality control.

Which decisions these results support

Paint system approval and supplier selection
Process control of the paint line and lot acceptance
Supporting outdoor and automotive durability claims
Detecting defects arising from surface preparation
Monitoring colour and gloss consistency lot by lot

Related Tests

Frequently Asked Questions About Coating Porosity

According to which standard is Coating Porosity made?

Coating Porosity is executed in scope ISO 29601, NACE SP0188, ASTM G62 and ASTM D5162. The limit values and acceptance criteria to be applied are defined in the product standard; The version valid for your product is confirmed in writing at the offer stage.

Which products does Coating Porosity apply to?

Mainly applied to External pipeline coatings, Tank and container inner coatings, Marine structures coatings and Protective coatings on concrete. Whether your product is included in the scope is determined by examining its technical specifications and intended use.

Under what conditions is Coating Porosity done?

The determining parameters are: Method — Low voltage wet sponge (typically on thin coatings) or high voltage spark detector (on thick coatings), Gerilim — Defined in the standard according to coating thickness; excessive voltage may damage the coating and Scanning speed — Constant speed defined in the standard. The full terms are in the "Test Conditions" section on the page and vary depending on the product standard.

What kind of sample is needed for Coating Porosity?

Coating thickness must be measured; voltage is selected accordingly, The coating must be fully cured and dry. and Subsurface grounding must be provided. The number of samples is determined according to the product standard and the scope to be reported.

How long does Coating Porosity take?

Hours to days depending on surface area. This duration belongs to the experiment itself; sample preparation, queuing and reporting are also evaluated. A realistic schedule for your product is given in the offer.

What is the Coating Porosity result called "appropriate"?

The acceptance criterion is given in the specification: generally no discontinuities are allowed and those found are repaired.