Frequently asked questions

Dry film thickness, or DFT, is the thickness of a coating measured above the substrate once the coating has dried and cured. It can describe a single layer or the combined build of multiple layers, and the figure you get depends on the application method and the type of process used to apply the coating.

Wet film thickness, or WFT, is the other side of the same measurement. It is the thickness of the paint while it is still liquid, before the solvents and other liquid content evaporate. DFT is what remains once all of that liquid has gone, so the two readings are related but never equal.

The distinction matters in practice. WFT can only be checked while the coating is being applied, which makes it the applicator's control reading. DFT is measured on the cured film and is the figure a specification is judged against.

For cured films on metal, a digital gauge such as the Elcometer 456 Coating Thickness Gauge gives reliable, accurate and repeatable DFT readings and is the standard instrument for coatings inspection and quality control work. For the wet reading, a wet film comb such as the Elcometer 154 is used as the paint goes on.

A DFT gauge, also called a DFT meter or coating thickness gauge, measures the thickness of a dry coating on a metal substrate without cutting into it. Two measurement principles do the work: electromagnetic induction on ferrous (magnetic) substrates such as steel, and the eddy current principle on non-ferrous metals. Many modern gauges combine both, so one instrument covers both substrate types.

For most inspection and quality control work a digital gauge is the default choice. The Elcometer 456 Coating Thickness Gauge is the benchmark here, built for reliable, accurate and repeatable readings across coatings inspection, manufacturing and QC. Where the substrate is shot or grit blasted steel, the Elcometer 456 IPC Industrial Protective Coating Thickness Gauge is designed specifically for that surface.

Electronic instruments are not always practical. In inflammable atmospheres such as oil and gas production, or for underwater inspection, a mechanical gauge is the safer tool. The Elcometer 211 Magnetic Coating Thickness Gauge, widely known as the banana gauge, remains one of the most popular mechanical options for exactly those conditions.

Dry film thickness is measured non-destructively with a digital coating thickness gauge, and the same gauge approach works on both magnetic steel and non-magnetic metals such as stainless steel or aluminium. What changes is the measurement principle the gauge applies.

• On magnetic substrates such as steel, the gauge uses electromagnetic induction to measure non-magnetic coatings.
• On non-ferrous metal substrates, the eddy current principle measures non-conductive coatings.

In practice the inspector places the probe on the cured coating and reads the thickness directly. A gauge such as the Elcometer 456 Coating Thickness Gauge is built for this: reliable, accurate and repeatable readings, designed for coatings inspection, manufacturing and quality control. Where electronic instruments cannot be used, the mechanical Elcometer 211 covers conditions such as inflammable atmospheres and underwater inspection.

For recording and reporting, ElcoMaster data management software collects readings from the gauge and is downloadable free of charge, which keeps the measurement record as defensible as the measurement itself.

Coating thickness splits into two measurements: wet film thickness, taken while the paint is still liquid, and dry film thickness, taken once the coating has cured. Both sit at the centre of the quality control process for any coated asset.

The required thickness is not the applicator's choice. It is usually specified by the paint manufacturer, the asset owner or an appointed third party, who set out the paint system and the specifications it must meet. The inspector's job is to prove the applied coating matches that specification.

The instruments range from simple to sophisticated to match the job. A wet film comb such as the Elcometer 154 checks the film as it is applied, while a digital gauge such as the Elcometer 456 Coating Thickness Gauge measures the cured film with the accuracy and repeatability that specification work demands.

Dry film thickness, or DFT, is the thickness of a coating measured above the substrate once the coating has dried and cured. It can describe a single layer or the combined build of multiple layers, and the figure you get depends on the application method and the type of process used to apply the coating.

Wet film thickness, or WFT, is the other side of the same measurement. It is the thickness of the paint while it is still liquid, before the solvents and other liquid content evaporate. DFT is what remains once all of that liquid has gone, so the two readings are related but never equal.

The distinction matters in practice. WFT can only be checked while the coating is being applied, which makes it the applicator's control reading. DFT is measured on the cured film and is the figure a specification is judged against.

For cured films on metal, a digital gauge such as the Elcometer 456 Coating Thickness Gauge gives reliable, accurate and repeatable DFT readings and is the standard instrument for coatings inspection and quality control work. For the wet reading, a wet film comb such as the Elcometer 154 is used as the paint goes on.

Dry film thickness, or DFT, is the thickness of a coating measured above the substrate once the coating has dried and cured. It can describe a single layer or the combined build of multiple layers, and the figure you get depends on the application method and the type of process used to apply the coating.

Wet film thickness, or WFT, is the other side of the same measurement. It is the thickness of the paint while it is still liquid, before the solvents and other liquid content evaporate. DFT is what remains once all of that liquid has gone, so the two readings are related but never equal.

The distinction matters in practice. WFT can only be checked while the coating is being applied, which makes it the applicator's control reading. DFT is measured on the cured film and is the figure a specification is judged against.

For cured films on metal, a digital gauge such as the Elcometer 456 Coating Thickness Gauge gives reliable, accurate and repeatable DFT readings and is the standard instrument for coatings inspection and quality control work. For the wet reading, a wet film comb such as the Elcometer 154 is used as the paint goes on.

Wet film thickness is measured by the applicator, during application, using a wet film comb or gauge. Timing is the critical part: the reading must be taken before significant solvent evaporation occurs, so that it genuinely represents the calculated wet film the specification is based on.

The comb is the everyday tool. The Elcometer 154 Wet Film Combs cover routine checks, while the Elcometer 115 Stainless Steel Wet Film Combs are made to last, supplied in metric or imperial, with four thickness ranges up to a maximum of 1250 micrometres, ten measurement steps per comb and an accuracy of 5% or 2.5 micrometres, whichever is greater.

Where more precision is needed, the Elcometer 3230 Wet Film Wheel is the step up. It consists of three wheels, with the smaller central wheel set eccentric to the two outer wheels, and gives a high precision reading that is easy to take as the coating goes on.

Specifications such as SSPC PA2 group dry film thickness gauges into two types. A Type 1 gauge is a mechanical instrument: a calibrated magnet is pulled away from the coated steel and the force needed to break contact indicates the thickness. The Elcometer 211, the well known banana gauge, is the classic example. It needs no batteries, works in explosive atmospheres where electronics are not permitted, and survives site conditions that would kill an electronic instrument.

A Type 2 gauge is electronic. It measures with an electromagnetic induction probe on steel, or an eddy current probe on non-ferrous metals such as aluminium, and reads to a far finer resolution. Modern Type 2 gauges such as the Elcometer 456 store thousands of readings, calculate the statistics your specification asks for and transfer data straight into inspection reports.

For specification work the Type 2 gauge is the usual choice because the readings are logged and repeatable. Keep a Type 1 gauge in the kit for hazardous areas, underwater work and quick checks where a battery or a fragile probe is a liability.

Verification is the routine you do yourself. Before each shift, and again whenever the gauge has been dropped or the readings look suspect, check the gauge against calibration foils or coated standards on a surface similar to the work. Most coating specifications require this check and expect it to be recorded in the inspection report. Adjusting the gauge to the substrate, a zero plus foil adjustment on bare blasted steel for example, is part of the same routine.

Calibration is the formal, traceable event. The gauge and its probe are checked against reference standards in a controlled environment and issued with a calibration certificate that third party inspectors will ask to see. The interval is set by your quality system; a year is the common default, shortened for gauges that live on site or take heavy use.

A gauge that fails verification between calibrations should come out of service, whatever its certificate says. Foil sets are inexpensive insurance against a day of unusable readings.

Three standards dominate dry film thickness work on coated steel. ISO 19840 governs measurement on rough surfaces, applies correction values for the blast profile and sets out sampling plans and acceptance criteria. SSPC PA2 is the North American counterpart, defining gauge types, spot measurements and the tolerance each must meet. ASTM D7091 describes the measurement practice itself for both ferrous and non-ferrous substrates.

They differ in detail: how many readings make a spot, how spots are spread over an area, what correction is applied for surface profile, and what counts as a pass. A reading plan that satisfies one will not automatically satisfy another, so confirm which standard your specification invokes before measuring rather than after.

Modern gauges hold these standards as counted batch modes, prompting for the right number of readings and producing the statistics the standard asks for.

Destructive DFT testing always carries a level of uncertainty, and a textured profile adds to it. Several choices the inspector makes feed directly into the final reading:

• the choice of blade
• the position of the cut
• the point along the cut where the measurement is taken
• the factor used to convert one reticule division to micrometres

The accepted way to manage this is averaging. At least three readings should be taken along a single cut, and the average DFT for each layer is then taken as the DFT for that area.

The surface profile itself also influences the result. On steel the effect is considered reasonably negligible, but on concrete the range from peak to valley can be significant, which is exactly why several readings along the cut length are averaged.

The standard instruments for this work are the Elcometer 121/4 Paint Inspection Gauge, a hand-held gauge for quick, versatile coating examination, and the Elcometer 141 Paint Inspection Gauge, which determines paint thickness on both single and multiple layer coatings. On concrete, where profile uncertainty is at its worst, the Elcometer 500 measures coating thickness non-destructively as an alternative.

All coating systems degrade with age to some extent, and acrylics are no exception. They are generally more resistant to UV and chemical exposure than many other chemistries, but different acrylic formulations age differently, so two acrylic systems of the same age will not necessarily have degraded by the same amount.

The mechanism that lowers the DFT reading is chalking. Acrylics, like epoxy-based systems, chalk to a degree over a prolonged period, and that surface loss means a measurement taken several years after application can read lower than the original film thickness.

Chalking is not the only ageing concern. Cracking of the film is the more serious finding, because judging the degree of degradation is not a simple task and environmental factors play a significant part in how a given system has aged.

The practical takeaway for asset owners: a DFT survey on an aged coating measures what is left, not what was applied. Periodic measurement with a non-destructive gauge such as the Elcometer 456 Coating Thickness Gauge tracks that loss over time and flags a system that is approaching the end of its service life.

Not realistically, provided the instructions are followed. The method leaves little room for steering a result: the inspector must use the correct blade, set the correct resolution, and choose a blade whose range sits closest to the total expected thickness. Within those rules, the cut shows what the cut shows.

In fact the method works against anyone trying to hide thin work. A cross-section cut makes it quite easy to determine whether a contractor has applied one coat instead of two, because the layers are visible in the cut itself.

Destructive DFT is a recognised inspection technique. It is acknowledged to be less accurate than other methods, but it remains widely used in situations where other techniques cannot be applied. The instruments built for it are the Elcometer 121/4 Paint Inspection Gauge, a hand-held gauge for quick, versatile coating examination and measurement, and the Elcometer 141 Paint Inspection Gauge, which determines paint thickness on both single and multiple layer coatings.

Surface profile is the peak-to-valley height of a prepared surface, measured with a surface profile gauge. It is distinct from surface roughness, which combines the profile measurement with the frequency of peaks across a linear length, known as the peak count.

Profile is worth measuring because it determines how a coating performs. The degree of profile on a surface affects adhesion, coverage and the overall volume of coating used. If the profile is too large, the amount of coating required increases and the peaks risk remaining uncoated, which allows rust spots to occur. If it is too small, there may be insufficient key for adequate adhesion, leading to premature coating failure.

The Elcometer 123 Surface Profile Gauge measures profile height directly. Where the work calls for digital convenience, the Elcometer 224 Digital Surface Profile Gauge measures on both flat and curved surfaces and is available with or without memory and Bluetooth.

The quickest method on site is a digital surface profile gauge. A hardened pointed probe drops into the valleys of the blast pattern while the gauge body rests on the peaks; the gauge reads the peak-to-valley height directly and logs each reading. This suits flat and gently curved steel and gives a recorded value per location.

Replica tape is the traditional alternative, and the only practical method on some awkward geometries. The tape is burnished onto the surface, takes a foam impression of the profile, and is then read with a micrometer or an optical reader. The compressed tape is itself a physical record that can be filed with the inspection report.

ISO 8503 surface comparators support both methods: a visual and tactile reference plate held against the blasted steel to grade the profile as fine, medium or coarse. Many specifications accept a comparator grading for general work and demand measured values for critical items, so carry the method your specification names.

Blast cleaning does two jobs: it removes mill scale and rust, and it cuts an anchor pattern into the steel for the coating to key into. The depth of that pattern, the surface profile, directly affects how well the coating grips. Too shallow and the mechanical bond is weak, inviting disbondment and premature failure. Too deep and the highest peaks can sit close to or proud of the finished coating surface, becoming the first points to rust.

Profile also drives paint consumption. A deeper pattern increases the true surface area and the volume of paint needed to achieve the specified thickness over the peaks, which is why estimators care about the number as much as inspectors do.

Coating specifications state a target profile range for the chosen system, and the inspector measures after blasting, before the first coat goes on. It is a cheap measurement that prevents an expensive failure.

Surface roughness is a component of surface texture. It is quantified by the deviations of a real surface from its ideal form, measured in the direction of the surface's normal vector. Large deviations mean a rough surface; small deviations mean a smooth one.

The instrument for measuring roughness is a stylus instrument, which traces the surface and records those deviations directly.

For workshop and shop floor checks, comparators offer a faster alternative: estimating roughness by touch and sight against reference specimens. The Rubert 100 Series Heavy Duty Surface Roughness Comparators carry between five and eight specimens per machining process, ranging from the roughest to the smoothest finish that process normally produces. The Rubert 130 Composite Set covers six common machining methods in one set of thirty specimens: turned, end-milled, horizontally milled, ground, lapped, and reamed or drilled.

A pull-off adhesion test measures how strongly a cured coating holds to its substrate. A loading fixture called a dolly is glued to the coating with a structural adhesive. Once the adhesive has cured, the tester grips the dolly and applies a steadily increasing tensile force until something lets go. The reading at failure, expressed in MPa, is the result the specification judges.

The number is only half the story. The face of the dolly is examined to classify the failure: adhesive failure between coating and substrate, cohesive failure within a coating layer, or failure of the glue itself, which means the coating exceeded the strength of the test and the result is a minimum. Recording the failure mode is what makes the result defensible.

The method is described in ISO 4624 and ASTM D4541. Hydraulic and automatic testers such as the Elcometer 506 apply the load smoothly and at a controlled rate, which the standards require, and log each pull.

Dew point is the temperature at which the moisture in the air condenses onto a surface, the same effect that mists a cold bottle taken out of the fridge. If steel sits at or below the dew point during preparation or painting, an invisible film of moisture forms on it. Freshly blasted steel flash rusts, and paint applied over that film traps moisture against the substrate, setting up blistering and adhesion failure long before the coating should have aged.

That is why nearly every coating specification requires the steel temperature to be at least 3°C above the dew point before and during application, and why climatic readings are taken through the shift, not just at the start: steel lags the air as temperatures change, so a safe morning margin can vanish by mid afternoon.

A dewpoint meter measures air temperature, relative humidity and surface temperature together, calculates the dew point and the margin, and logs the readings for the inspection record.

A holiday is any discontinuity in a coating that exposes the substrate: a pinhole left by a burst bubble, a void, a crack, or a spot of missed coverage. Each one is a future corrosion cell, which is why pipelines, tank linings and other immersed or buried assets are tested for them before going into service.

Thin coatings are tested with a low voltage wet sponge detector. A damp sponge is wiped over the surface; where moisture reaches the substrate through a flaw, a circuit completes and the instrument alarms. Thicker, insulating coatings such as pipeline linings need a high voltage spark tester, where an electrode passed over the coating discharges through any discontinuity.

The test voltage is not guesswork: it is calculated from the coating thickness and the standard being worked to, high enough to find flaws without breaking down sound coating. Detectors such as the Elcometer 270 and the high voltage 266 cover the two regimes between them.