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Dry film thickness — why it's the number that fails jobs

Reference material, not engineering advice. Every technical claim traces to the cited standard or reference. Technically reviewed August 2026.

Published 2 August 2026 · Core Coatings technical desk

Ask any coating inspector: more jobs get rejected on dry film thickness than on anything else. Not because it is hard to achieve, but because it is the only property on the whole system that gets measured directly, at every point, by someone holding a gauge.

Everything else on an inspection is pass or fail. DFT produces a number, and a distribution of numbers, and the rules for accepting that distribution are more specific than most people realise.

What DFT actually measures

The definition is precise and it is the source of most of the confusion:

Dry film thickness — the thickness of a coating remaining over the peaks of a rough surface when the coating has hardened.

Blast-cleaned steel is not flat. It has a profile of peaks and valleys, typically 40 to 75 µm from major peak to major valley on structural work. A magnetic or eddy-current gauge does not know that. Adjusted on a smooth plate, it measures from somewhere down in the valleys, so every reading on a blasted surface reads high.

ISO 19840 handles this with a correction value subtracted from each instrument reading:

Where the profile is unknown, 25 µm is the default.

That is a 25 µm error on every single reading if it is ignored. On a system specified at 75 µm, that is a third of the film.

ISO 19840 is explicit that if individual readings based on adjustment to a smooth flat steel surface are used without correction values, that method does not conform to the standard. It applies where the nominal dry film thickness is 40 µm or greater.

The three numbers in a specification

• NDFT — nominal dry film thickness. The DFT specified for each coat, or for the whole system, to achieve the required durability.
• Mean DFT. The arithmetic mean of all individual dry film thicknesses in the inspection area.
• Maximum DFT. The highest acceptable dry film thickness above which the performance of the paint or the system might be impaired.

The acceptance rules

Under ISO 19840:

1. The arithmetic mean of all individual dry film thicknesses must be equal to or greater than the NDFT.
2. All individual dry film thicknesses must be equal to or above 80 per cent of the NDFT.
3. Individual readings that fall between 80 per cent of NDFT and NDFT are acceptable, provided the number of those readings is less than 20 per cent of the total taken.
4. No individual reading may exceed the specified maximum dry film thickness. Where no maximum is stated, guidance comes from ISO 12944-5.

Read rule 3 carefully, because it is the one that catches people. Hitting the average is not enough. If more than one reading in five sits in the 80-to-100 per cent band, the area fails even though the mean is fine.

Australian and New Zealand project specifications do not always adopt ISO 19840 wholesale. One published government specification instead requires that, in a multi-coat system, the average DFT of each coat be not less than 80 per cent of the specified nominal and not thicker than 120 per cent of any specified maximum, with the average total system DFT not less than 80 per cent of nominal and not thicker than 200 per cent of nominal — measured to AS 3894.3.

Those two rule sets are genuinely different. Which one applies to your job is a question for your specification, not an assumption.

How many readings

ISO 19840 sets a sampling plan by the size of the inspection area:

Areas above 1,000 m² or m should be divided into smaller inspection areas.

The standard also requires the number to be increased for areas with difficult configuration, and requires additional random measurements on each difficult area — stiffeners, brackets, supports, attached piping — over and above the readings taken across the inspection area.

That is not an administrative detail. Those are precisely the locations where the film is thin.

Where the low readings actually are

If a job is going to fail on DFT, it will fail at:

• Edges, where wet film pulls back as it cures
• Welds, where the profile is different and access is awkward
• Corners and internal angles, which get sprayed twice and read high, or once and read low
• Back faces and blind sides, which get one pass from the wrong angle
• Stiffeners and brackets, which the gun reaches last

The specifications know this. Sharp edges are required to be rounded to a minimum 2 mm radius before blasting, and difficult areas are singled out for extra measurement.

When too thick is the failure

Excess film is a defect in its own right. In one published specification, areas of inorganic zinc silicate applied at excessive thickness, resulting in mud cracking through to the substrate, must be totally removed by abrasive or bristle blasting and repaired the same way as under-thickness areas.

That is a full strip and re-do, on steel that had more paint on it than the specification asked for.

What this means for you

DFT is the one number in the whole process that is entirely inside your control — set by the applicator, the gun, the pass pattern and how carefully the awkward areas get treated. It is also the one number the inspector will take fifty times.

If you are getting inconsistent readings, or you are not sure which acceptance rule your specification is actually running, the Paint Audit asks how you apply and what has gone wrong in the last twelve months, and the read comes back with the questions your answers raise.

References

• ISO 19840:2012, Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces
• FROSIO, Coating Dry Film Thickness (DFT) Measurements and Acceptance Criteria
• NZ Transport Agency, S9 Specification for coating steelwork on highway structures, October 2020
• ISO 8503-1:2012, Preparation of steel substrates before application of paints and related products — Surface roughness characteristics of blast-cleaned steel substrates — Part 1: Specifications and definitions for ISO surface profile comparators
• AS 3894.3-2002, Site testing of protective coatings — Determination of dry film thickness

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