If you're managing a construction or maintenance project that involves protective coatings — whether it's structural steel, a bridge, a tank, or industrial equipment — you've probably heard the term dry film thickness, or DFT. But what does it actually mean, and why should you care whether it's measured correctly?
This guide breaks it down in plain language, so you can make informed decisions about your project.
What Is Dry Film Thickness?
When a coating — paint, primer, or protective finish — is applied to a surface and dries, what remains is called the dry film. Dry film thickness is simply the measurement of how thick that dried layer is, usually expressed in microns (µm).
Most protective coating systems are made up of multiple layers: a primer, an intermediate coat, and a topcoat. Each layer has a specified DFT range set by the coating manufacturer and the project specification. Too thin, and the coating won't provide the protection it was designed for. Too thick, and you can get cracking, solvent entrapment, or adhesion problems down the track.
💡 Rule of thumb: A coating applied at the right thickness performs as tested and warranted. Outside that range — either way — and you're taking on risk the manufacturer hasn't accounted for.
How Is DFT Measured?
DFT is measured using a calibrated electronic gauge — either a magnetic pull-off gauge or, more commonly today, an electronic digital gauge. The inspector places the probe on the coated surface and gets an instant reading in microns.
It sounds simple, but there's a lot that can go wrong:
- Gauges need to be calibrated correctly for the substrate (steel vs aluminium behave differently)
- Readings need to be taken at multiple locations — not just the easy-to-reach spots
- The inspector needs to understand how to interpret a spread of readings against the specification
- Results need to be documented properly so there's a record if issues arise later
Why Does Independent DFT Testing Matter?
Here's the issue that many project managers don't think about until it's too late: the contractor applying the coating has a financial incentive to apply it quickly. Slow down for proper coverage checks, and the job costs more. That's not an accusation — it's just reality.
When the same contractor who applied the coating is also reporting the DFT readings, there's a conflict of interest. An independent inspector has no stake in the outcome of those measurements. Their job is simply to record what's there.
- Independent results carry weight if a dispute arises later
- You get an unbiased record for handover documentation and asset registers
- Issues are caught during application — not years later when corrosion appears
- Coating warranties often require documented independent inspection
When Should DFT Be Checked?
Ideally, DFT should be checked at each coat — not just at the end of the job. By the time the topcoat is on, it's too late to go back and fix a primer that was applied too thin.
For larger projects, a formal inspection hold point should be built into the programme before each coat is applied. For smaller jobs, a final inspection after the full system has cured is the minimum you should expect.
Projects Where DFT Testing Is Especially Important
- Structural steelwork — especially in coastal or industrial environments
- Intumescent (fire-resistant) coatings, where thickness directly affects fire rating
- Water and wastewater infrastructure
- Industrial plant and storage tanks
- Bridges and transport infrastructure
- Any project where the coating system carries a manufacturer's warranty
What Standards Apply in New Zealand?
DFT testing in New Zealand generally follows AS/NZS 1580 test methods, along with international standards such as ISO 19840 for corrosion protection of steel structures. Project specifications will typically reference one of these, along with the coating manufacturer's own data sheet requirements.
A qualified coating inspector — such as one certified under the AMPP (formerly NACE/SSPC) programme — will be familiar with these standards and how to apply them on site. When searching for a coating inspector in NZ, you may see both "AMPP certified" and "NACE certified" used — these refer to the same qualification, as NACE merged with SSPC to form AMPP in 2021.
Summary
Dry film thickness testing is a straightforward but critical quality check. It confirms that the coating system has been applied as specified, giving you confidence that the asset is protected for the long term. When that testing is done by an independent inspector — someone with no connection to the contractor — you get results you can rely on and documentation that holds up if questions are ever raised.