Film thickness is the depth of a coating layer applied to a substrate, measured wet during application or dry after cure. Adhesion coating is the strength of the bond between that coating and the surface beneath it. Together, these two properties define what engineers call coating quality, and they decide how long a protective or marine system will actually protect.

A coating can be the right chemistry, applied at the wrong film build, and still fail early. It can be applied at the correct thickness but delaminate because the surface was not prepared for adhesion. This article explains what dry film thickness and adhesion are, how they are measured and verified against standards such as ISO 19840 and ISO 4624, and why both must be right for a coating system to perform. Elok Jaya Perkasa, a Jotun Authorized Dealer in Bali, wrote this guide for engineers, specifiers, and procurement teams who need to specify and verify coating work, not just buy paint.

What Is Film Thickness?

Film thickness is the measure of how deep a coating layer sits on the substrate. It exists in two forms, and confusing the two is a common source of specification error.

Wet film thickness (WFT) is the depth of the coating immediately after application, before any solvent or water leaves the film. Dry film thickness (DFT) is the depth of the cured film that remains after drying. DFT is the number that matters for performance, because it describes the actual protective barrier left on the surface.

The relationship between the two is straightforward in principle. Wet film thickness converts to dry film thickness according to the solids content of the coating by volume. A coating with high solids by volume leaves a dry film close to its wet film, while a lower solids coating loses more depth as its volatile content evaporates. The exact conversion depends on the product data for the specific coating, so DFT is not something to guess. It is something to measure after the film has cured.

For a specifier, the important point is that WFT is a field control during application, while DFT is the verification of what was actually delivered. Both have a role, and neither replaces the other.

Why Dry Film Thickness Determines Coating Quality

Dry film thickness is the property that determines whether a coating system has enough material in place to do its job. Every coating system is designed around a specified DFT, and that value is not arbitrary.

The specified DFT is set by the coating system design to deliver specific protective functions. For a barrier coating on steel, the film must be thick enough to slow the passage of water and oxygen to the substrate. For a primer, the film must cover the full profile of a blast-cleaned surface so no peak of the profile protrudes and becomes a corrosion starting point. For a topcoat, the film must be thick enough to hold its colour, resist UV, and shield the layers beneath it.

When DFT falls below specification, protection is incomplete. A film that is too thin leaves high spots on a rough surface exposed, allows moisture through more quickly, and wears away faster under erosion or UV. When DFT is excessive, the risks are different but just as real. Over-thick films can trap solvent, sag, crack under stress, or fail to cure properly through their full depth. Both directions of error shorten service life.

This is why the specification for a protective system states a nominal DFT and a tolerance around it, and why verification is a defined step in the job, not an afterthought.

How Dry Film Thickness Is Measured

Dry film thickness is verified with an instrument, not by eye. The standard framework for measuring DFT on protective coatings is ISO 19840, which covers measurement of the dry film thickness of coatings on rough, blast-cleaned steel surfaces.

The method described by ISO 19840 uses a magnetic or electronic gauge on ferrous substrates. The gauge measures the distance from the probe to the steel beneath the coating. Because a blast-cleaned surface has a profile, a single reading is not enough. The standard approaches the measurement as a statistical exercise: multiple readings are taken across a defined area, and the result is evaluated against the specified DFT and its tolerance.

In practice, DFT verification works like this. An inspector selects measurement points across the coated area. The number and pattern of points follow the inspection plan for the job. Each reading is recorded. The set of readings is then compared against the specification: the nominal DFT, the tolerance band, and any rules for how many individual readings may fall outside the band and by how far.

The key discipline is this: DFT is verified against the specification for the system, not against a guess. Different systems carry different specified DFT values depending on the chemistry, the number of coats, and the service environment. The number that matters is the one written in the coating schedule or product data for your specific system.

What Is Adhesion Coating and Why It Matters

Adhesion coating is the bond that holds the coating to the substrate. A coating with poor adhesion will separate from the surface even if its thickness is correct, which is why adhesion and thickness are treated as a pair in coating quality.

Adhesion comes from two mechanisms. Mechanical adhesion happens when the coating flows into the roughness and profile of the surface and locks onto it as it cures. Chemical or specific adhesion happens at the molecular level between the coating and the substrate. Both mechanisms depend on one thing the coating cannot control: the condition of the surface it is applied to.

Adhesion fails when the interface between coating and substrate is weak. Contaminants left on the surface, such as oil, dust, salt, or rust, form a weak layer between the two. Moisture, uncured regions, or an incompatible primer all weaken the bond. The result is failure that starts at the interface: blistering, peeling, flaking, or complete delamination.

This is why adhesion cannot be specified by film thickness alone. The correct film build on a contaminated surface still fails. Adhesion is a property of the whole system: the surface preparation, the primer, and the application conditions together.

How Adhesion Is Tested

Adhesion is verified with a pull-off test described in the framework of ISO 4624, which covers the tensile pull-off method for measuring the adhesion of a coating.

The method works on a simple principle. A dolly, a small metal disc, is bonded to the surface of the cured coating with adhesive. Once the adhesive has set, a portable tester applies a controlled tensile force that pulls the dolly away from the surface. The test measures the force required to detach the coating, and the failure is examined to see where it occurred.

The location of the failure matters as much as the force. If the coating separates cleanly from the substrate, the adhesion between coating and substrate is the weak point. If the coating splits within itself, the cohesive strength of the coating is the limiting factor. If the adhesive on the dolly fails, the test itself is invalid and must be repeated. Reading where the failure happened tells an engineer what to fix.

Pull-off testing is destructive, so it is used on representative areas and as a verification step, not continuously across a whole structure. The result is evaluated against the adhesion requirement set out in the specification for the system, which in turn reflects the coating chemistry and the service conditions.

How Film Thickness and Adhesion Work Together

Film thickness and adhesion are not separate concerns. They interact, and a failure in one often shows up as a failure in the other.

The clearest connection is the surface profile. Surface preparation creates a profile, a controlled roughness, on the substrate. That profile is what gives the coating its mechanical grip, which is the foundation of adhesion. The same profile also demands a minimum film thickness: the coating must be thick enough to cover the peaks of the profile completely. If the DFT is too low for the profile, the peaks stand proud, the coating thins over them, and those points become early corrosion sites. Adhesion and thickness both trace back to the same prepared surface.

A second connection is stress. A thick film builds internal stress as it cures and as it responds to temperature change. That stress is carried by the adhesion at the interface. If adhesion is weak, a thick film is more likely to crack or delaminate than a thin one. In this sense, more thickness is not automatically better: thickness must be matched to the adhesion the system can sustain.

This is why a coating system is specified as a whole: surface preparation standard, primer, intermediate and topcoat products, and the DFT for each coat. Each element constrains the others. For a fuller picture of how preparation sets the foundation for everything above it, read our guide to surface preparation.

Common Causes of Film Thickness and Adhesion Failure

Most field failures trace back to a small number of causes. Knowing them helps a specifier write a tighter specification and an inspector catch problems early.

Thin film. Under-application is the most common thickness failure. It happens when the applicator applies too little material, spreads it too far, or fails to apply the specified number of coats. The result is a film that does not meet the specified DFT and a system that under-performs from day one.

Thick film. Over-application causes sagging, solvent entrapment, and cracking. It is especially common on edges and welds, where the natural tendency is to load material. Thick films also take longer to cure, which delays the next coat and the return to service.

Contaminated surface. Oil, salt, dust, and rust left on the substrate defeat adhesion before the first coat goes on. This is the single most common adhesion failure, and it is a preparation problem, not a coating problem.

Wrong conditions. Humidity, low temperature, and condensation during application all interfere with curing and adhesion. A coating applied outside its application window may look fine and fail later.

Incompatible layers. A topcoat applied over an incompatible primer, or a fresh coat over an old, chalky one, can fail at the interface between layers. Adhesion between coats is part of the system design and must be respected.

Skipping verification. When DFT is never measured and adhesion is never tested, failures are discovered in service instead of in the yard. The cost of verification is small compared with the cost of rework.

Specifying Film Thickness and Adhesion for Your Project

A coating specification that produces reliable quality has a few firm elements.

State the specified DFT for each coat and the total system. The nominal DFT and its tolerance must come from the coating system design and the product data. Do not leave thickness to the applicator’s judgment.

State the surface preparation standard. Adhesion depends on the profile and cleanliness of the surface. The preparation standard and the DFT must be consistent with each other.

State the verification method and acceptance criteria. Name the measurement framework, such as ISO 19840 for DFT, and the adhesion method, such as ISO 4624. Define what readings are acceptable and what happens when they are not.

State the application conditions. The coating system only cures properly inside its application window. Temperature, humidity, and surface condition limits belong in the specification.

Keep records. DFT readings and adhesion results are part of the project record. They are the evidence that the work met specification, and they matter to owners, insurers, and anyone who maintains the asset later.

For steel structures, where film thickness and adhesion carry the full corrosion-protection duty, see our article on anti-corrosion coating for steel structures. The same principles apply, and the stakes are highest there.

Frequently Asked Questions

What is dry film thickness?

Dry film thickness is the depth of a coating layer after it has cured and all volatile content has evaporated. It is the film that actually remains on the surface and is the number used to verify that a coating system meets specification.

Why does film thickness matter for coating quality?

Film thickness matters because a coating must reach its specified DFT to perform. A film that is too thin exposes the surface and wears away early. A film that is too thick can sag, crack, or fail to cure. The specified DFT and its tolerance define the correct result.

What is ISO 19840?

ISO 19840 is a standard framework for measuring the dry film thickness of coatings applied to rough, blast-cleaned steel surfaces. It describes how measurements are taken and evaluated so that DFT is verified statistically rather than by a single reading.

What is ISO 4624?

ISO 4624 is a standard framework for the pull-off adhesion test. A dolly is bonded to the cured coating and pulled with controlled tensile force, and the force and failure location are recorded to evaluate the coating’s adhesion.

How does adhesion differ from film thickness?

Film thickness is the depth of the coating. Adhesion is the strength of the bond between the coating and the substrate. Both are required for coating quality: correct thickness on a poorly bonded surface still fails, and good adhesion at the wrong thickness still under-performs.

Can adhesion be tested without damaging the coating?

The pull-off test described by ISO 4624 is destructive, so it is applied to representative areas rather than across an entire structure. Non-destructive methods exist for some checks, but pull-off remains the standard reference for quantifying adhesion.

Why do thin films fail first on rough surfaces?

A blast-cleaned surface has a profile of peaks and valleys. The coating must be thick enough to cover the peaks completely. If DFT is too low, the peaks stand proud of the film and become early corrosion sites. This is why surface profile and minimum DFT are specified together.

Technical Consultation: Verify Your Coating Specification

Film thickness and adhesion are the two properties that decide whether a coating system earns its place on your asset or becomes an expensive rework. Getting them right means reading the product data, writing a clear specification, and verifying the result against standards, not assumptions.

If you are specifying or reviewing a protective, marine, or industrial coating system in Bali, Elok Jaya Perkasa can help. As a Jotun Authorized Dealer, we work with project teams to define the correct coating system, the specified DFT for each coat, the surface preparation standard, and the verification approach for the job. We are happy to discuss your substrate, exposure, and service requirements and to connect you with the right technical support. Before you commit to any supplier, you may want to verify their authorized dealer status.

Contact Elok Jaya Perkasa for a technical consultation on film thickness, adhesion, and coating quality for your project. No obligation: just a clear conversation about what your surface needs.

 

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