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.

First published in 1939, one of the oldest specifications in the Committee D01 arsenal is ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus. It is still in active use today in a wide variety of markets including automotive, marine, aerospace, and building construction to evaluate, using a standardized approach, the comparative durability and performance of substrates in corrosive environments. This is despite some experts suggesting that the test is outdated and convenient rather than scientifically rigorous.

Corrosion is a significant challenge for any industry that uses metallic substrates. Protective metal coatings and structural integrity can be adversely affected by corrosion. Salt spray testing is a method designed to provide a controlled corrosive environment using a salt fog chamber to evaluate the corrosion resistance of metallic and nonmetallic coated metal substrates.

On the surface (no pun intended), this test is easy to perform. It is quick, cost effective, and laden with decades of historical comparative data on a wide variety of substrates. Samples are placed in a sealed chamber within which is generated a continuous fine 5% neutral aqueous sodium chloride mist onto samples placed at a 45° angle. Temperature, humidity, pH, and airflow are all specified within the practice and regulated within the test chamber to provide as consistent an exposure environment as possible. Exposure hours can vary from 24 hours to over 1,500 hours, depending upon the level of corrosion resistance required.

Here begins the controversary about the real influence of salt spray testing. The practice clearly states that this is a test that is designed to “produce relative corrosion resistance information.” However, over the decades, a common misconception has grown that the test can predict real-world product service life. Often customers self-correlate hours to salt spray failure with product life and even try to reject products or extract warranty based on hours to failure.

Most coating and corrosion experts dispute this premise. Real-world service life includes an uneven cycle of humidity, UV exposure, temperature variations, pollutants, cleaning solutions, vibration, and other environmental factors that are not captured in continuous salt fog chambers.

Rather than predicting service life, salt spray testing is well suited for comparative evaluations of a host of coatings, plated finishes, pretreatment efficacy, pre- and postcoated sheet and formed parts, and manufacturing quality in a controlled environment.

If predictive testing is required, it is necessary to augment neutral salt spray (NSS) testing by expanding into cyclic and real-world testing. Those tests are also ASTM standards. Included are acetic salt spray (AASS), copper accelerated salt spray (CASS), acidified seawater cycles (SWAAT), acidified acid fog/drying/humidity cycles (MASTMAASIS), and prohesion (wet fog/dry off cycles). Continuous testing (NSS, AASS, CASS) in constant fog environments is useful for benchmarking and comparative testing, but poor real-world simulations. Cyclic tests (SWAAT, MASTMAASIS, prohesion) that alternate wet/dry/humid and wet phases more closely simulate real-world environments. Table 1 further compares the various corrosion tests.

Salt spray testing is not a dinosaur approaching extinction. Rather, this 86-yearold test provides a foundational benchmark for product behavior in a corrosive environment. It is a consistent, reproducible method for comparative benchmarking evaluation of corrosion performance under continuous corrosive chamber conditions, useful for monitoring process quality and consistency, evaluating plated finishes, supporting root-cause failure analysis, benchmarking coatings, and pretreatment efficacy. However, the caveat is that the test does not provide real-world long-term performance as it does not include other environmental influences. Predictive testing should be augmented by cyclic and/or real-world environmental testing.

Cynthia A. Gosselin, Ph.D.,Ěýis director at The ChemQuest Group, . Email: cgosselin@chemquest.com.

The idea of ASTM standards written for market specificity was introduced in the ASTM article from the January-February 2025 issue of CoatingsTech that discussed the requirements for coil coated building panels. Another example falls under the authority of ASTM Committee A05. Two specifications are written precisely for the sheet metal framing industry. Studs, purlins, joists, girts, and tracks are examples of these products that must conform to detailed strength, coating, and corrosion parameters to ensure the integrity of buildings.

The first one is ASTM A1003/A1003M Standard Specification for Steel Sheet, Carbon, Metallic- and Non-metallic Coated Cold-Formed Framing Members. This standard governs the steel grades, strength levels, metallic and nonmetallic coating requirements, testing reporting information, and ordering parameters for the entire industry. Steel producers, coil coaters that apply inorganic and organic topcoats (prepainted) and roll formers (post-painted) are the main targets of this standard. Customers are assured through the ordering and testing processes outlined in this standard that the products conform to all requirements across the steel-coating spectrum.

The adjunct to ASTM A1003/1003M is ASTM A1004/A1004M Standard Practice for Establishing Conformance to the Minimum Expected Corrosion Characteristics of Metallic, Painted-Metallic and Nonmetallic-Coated Steel Sheet Intended for Use as Cold-Formed Framing Members. This practice details the laboratory procedures used for assessing corrosion performance to verify that coated materials listed in A1003/A1003M satisfy the minimum corrosion requirements. This standard is normally used to verify that potential new material additions to A1003/A1003M meet the minimum requirements. However, the testing parameters within A1004/A1004M have also proven useful for settling claim disputes, verifying coating compliance of commercial materials, and ensuring that the metallic (steel mills) and nonmetallic coating operations (coil coaters and roll formers) remain in compliance with base metal, coating weight, and film thickness requirements that ultimately govern product durability in the field. Ordering requirements assist users in obtaining the material tailored to their needs.

These two ASTM documents are germane to AISI S903-13 Cold-Formed Design Manual—Standard Methods for Determination of Uniform and Local Ductility. Any changes to the ASTM standards are scrutinized by the Iron and Steel Institute Committee to ensure compatibility with industry building and inspection codes.

ASTM A1003/1003M updated in 2023 via the 5-year review process incorporated new base metal or coating materials that qualified for commercial use. The standard contains material details from base metal to topcoat requirements. Included are product analyses of steel heats used in the manufacture of the base metal, sheet thickness, and mechanical property requirements for the various structural, nonstructural, high, or low ductility steels. Metallic coating chemistry and weight [mass] for each product iteration are also identified. Paint (topcoat) chemistries, film thickness, flexibility, adhesion, and corrosion requirements are called out for both prepainted and post roll-formed parts to ensure durability. Each coil is required to have a ticket with a test report verifying all the physical and material properties of the base metal, metallic, and nonmetallic coatings as desired by the end customer.

In A1004/A1004M, the practice involves exposing flat and formed samples of metallic coated, painted metallic coated, or painted nonmetallic coated sheet steel to salt spray testing according to ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus.

Each type of material must meet the minimum corrosion resistance requirements for the specific substrate/metallic/nonmetallic coating combination after testing for 100–500 hours of salt spray. Generally, metallic coated fully painted samples are exposed for at least 500 hours, while those coated with nonmetallic thin films are closer to the 100-hour range.

Tested panels are evaluated according to ASTM D1654 Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments Procedures A (scribed), B (unscribed), and D (formed). Scribed panels can incorporate a 40° diagonal or X-scribe depending upon the agreement between the producer and user. Photographic standards provided in D714 Standard Test Method for Evaluating Degree of Blistering of Paints are used in the evaluation of unscribed panels. Formed panels are evaluated in bends, dimples, or other areas of interest for blistering and/or delamination from scribe as agreed upon by the producer and user.

These two product standards draw heavily upon a variety of individual technical ASTM Committee A05 and D01 standards governing everything from base metal chemistry, metallic coating thicknesses and chemistries, organic topcoat characteristics, and testing parameters that ensure every producer has a blueprint for supplying the exact material required by the building industry. This blending of committee standards allows for material uniformity across the industry that can be confidently verified as AISI building codes evolve.

Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group, www.chemquest.com. Email: cgosselin@chemquest.com.

Assuming that paint chemistry, film thickness, and application techniques are matched to the intended service environment, the degree of cure is the single most important characteristic required for a painted system to perform well. During the coating development phase, proper curing parameters are determined through a variety of sophisticated techniques monitoring the glass transition temperature. Thermomechanical Analysis (ASTM E1545) and/or Differential Scanning Calorimetry/Differential Thermal Analysis (E1356) are the usual methods for dissecting the degree of cure on a molecular level to inform the needed curing temperature and time information. Unfortunately, each of these techniques takes a fair amount of time that is inconsequential in a laboratory environment, but impractical for the fastpaced need of a production setting.

Testing has been developed to harness the laboratory curing information by correlating the glass transition temperature with a solvent (resistance) rub test. The standard for this testing is ASTM D5402 Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs.

This is a quick way for a production quality lab to simulate the degree of cure by measuring the number of double solvent rubs it takes to damage the surface. This manual method is quick, surprisingly simple, and exhibits a 95% confidence limit for reproducibility between round robin operators. Coating thickness is also an important variable. Thicker coatings provide higher rub numbers because it takes longer to expose the substrate. Very thin films with pencil hardness of 6H fall a little outside of the 95% confidence limit, at 90%. The film thickness range required number of rubs and type of solvent are listed on the Technical Data Sheet.

Historically, many quick property tests were developed within the coil coating industry where 40,000+ pound coils take only minutes to run through the line, with mere seconds of curing time. This production cycle requires the absolute minimal test completion time. In 1980, the test was codified as National Coil Coating Association No. 11-18. Revised in June 1996, it became NCCA Technical Bulletin 4.2.11 Test Method for Evaluation of Solvent Resistance by Solvent Rub. In 1993, ASTM D5402 was written for use in the laboratory, the field, or the factory.

Coatings that chemically change during the curing process become more resistant to solvents as they cure. They reach specific levels of solvent resistance prior to topcoating or being placed into service. Epoxies, vinyl esters, polyesters, alkyds, and urethanes are typical chemistries that fit this paradigm. In this standard, no solvent is specified. Rather, the producer and user agree on the solvent and the number of double rubs based on formulation chemistry.

ASTM Test Method D4752 Standard Practice for Measuring MEK Resistance of Ethyl Silicate (Inorganics) Zinc-Rich Primers by Solvent Rub is the preferred solvent rub method for ethyl silicate zinc-rich primers as the cure is a reaction with moisture, providing a binder. In this case, MEK is the specified solvent correlating very well with the chemical changes occurring during cure as identified by diffuse reflectance infrared spectroscopy.

The test is remarkably simple. First, the dry film thickness is verified to be within the required range. Cheesecloth is saturated with the solvent of choice. An index finger is placed in the center of the cloth, with the thumb and remaining fingers holding the cloth in place. The index finger is held at an approximately 43° angle as the cloth is rubbed onto the paint, first away from the operator, then back over the same line. The complete forward and back motion takes about one second. The testing continues, adding solvent as needed without lifting the finger off the substrate until either the required double rubs are achieved or the substrate surface is exposed. If the substrate is exposed before the required number of rubs, this is reported as a failure.

In addition to reporting the number of double rubs, sometimes a rating system is used to further describe any damage to the surface, such as burnishing, marring, or depressions in the film, particularly in zinc-rich systems.

In 2013, a mechanical rubbing machine was developed and codified through ASTM D7835/D7835M Standard Test Method for Determining the Solvent Resistance of an Organic Coating using a Mechanical Rubbing Machine. It provides consistent stroke length, rate, pressure, and contact area not subject to variables such as human fatigue and tabulates the number of rubs achieved.

Whether conducted manually or by machine, this simple test is a rapid, effective way to ascertain that the required level of cure is achieved for optimal service life.

Cynthia A. Gosselin, Ph.D.,Ěýis director at The ChemQuest Group, . Email: cgosselin@chemquest.com.

One of the more discussed paint adhesion and hardness tests involves “scratching” paint off a substrate. Not only does this type of testing depict a qualitative level of adhesion, but in many instances, it also provides a relative idea of degree of cure and hardness of the organic coating.

The genesis of scratch testing is found in the very old (pre-1960) National Coil Coating Testing Manual, Appendix A–Test Method NMCTP–17-92. Scratching could be preceded by environmental testing or conducted immediately after painting. The paint surface was scratched through the paint film with the edge of a nickel—hence also known as the “Nickel Adhesion Test.”

Every paint line superintendent had their favorite nickel in their pocket that, according to their experience, produced consistent results. A photographic standard was used to determine the quality of adhesion as depicted by the thickness of the paint removal line. (Figure 1) The thicker the substrate exposed within the scratch line, the poorer the adhesion. Acceptable adhesion levels were determined between the paint vendor, coated product producer, and the final customer to ensure success in the field.

FIGURE 1 Nickel adhesion photographic standard.

While the Nickel Adhesion Test provided surprisingly consistent relative results, more sophisticated and reliable quick adhesion and hardness testing was needed. In 1974, ASTM published ASTM D3363-74 Standard Test Method for Film Hardness by Pencil Test. This test method was a quick, inexpensive determination of film hardness. Relative adhesion and degree of cure could be inferred during testing. In 1980, the National Coil Coating Association issued an update of the ASTM specification as Technical Bulletin 4.2.5 (NCCA No. 11-12). The next iteration of this specification took place in June 1996 when a revised version was published in a compilation of testing for the coil coating industry. This revision was heavily influenced by the methodology utilized by ASTM.

Currently, ASTM D3363-74 (22) is an updated staple within myriad testing protocols included in ASTM Volume 06.01, routinely undergoing the rigorous five-year review process.

Pencil hardness testing involves holding a variety of flattened pencil leads of differing hardness at a 45° angle. The leads are pushed into the paint, away from the operator, using a 6.5mm (0.25″) stroke.

The process starts with the hardest pencil and continues down the hardness scale to the point where the lead will not cut into the film (hardness) or will not mar the film (scratch hardness). Any defacement of the film other than a gouge (cut) is considered a scratch. Both endpoints are reported to provide a full view of the paint film.

Calibrated pencil drawing leads or equivalent calibrated wood pencils are used. These leads meet the following hardness scale, with the difference between two adjacent leads considered as one unit of hardness.

Sometimes the point at which gouging occurs points to the relative degree of cure, which can affect hardness. High-reflectance white paint is one such product. The system required an H-pencil standard for the paint to adhere without incurring damage during the stamping process. Anything less results in paint peeling in the die, even if volume solids, paint film, color, reflectance, and crosshatch adhesion are perfect. Using the strict H-pencil criteria with mutually calibrated leads allows for the verification of enough cure to withstand the stamping process and a significant decrease in rejected material.

However, critics of this type of human/manual test are always looking for a combination adhesion/hardness test that can be perfectly calibrated and demonstrate excellent precision, repeatability, and reliability. There have been many attempts, but nothing comes close. Often alternative testing has proven too complicated and time-consuming, or it requires expensive, hard-to-calibrate equipment. That may be fine for a research laboratory, but for production labs and field evaluation, it is not practical.

To determine just how good (or bad) this test is, results of precision testing were added to the ASTM standard. A detailed interlaboratory study was conducted, analyzing data both within and between different laboratories. The study found that an interlaboratory standard deviation was 0.52 and 0.61 between laboratories. Based on this study, criteria were developed to judge the acceptability of results at a 95% confidence level. If two operators using the same panels and pencils produce results that differ by more than one pencil unit, the repeatability and reproducibility should be considered suspect.

This surprisingly good result makes pencil hardness one of the quickest and most relevant adhesion/hardness tests within the industry. Yes, it is qualitative and relative (not mathematically absolute), but good enough to provide confidence in the ultimate success of the product in the field.

Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group, www.chemquest.com. Email: cgosselin@chemquest.com.