Microns by the Manual: A Guide to ASTM Paint Film Thickness Measurement Techniques

By Cynthia A. Gosselin, Ph.D., The ChemQuest Group

film thickness is intrinsic to metallic, organic (paint), organometallic, and polymeric films. Every application has a prescribed ideal thickness to ensure durability, performance, and fitness for service. Film thickness governs gloss, color, hardness, adhesion, surface finish, scratch resistance, corrosion resistance, and durability.

ASTM standards have wide-ranging procedures for measuring film thickness, including wet, thick, thin, translucent, clear, and highly pigmented systems. Techniques can be destructive cut-andmeasure methods or nondestructive magnetic, eddy current, noncontact capacitance, ultrasonic, optical interference (ellipsometry), gravimetric, and micrometer measurements. The selected method depends upon the expected film thickness range, substrate (ferrous/nonferrous) and whether the film is a coating, plastic sheet, foil, or a multilayered system. Other selection criteria include the type of equipment, measurement location (lab or field), and cost.

Wet Film Thickness Measurements

Wet film thickness measurements are taken before the film dries during production, quality control, or when nondestructive testing of the dry film is not possible. A simple rule of thumb governs dry film thickness:

Three ASTM standards govern the measurement of wet film thickness:

• D1005-95(2024) Standard Test Method for Measurement of Dry-Film Thickness of Organic Coatings Using Micrometers. This standard is used to measure uncured coating thickness in paint and industrial coatings for either free films or on painted substrates. This technique is often used in the developmental stages to correlate wet-to-dry films, especially in volume-to-value modeling.
• ASTM D1212-91(2025) Standard Test Methods for Measurement of Wet Film Thickness of Organic Coatings. This standard consists of Method A: Wet film gauge and Method B: Notched gauge.
• ASTM D4414-25 Standard Practice for Measurement of Wet Film Thickness by Notch Gages. This procedure is not as accurate as those in D1212, rather is best suited for nonuniform surfaces like concrete block that are too rough for other gauges. It is also useful for shop and field applications where part size and shapes such as ellipses, corners, and thin edges are not suitable for measurement by other gauges.

Dry Film Thickness Measurements

The following standards use x-ray fluorescence to determine coating weight of the first “layer” of a painted system commonly known as pretreatment. Accurate application of pretreatment provides the foundation for adhesion and durability.

• D5723-95(2024) Standard Practice for Determination of Chromium Treatment Weight on Metal Substrates by X-Ray Fluorescence
• D7639-22 Standard Test Method for Determination of Zirconium Treatment Weight or Thickness on Metal Substrates by X-Ray Fluorescence

Nondestructive Measurements

• D7091-22 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals (Replaced D1186 in 2006). This standard practice describes the three operational steps necessary to ensure accurate coating thickness measurements—calibration, verification, and adjustment of measuring gauges, as well as the proper methods for both ferrous and nonferrous substrates.
• ASTM D8331/D8331-25 Standard Test Method for Measurement of Film Thickness of Thin-Film Coatings by Non-Destructive Means Using Ruggedized Optical Interference. This method is the newest system in the film thickness measurement arsenal. Ruggedized optical interference translates signal outputs in coating film thicknesses using digital formulas (or “recipes”) which are reproducible from one instrument to another. This is also a reliable continuous monitoring technique for coil coating.

Destructive Measurements

• D4138-07a(2022) Standard Practices for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means. This standard prescribes a cutting tool (aka Tooke Gauge) to scribe through the coating to the substrate. A built-in microscope or magnifier measures the thickness of the exposed cross-section groove. While precision can depend upon operator skill and experience, this is a particularly good method for field inspections, especially when verifying total film thickness in regulated or warranted products (e.g., prepainted roofs).
• ASTM 5796-24 Standard Test Method for Measurement of Dry Film Thickness of Thin-Film Coil-Coated Systems by Destructive Means Using a Boring Device. This standard describes a method whereby a precision boring tool creates a shallow-angle crater through the paint film, exposing concentric rings corresponding to each coating layer. Film thickness is calculated for each layer, assuming contrast between layers is sufficient.

Both nondestructive and destructive standard methods measure paint film thickness for quality control, variability mapping, continuous profiling, warranty claims, and field inspections. From the simple micrometer to high-tech ruggedized optical interference, there is an ASTM film thickness measurement standard best suited to any application.