Coating Film Thickness Measurement Explained: Wet Film, Dry Film, and Contactless Methods (ISO 2808)

Why film thickness is a critical quality control parameter

Film thickness is one of the most important quality checks for any applied coating. Get it wrong in either direction and the coating fails to do its job. Too thin, and it will not provide the protection, appearance, or function the specification calls for. Too thick, and you waste material, drive up cost, and risk adhesion failures, cracking, or poor cure. 

Measuring film thickness lets you: 

• Verify compliance with a specification. Most coating contracts set a minimum and maximum dry film thickness. 
• Confirm corrosion protection. Anti-corrosion coatings must reach a minimum film build to perform across their rated service life. 
• Control material consumption and application cost. 
• Confirm proper cure and avoid defects such as sagging, wrinkling, or solvent entrapment. 

This is the heart of what we do: helping you protect the integrity of your brand and product through accurate, repeatable measurement. 

Wet film measurement: checking thickness at application

Wet film thickness (WFT) is measured immediately after application, before the coating dries or cures. Checking WFT at the point of application means an out-of-spec coating can be corrected straight away, rather than discovered after curing. Two instrument types are common. A comb gauge (or notch gauge) is a flat plate with stepped teeth; you press it into the wet film and read the highest wetted tooth. A wheel gauge rolls through the film, using an eccentric inner rim to register thickness. 

Our wet film range from TQC Sheen includes the Wet Film Thickness Gauge Stainless Steel (three models spanning 20-370 µm, 25-2000 µm, and 50-10,000 µm), the aluminum Wet Film Measuring Comb for general-purpose use, the pocket-sized Plastic Wet Film Comb for quick field and lab checks, and the Wet Film Thickness Wheel for wet lacquers and coil-coated surfaces. Comb-type wet film measurement is covered by ASTM D1212 and ASTM D4414. 

Converting wet film to dry film

One of the most searched questions in coating QC is how to convert a wet film reading into an expected dry film result. The relationship depends on the coating’s volume solids content, the proportion of the coating that stays on the surface after solvents evaporate. The formula is: DFT = WFT x (volume solids % / 100) 

For example, a 100 micron wet film of a 50 percent solids coating will yield approximately 50 microns dry film thickness. Always contact your coating supplier for exact solids content data. This principle underpins our film applicators range, and you can run the numbers both ways with our paint consumption calculator. 

Dry film measurement: methods by substrate

Dry film thickness (DFT) is measured after the coating has cured. The right method depends on what is underneath the coating: 

• Magnetic induction for ferrous substrates such as steel and iron. 

• Eddy current for non-ferrous metals such as aluminum, copper, and brass. 

• Ultrasonic for non-metallic substrates such as wood, concrete, plastic, and composites. 

Most modern gauges combine magnetic induction and eddy current with automatic substrate detection. Our Coating Thickness Gauge Fe and NFe auto-detects the substrate and measures 0-1300 µm. The QNix 4500 was developed for automotive work and reads up to 3000 µm on both Fe and NFe substrates, while the QNix 4200 Ferro measures non-magnetic coatings on steel and iron only. The DeFelsko PosiTest DFT Fe/NFe offers handheld, non-destructive measurement on both metal types. For non-metals, the Coating Thickness Gauge for All Substrates (PosiTector 200 series) uses ultrasonic measurement on wood, concrete, plastic, composites, and fiberglass. Non-destructive dry film measurement on metals is covered by ASTM D7091, and both ISO 2808 and ASTM D7091 sit behind these methods. 

Contactless measurement: wet and cured in one instrument

The Coatmaster Flex occupies its own category. It measures both wet and cured or dried coatings using the same instrument, without contact, on metal, paper, rubber, ceramic, coil, glass, plastic, and wood. It uses Thermal Layer Testing (TLT), a patented technique that does not rely on magnetic induction or ultrasound, and it is insensitive to the angle and distance of measurement. It suits in-line QC where measuring before the oven lets you rework out-of-spec coatings early, and where non-contact measurement protects delicate or moving surfaces. 

When to use which instrument

A simple guide: 

• Wet, at application: comb or wheel gauge. 
• Cured, on steel or iron: magnetic induction gauge. 
• Cured, on aluminum or other non-ferrous metal: eddy current gauge (or an auto-detecting Fe/NFe gauge). 
• Cured, on wood, concrete, or plastic: ultrasonic gauge. 
• Wet or cured, non-contact, in-line: Coatmaster Flex. 

Surface preparation context

Substrate profile affects DFT readings. A rougher blast-cleaned surface holds coating in its valleys, so the profile has to be accounted for when interpreting thickness. Surface profile is assessed separately, often using replica tapes measured with the Film Thickness Gauge (FTG2000), or with a combined Surface Roughness and Coating Thickness Gauge. Surface cleanliness before painting is assessed visually under ISO 8501. 

Why it matters

Film thickness measurement is a Qualicoat test requirement for architectural aluminum, and it is central to quality across automotive, marine, and industrial infrastructure work. You can read more on our Qualicoat standards page. Across our coatings testing brand, TQC Sheen, we help you measure film thickness accurately at every stage, so you can protect the integrity of your customers’ brands and products.