Dry Film Thickness (DFT) Measurement

Every coating system is designed around a target thickness. Corrosion resistance, chemical resistance, cure behaviour, coverage and cost all depend on getting it right. Dry film thickness (DFT) measurement is how applicators control film build during application and how inspectors confirm the finished work meets the specification.

Thickness is expressed in mils (thousandths of an inch) in North America and micrometres (µm) elsewhere; 1 mil equals 25.4 µm, so a 10-mil coat is 254 µm.

Why film thickness matters

A film that is too thin may not cover the peaks of the surface profile, leaves less barrier between the substrate and the environment, and is more prone to pinholes and early rusting. A film that is too thick can trap solvent, sag, cure slowly or crack. Inorganic zinc-rich primers are a classic example: applied well above the recommended range, they are prone to mud cracking. Over-application also wastes material and can push a job over budget.

Predicting DFT from wet film thickness

Applicators check wet film thickness (WFT) during application with a notch gauge (ASTM D4414), pressing it into the fresh film and reading the last wetted tooth. Wet and dry thickness are linked by the coating’s volume solids:

• DFT = WFT × volume solids (as a decimal)
• With thinner added: WFT = DFT × (1 + thinner fraction) ÷ volume solids

For example, a coating with 60% volume solids and a target of 6 mils (150 µm) DFT needs about 10 mils (254 µm) wet. Thinned 10% by volume, the same target needs about 11 mils (280 µm) wet. The calculators in the Coatingpedia tools section can run this arithmetic for any product. Wet readings are a control tool, not an acceptance test — final acceptance is always based on dry measurements.

Gauge types and when to use them

ASTM D7091 covers non-destructive magnetic and eddy-current gauges on metal substrates, and ISO 2808 describes the full range of film-thickness methods. SSPC-PA 2 groups magnetic gauges as Type 1 (magnetic pull-off) and Type 2 (electronic).

Calibration, verification and adjustment

Three separate steps keep a gauge honest, and they are often confused:

• Calibration is done by the manufacturer or an accredited laboratory against traceable standards, with a certificate and a recalibration interval.
• Verification of accuracy is done by the user, typically at the start and end of each shift, by measuring coated thickness standards that bracket the expected DFT.
• Adjustment aligns the gauge to the actual job surface. On blasted steel, the gauge “sees” partway into the profile, so readings on a rough surface include a profile effect.

Under SSPC-PA 2, the profile effect is handled either by measuring a base metal reading (BMR) on the prepared, uncoated steel and deducting it, or by adjusting a Type 2 gauge with plastic shims placed on the prepared surface. ISO 19840 allows fixed correction values when no uncoated reference area is available — 10 µm for a fine profile, 25 µm for medium and 40 µm for coarse.

Sampling plans and acceptance rules

Individual readings vary across any real surface, so standards define how many readings to take and how much scatter is acceptable.

SSPC-PA 2 also defines several coating thickness restriction levels, so a specification can tighten or loosen the tolerance from the default. Under ISO 12944-5 practice, maximum DFT is commonly limited to around three times the nominal DFT (NDFT) unless the manufacturer states otherwise. For areas under PA 2, surfaces up to 300 ft² (about 28 m²) are measured in every 100 ft² section; larger surfaces are sampled by randomly selected 100 ft² areas.

Taking measurements step by step

1. Check the gauge. Confirm the calibration certificate is current and verify accuracy on thickness standards near the target DFT.
2. Adjust for the surface. Record base metal readings or adjust with shims on the prepared, uncoated substrate.
3. Confirm the film is ready. Measure only when the coating is hard enough not to be indented by the probe; soft films read low.
4. Measure systematically. Follow the required sampling plan, keeping the probe perpendicular and away from edges, holes and corners.
5. Record and report. Log spot or reading values, locations, gauge serial number and verification results, and flag non-conforming areas for correction.

Concrete and other non-metallic substrates

Magnetic and eddy-current gauges cannot measure coatings on concrete floors, wood or plastics. Ultrasonic gauges are the usual non-destructive option, but rough or porous substrates make the coating–substrate interface less distinct and readings less precise. On concrete, applicators often control thickness by spread rate — measuring the area covered by a known volume — backed by wet film checks and occasional destructive verification. Small test chips or cut sections examined under magnification can also confirm the thickness of each layer.

Frequently asked questions

Should DFT be measured on each coat or only on the finished system?

Best practice is to measure after each coat. Measuring only the total cannot tell you whether the primer or intermediate coat was thin, and those layers often carry the main corrosion-protection function.

Why do my readings on blasted steel seem too high?

Most likely the gauge has not been adjusted for the surface profile. Deduct a base metal reading or adjust the gauge with shims on the prepared surface, as SSPC-PA 2 describes.

Is a thick coating always better protected?

No. Exceeding the manufacturer’s maximum can cause solvent entrapment, cracking, poor cure and adhesion loss. Stay within the range on the product data sheet.

Does DFT measurement replace holiday testing?

No. A film can meet thickness requirements and still contain pinholes. Linings for immersion service are commonly checked by holiday testing as well.