DFT stands for dry film thickness. It’s the thickness of a coating layer after the solvents have evaporated and the film has fully cured — measured in micrometres (µm) or sometimes mils (thousandths of an inch, used mainly in North American specifications).
That’s the short answer. But if DFT were just a measurement, it wouldn’t show up in almost every coating specification ever written. The reason it matters so much is what it represents: the difference between a coating system that performs as specified and one that fails in service — sometimes within months.
Why DFT Controls Everything
Corrosion protection works because a coating film creates a physical barrier between steel and the environment. Thicker film, longer diffusion path for water and oxygen, better protection. It’s not a linear relationship, but the principle holds — and there are diminishing returns at both extremes.
Apply too little DFT and the barrier is compromised. Pinholes and thin spots become the weak points where corrosion initiates. Apply too much — particularly with certain epoxy and intumescent systems — and you get mud-cracking, solvent entrapment, and delamination. Both directions cause failure. This is why coating specifications define both a minimum and a maximum DFT, not just a target.
For a zinc-rich primer, the DFT window is critical because the zinc pigment needs to be at the right density for galvanic protection to work. Too thin and there’s not enough zinc. Too thick and the primer becomes brittle. Similar constraints apply to intumescent coatings, glass flake epoxy systems, and virtually every other specialist coating type.
WFT vs DFT — the Relationship
Wet film thickness (WFT) is what you measure during application — before the solvents evaporate. DFT is what you measure after. The relationship between them depends on the coating’s volume solids content.
The formula is straightforward: DFT = WFT × (volume solids % ÷ 100). So a coating with 60% volume solids applied at 200 µm WFT will give roughly 120 µm DFT. A 100% solids (solvent-free) coating gives a 1:1 ratio — 200 µm WFT becomes 200 µm DFT.
In practice, applicators use a wet film comb gauge during spraying to check WFT in real time. It’s not as accurate as a calibrated DFT gauge on the cured film, but it’s the only in-process check available. Good applicators know the WFT target for their product and check it every few square metres.
How DFT Is Measured
On steel substrates, DFT is measured using a magnetic induction gauge — a handheld device that detects the distance between the probe tip and the steel surface beneath the coating. It’s non-destructive and fast. Calibrate it against uncoated steel first (zero the gauge on bare metal), then measure the coated surface.
The readings you get vary. Even on a well-applied coat, DFT naturally varies across a surface — by 10–20% is normal. This is why specifications define measurement protocols rather than single-point pass/fail.
For a step-by-step field procedure, see the guide to measuring dry film thickness in the field.
SSPC-PA 2: The Standard Measurement Protocol
SSPC-PA 2 is the most widely referenced measurement standard for field DFT inspection. The method requires a minimum number of spot measurements per defined area, with each spot being an average of several individual gauge readings. The protocol distinguishes between readings that are acceptable (within the specified range), low (below minimum but within a tolerable band), and unacceptable (too low to provide specified protection).
Some projects reference ISO 19840 instead — particularly in European and offshore contexts. The principle is the same: multiple readings per area, statistical evaluation, documented results. Which standard applies to your project is usually specified in the coating specification document itself.
DFT in a Multi-Coat System
Most industrial coating systems consist of several coats — primer, intermediate, topcoat — each with its own DFT requirement. The total system DFT is the sum of all individual coats.
| Coat | Typical Function | Typical DFT Range |
| Zinc-rich primer | Galvanic corrosion protection | 50–80 µm |
| Epoxy intermediate (1–2 coats) | Barrier, build, adhesion bridge | 100–200 µm per coat |
| Polyurethane topcoat | UV resistance, weathering, aesthetics | 50–80 µm |
| Total (C5 system example) | — | 300–440 µm |
Each coat needs to be checked individually — not just the final total. An over-thick primer coat doesn’t compensate for a thin topcoat, and vice versa. The individual coats have different functions and different performance requirements.
What Happens When DFT Is Wrong
Low DFT is the more common problem in practice. It usually comes from one of three places: insufficient coating applied during spraying, excessive thinning of the product, or inadequate overlap at spray passes.
The consequences aren’t always immediate. A coating at 80% of specified DFT might look fine and pass a visual inspection. Problems typically show up 18–36 months into service — blistering, rust bleed, or early topcoat failure. By then the cause is hard to trace and the coating has to be replaced.
High DFT — usually from over-application in corners, recesses, or areas with poor spray technique — causes different problems. Solvent entrapment leads to bubbling. Excessive intumescent thickness cracks on thermal cycling. Some epoxy systems develop internal stress cracking if applied too thick per coat. And it wastes material, which on a large project adds up quickly.
A Few Questions Worth Answering Directly
Can I measure DFT through multiple coats at once?
Yes — a magnetic induction gauge reads total DFT above the steel, regardless of how many coats are present. To get individual coat DFTs, you need to measure after each coat. Once a subsequent coat is applied over a previous one, you can only determine the individual coat thickness if you know the previous total. This is why DFT inspection is done at each hold point, not just at the end.
What does ‘nominal’ DFT mean in a specification?
Nominal DFT is the target thickness — what the system is designed to be applied at. It’s not the minimum; it’s the intended mid-point. Most specifications allow a spread around nominal — something like -10% to +25% — with the understanding that local variation is unavoidable. The minimum acceptable DFT (sometimes called MDFT or minimum DFT) is the absolute floor below which no reading should fall.
Does DFT matter differently for different coating types?
Yes, and this is where it gets specific. For intumescent coatings, DFT is directly linked to fire resistance period — the calculation is precise and product-specific. For zinc-rich primers, DFT affects the galvanic protection mechanism. For glass flake epoxy, DFT affects osmotic blistering resistance. Each coating type has a distinct performance-to-thickness relationship, which is why generic ‘apply more for better protection’ thinking doesn’t always apply.
Related Reading
- How to measure dry film thickness in the field: How to Measure Dry Film Thickness
- Steel structure coating inspection checklist — surface prep, DFT hold points, recoat intervals, and touch-up procedures.
- How to calculate intumescent coating thickness — section factor, fire curve, and DFT table lookup explained step by step.
- Surface preparation for industrial coatings — ISO 8501, SSPC standards, and practical selection guide.
Send your steel schedule, coating system, and inspection requirements via the project inquiry form and our technical team will advise on DFT specification and system selection.
