ASTM D7091

Introducing ASTM D7091 – Industrial Physics

ASTM D7091 is the ASTM standard practice for non-destructive measurement of dry film thickness (DFT) of coatings on metal. Uniquely, it covers both of the common measurement techniques in a single document: the magnetic method for non-magnetic coatings on ferrous metals like steel, and the eddy-current method for non-conductive coatings on non-ferrous metals like aluminium.

Rather than defining a brand-new instrument, ASTM D7091 sets out how to use these coating thickness gauges properly – how to verify their accuracy, how to adjust them, how often to measure, and how to report results. It gives quality, R&D, and operations experts a consistent, repeatable way to confirm a coating has been applied to specification, whatever the metal underneath.

What ASTM D7091 Covers

ASTM D7091 describes the use of magnetic and eddy-current gauges for dry film thickness measurement. Key elements of the practice include:

• Non-destructive measurement of non-magnetic coatings on ferrous metals using magnetic gauges
• Non-destructive measurement of non-conductive, non-magnetic coatings on non-ferrous metals using eddy-current gauges
• Gauge types covered, including Type 1 magnetic pull-off and Type 2 electronic gauges
• Procedures and recommended frequency for verifying gauge accuracy and for adjusting or optimising the equipment
• The advantages and limitations of coating thickness gauges, plus reporting recommendations

The practice is not intended for coatings that deform easily under the probe, and it notes that metal-filled coatings can give false readings. For how many measurements to take across an area, it points to SSPC-PA 2, and it’s the governing project specification that sets the minimum and maximum thickness for each layer and for the total system.

The current version is ASTM D7091-22 (approved June 2022), which replaced the 2021 edition. It was originally approved in 2005 and sits under ASTM Committee D01 on Paint and Related Coatings. It’s worth confirming the current revision at the time of publishing, as ASTM revises its standards periodically.

Why Coating Thickness Matters

Dry film thickness is one of the most important checks in any coating operation. Too little coating and the part may not be properly protected against corrosion or wear. Too much and you waste expensive material, risk cracking or poor adhesion, and add unnecessary cost. Many high-performance coating warranties depend, in part, on achieving the correct thickness for each layer and for the total system – so being able to measure and prove it matters commercially as well as technically.

A reliable DFT measurement helps quality, R&D, and operations experts to:

• Confirm coatings meet the specified thickness for corrosion protection and durability
• Measure confidently on both ferrous and non-ferrous parts using one consistent practice
• Support coating warranties and meet customer, contract, and specification requirements
• Control coating usage and cost by avoiding both under- and over-application
• Provide auditable, repeatable evidence backed by verified, correctly adjusted gauges

Because coating thickness is easy to get wrong by eye but simple to prove with a properly verified gauge, a standardised practice like ASTM D7091 protects the integrity of both the coating and the customer relationship – across protective coatings, automotive, marine, industrial, and general metal finishing.

How ASTM D7091 Testing Works

ASTM D7091 works with non-destructive coating thickness gauges that measure the gap between the gauge probe and the base metal, and display that gap as coating thickness. On ferrous metals, a magnetic gauge senses the change in magnetic attraction or field caused by the coating holding the probe away from the steel. On non-ferrous metals, an eddy-current gauge uses a high-frequency field to sense the distance to the conductive substrate. In both cases, nothing is cut or removed, so the part stays fully usable.

Central to the practice is gauge accuracy: the standard describes how to verify a gauge against known reference standards and adjust it before and during use, and how often to check it. Measurements are then taken and reported following the practice’s recommendations, with measurement frequency across an area guided by SSPC-PA 2. Verifying, adjusting, and reporting this way is what makes the result repeatable and defensible.

ASTM D7091 Testing Equipment

Industrial Physics offers a coating thickness gauge from TQC Sheen that supports ASTM D7091 dry film thickness measurement on both ferrous and non-ferrous metals.

TQC Sheen PosiTector 6000 Coating Thickness Gauge

The TQC Sheen PosiTector 6000 is a rugged, fully electronic coating thickness gauge that fits ASTM D7091 particularly well, because the practice covers both measurement methods and the gauge uses both. It applies the magnetic method on ferrous substrates like steel and cast iron and the eddy-current method on non-ferrous metals like aluminium and copper, automatically recognising the substrate and measuring accordingly.

Typical features include:

• Magnetic and eddy-current measurement in one gauge, with automatic substrate recognition
• Suitable for a wide range of coatings, including paint, powder coatings, galvanizing, anodizing, and plating
• A wide range of interchangeable Ferrous, Non-Ferrous, and Combination probes
• Standard and Advanced models, with the Advanced model adding a colour touchscreen, real-time graphing, and compliance modes for SSPC-PA 2, ISO 19840, and PSPC 90/10
• Rugged, weatherproof design with built-in temperature compensation for reliable readings in the field

The PosiTector 6000 conforms to a wide range of national and international standards, including ASTM D7091, ASTM B499, and ASTM E376, alongside ISO 2178, ISO 2360, ISO 2808, and ISO 19840. If you’re not sure which model or probe best fits your substrates and coatings, our experts can help guide you to the right choice.

Your Global Test and Measurement Partner for ASTM D7091

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 ASTM D7091. Whether you’re checking corrosion protection on steel, measuring a coating on an aluminium part, or controlling paint usage on a production line, we can help you achieve accurate, repeatable, and reliable dry film thickness results.

Get in touch to talk through your coating thickness measurement requirements.