ISO 19840

Introducing ISO 19840 – Industrial Physics

ISO 19840 is the international standard that sets out how to measure – and how to accept or reject – the dry film thickness (DFT) of protective paint systems applied to rough steel surfaces. It is the go-to standard for corrosion-protection coatings on blast-cleaned steel, where the roughened surface profile makes a straight gauge reading misleading.

Rather than defining a new measuring instrument, ISO 19840 defines the procedure around the measurement: how to adjust the gauge, how to correct for surface roughness, how many readings to take and where, and the rules for deciding whether a coated structure passes. It gives quality, R&D, and operations experts a consistent, defensible way to confirm a protective coating has been applied to specification.

What ISO 19840 Covers

ISO 19840 specifies a procedure for verifying dry film thickness against the nominal (specified) dry film thickness on rough surfaces. Key elements of the method include:

• Adjustment of the coating thickness gauge at zero and at a known thickness on a smooth steel surface
• Application of a correction value to allow for surface roughness, since readings on a rough profile read higher than the true thickness above the peaks
• Definition of inspection areas and sampling plans – how many measurements to take and how they are distributed
• Clear acceptance and rejection criteria based on minimum and maximum specified levels
• Use of permanent-magnet, electromagnet, and eddy-current type gauges

The standard applies only where the nominal dry film thickness is 40 micrometres (µm) or greater. It supplements the ISO 12944 series on corrosion protection by protective paint systems, and works alongside the base measurement methods in ISO 2808.

The current version is ISO 19840:2012 (the second edition, published September 2012), which cancelled and replaced the 2004 first edition. It was reviewed and confirmed in 2023, so it remains current. It’s worth confirming the current revision at the time of publishing, as ISO reviews its standards on a regular cycle.

Why Dry Film Thickness on Rough Steel Matters

For steel structures – bridges, offshore platforms, storage tanks, pipelines, and heavy plant – the protective paint system is the main defence against corrosion. If the coating is too thin, corrosion protection and service life are compromised. If it’s too thick, material is wasted and the coating may crack or lose adhesion. On blast-cleaned steel, the rough surface profile means a raw gauge reading overstates the real coating thickness, so a standardised correction and acceptance procedure is essential to judge the job fairly.

A robust DFT verification helps quality, R&D, and operations experts to:

• Confirm protective coatings meet the specified thickness for the required corrosion-protection life
• Judge large, complex structures consistently using defined inspection areas and sampling
• Settle accept or reject decisions with clear, agreed criteria rather than opinion
• Control coating usage and cost by avoiding both under- and over-application
• Provide auditable evidence for contract, warranty, and specification compliance

Because coating thickness on rough steel is easy to dispute but hard to argue with once it’s measured and corrected the agreed way, ISO 19840 protects the integrity of both the coating and the customer relationship – particularly across protective coatings, marine, oil and gas, and heavy industrial applications.

How ISO 19840 Testing Works

A non-destructive coating thickness gauge is first adjusted to zero and to a known thickness on a smooth, flat steel reference. Because the real structure has a rough, blast-cleaned profile, every reading taken on it will sit higher than the true thickness above the surface peaks – so a correction value is subtracted from the readings to give the ISO 19840 dry film thickness.

Measurements are then taken across defined inspection areas following the standard’s sampling plan, rather than at a few random spots. The set of corrected readings is assessed against the specified minimum and maximum thickness using the standard’s acceptance rules, giving a clear pass or fail for the area. Taking readings this way – adjusted, corrected, and sampled to a defined plan – is what makes the result repeatable and defensible.

ISO 19840 Testing Equipment

Industrial Physics offers a coating thickness gauge from TQC Sheen that supports ISO 19840 dry film thickness measurement on rough steel surfaces.

TQC Sheen PosiTector 6000 Coating Thickness Gauge

The TQC Sheen PosiTector 6000 is a rugged, fully electronic coating thickness gauge that measures coatings on both ferrous and non-ferrous metals. The Advanced model includes a dedicated ISO 19840 compliance mode, which analyses a larger area using the statistical approach the standard requires – making it well suited to protective-coating inspection on steel structures.

Typical features include:

• A built-in ISO 19840 mode on the Advanced model for statistical area analysis, plus SSPC-PA 2 and PSPC 90/10 compliance modes
• Magnetic and eddy-current measurement, with automatic recognition of ferrous and non-ferrous substrates
• Scan modes for fast, continuous readings across large areas
• Rugged, weatherproof design with built-in temperature compensation for reliable readings in the field
• WiFi and Bluetooth connectivity and real-time graphing on the Advanced model for straightforward reporting

The PosiTector 6000 conforms to a wide range of national and international standards, including ISO 19840, ISO 2178, ISO 2360, and ISO 2808, along with ASTM methods such as D7091 and E376. If you’re not sure which model or probe best fits your structures and coatings, our experts can help guide you to the right choice.

Your Global Test and Measurement Partner for ISO 19840

Industrial Physics supports quality, R&D, and operations experts around the world with the paint and coating test and measurement expertise needed to work confidently with standards like ISO 19840. Whether you’re inspecting corrosion protection on a steel structure, settling an accept or reject decision, or controlling coating usage on a large contract, we can help you achieve accurate, repeatable, and reliable dry film thickness results.

Get in touch to talk through your dry film thickness measurement requirements.