When a protective coating fails early, the cause often comes down to one measurable factor: thickness. Dry film thickness testing is the process that confirms a coating system has been applied within the range specified by the manufacturer and the project engineer, and it is one of the most important quality control steps on any industrial coating project. For water storage tanks, treatment facilities, pipelines, and other steel infrastructure across Nevada and the Western United States, getting coating thickness right can mean the difference between decades of reliable protection and premature corrosion. Resource Development Co, a Reno, NV general contractor with more than 50 years of experience in welded steel tanks and industrial coatings, relies on careful measurement to deliver coating systems that perform as intended. This guide explains what the testing involves, why it matters, and how it fits into a successful project.
What is dry film thickness testing?
Dry film thickness, commonly abbreviated as DFT, refers to the thickness of a coating after it has cured and all solvents have evaporated. It is typically measured in mils (thousandths of an inch) or microns. Every coating product has a specified DFT range, and multi-coat systems often have separate targets for the primer, intermediate, and topcoat layers, as well as a total system thickness.
Dry film thickness testing uses specialized gauges to measure the cured coating at multiple locations across a surface. The readings are then compared against the project specification to confirm that the coating is neither too thin nor too thick. Because a coating's protective properties depend heavily on reaching the correct thickness, these measurements provide objective evidence that the work meets requirements.
Why dry film thickness testing matters
Coating manufacturers design their products to perform at specific thicknesses. When a coating is applied too thin, it may not provide an adequate barrier against moisture, oxygen, and chemicals. Thin areas can also leave the peaks of a blasted steel profile exposed or barely covered, creating weak points where rust can begin. On a potable water tank or wastewater structure, that early corrosion can spread quickly beneath the surrounding coating.
Excess thickness causes its own problems. Overly thick films may crack, sag, or fail to cure properly, and they can trap solvents that later lead to blistering or poor adhesion. Excess material also adds unnecessary cost. Dry film thickness testing helps crews and inspectors identify both conditions while there is still time to correct them.
For owners, the testing provides documentation. A complete record of thickness readings supports warranty claims, satisfies regulatory and contract requirements, and creates a baseline for future maintenance evaluations. Resource Development Co views these records as a key part of delivering long-term value on every coating project.
How dry film thickness is measured
Several methods are available, and the right one depends on the substrate, the coating type, and whether the test can damage the finished surface.
Magnetic pull-off gauges
Mechanical gauges, sometimes called Type 1 gauges, use a permanent magnet and a calibrated spring to measure the force needed to pull the magnet away from a coated steel surface. The thicker the coating, the less force is required. These gauges are durable and do not require batteries, making them a longtime field option.
Electronic gauges
Electronic gauges, known as Type 2 gauges, use magnetic induction for steel substrates or eddy current technology for non-ferrous metals such as aluminum. They offer fast, precise readings and can store data for reporting. Many modern units allow technicians to capture hundreds of measurements on a large structure and export them directly into project records, which makes dry film thickness testing on large tanks far more efficient.
Destructive methods
In some cases, such as coatings applied over concrete or when layer-by-layer thickness needs to be verified, inspectors may use destructive techniques that cut through the coating to view it under magnification. Because these methods damage the finish, the test area must be repaired afterward.
During application, painters also use wet film thickness gauges to estimate how much material they are laying down. While wet readings help crews stay on target in real time, only dry film thickness testing can confirm the final cured result.
Industry standards that guide the process
Consistent measurement depends on following recognized standards. SSPC-PA 2, published by the Association for Materials Protection and Performance (AMPP), is widely used to define how many readings to take, where to take them, and how to determine whether an area conforms to the specification. ASTM D7091 covers the use of magnetic and electromagnetic gauges on ferrous and non-ferrous metals. For steel water tanks, AWWA D102 addresses coating systems, and coatings in contact with drinking water must also be certified to NSF/ANSI/CAN 61.
Gauge accuracy is just as important as the procedure itself. Technicians should verify their instruments against certified reference standards or shims before and during use. A gauge that is out of calibration can produce readings that look acceptable when the coating is not.
Common issues uncovered during testing
Dry film thickness testing frequently reveals problems that are difficult to see with the naked eye. Edges, welds, bolts, and corners are notorious for thin coverage because coatings tend to pull away from sharp surfaces. Stripe coating, which involves brushing an extra layer onto these areas, is a common solution. Complex geometries such as ladders, piping, and internal tank supports can also be challenging to coat evenly.
Heavy overlaps from spray application can create areas of excess build, while rushed recoat windows may prevent individual layers from reaching their targets. When readings fall outside the acceptable range, crews can apply additional coats to thin areas or address excessive thickness according to the manufacturer's guidance. Catching these issues during the project is far less costly than discovering them after the structure returns to service.
Where DFT testing fits into a coating project
Measurement is most effective when it is built into every phase of the work rather than saved for the end. Surface preparation comes first, including abrasive blasting to the specified cleanliness and profile. After each coat is applied and cured, dry film thickness testing confirms that layer before the next one goes on. A final round of measurements verifies the total system thickness, and on immersion services such as potable water tanks, holiday testing is often performed to locate pinholes or voids in the film.
Resource Development Co approaches coating work with this layered quality control in mind. The company can apply virtually all major manufacturers' product lines, including Sherwin-Williams, Carboline, Tnemec, Devoe, Sauereisen, Raven, and Enduraflex. That broad experience means the team understands the specific thickness requirements and application characteristics of a wide range of coating systems.
Because Resource Development Co has also erected more than 500 AWWA D100 welded steel water storage tanks throughout the Western United States since 1972, its crews understand how coatings interact with welded seams, roof structures, and tank interiors. That construction knowledge helps prevent the thin spots and coverage gaps that measurement so often reveals.
Choosing a contractor that prioritizes quality control
When selecting a coatings contractor, owners should ask how thickness will be measured, which standards will be followed, and how results will be documented. A qualified contractor should be able to explain its inspection procedures, gauge calibration practices, and plan for correcting nonconforming areas.
Resource Development Co holds contractor licenses in Nevada and California and has earned repeated safety recognition, including an award for more than 250,000 hours worked in 2024. Its services extend across welded steel tanks, industrial coatings, heavy civil water and wastewater construction, and residential and commercial concrete, allowing Resource Development Co to manage coating work as part of a larger project or as a standalone rehabilitation effort.
How Resource Development Co delivers lasting coating performance
A coating system is only as strong as its thinnest point. Consistent dry film thickness testing gives owners confidence that their investment will protect steel structures from corrosion for years to come. Resource Development Co is a Reno, NV general contractor serving communities throughout Nevada, California, and the Western United States, with more than five decades of experience in welded steel storage tanks, industrial coatings, and heavy civil water and wastewater construction. Whether you are recoating an existing tank or planning a new build, the team is ready to help. Call the office at 775.356.8004, email info@resourcedevelopmentco.com, or contact us to get your project started.
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