Article 15: Wet Film and Dry Film Thickness Control

Applying Enough Coating to Protect the Steel - Without Creating Problems from Excessive Film

Why Film Thickness Matters

Protective coatings are designed to perform within a specified thickness range. That range is established according to the coating formulation, surface profile, service environment, number of coats, and expected system performance.

Thickness is generally expressed in mils or micrometers. One mil equals one-thousandth of an inch. One mil is approximately 25.4 micrometers.

A complete coating system may have separate requirements for the primer, intermediate coat, finish coat, stripe coat, and total system.

Thickness affects:

• Coverage of the surface profile
• Barrier protection
• Resistance to moisture and chemicals
• Curing and solvent release
• Flexibility and cracking resistance
• Intercoat adhesion
• Material consumption and project cost

Wet Film Thickness and Dry Film Thickness

Wet film thickness, commonly abbreviated WFT, is the thickness of a liquid coating immediately after application and before solvent or water leaves the film.

Dry film thickness, commonly abbreviated DFT, is the thickness remaining after the coating has dried or cured sufficiently for measurement.

Wet film measurements provide immediate process control for the applicator. Dry film measurements provide evidence of the thickness that remains on the structure.

Using Volume Solids to Estimate Wet Film Thickness

Volume solids represent the percentage of the liquid coating expected to remain as dry film under the stated conditions. The remainder leaves the film during drying or curing.

The theoretical wet film thickness can be estimated when the required dry film thickness and volume-solids percentage are known.

This is a theoretical estimate. Actual results can be affected by surface profile, overspray, transfer efficiency, application technique, material loss, evaporation during application, and permitted thinning.

If thinner is added, the wet-film target may need to be adjusted because the volume of liquid increases while the amount of coating solids remains essentially unchanged. Follow the manufacturer's approved calculation or project procedure.

The Wet Film Thickness Gauge

A wet film thickness gauge is a flat, notched tool with teeth of different lengths. It is pressed into the freshly applied coating while the film is still wet.

Some teeth contact the substrate, some become coated, and the next higher tooth remains dry. The wet film thickness lies between the highest wetted tooth and the lowest adjacent dry tooth.

Basic Wet Film Measurement Procedure

1. Select a representative, freshly coated area.
2. Hold the gauge perpendicular to the surface.
3. Press it firmly into the coating until the outer support points contact the substrate.
4. Remove the gauge without sliding it across the film.
5. Identify the highest wetted tooth and the next dry tooth.
6. Record or communicate the result.
7. Clean the gauge before the coating cures.

Wet film readings should be taken promptly. Fast solvent release, water evaporation, hot surfaces, wind, and rapid curing can change the wet film before the measurement is made.

Where Wet Film Gauges Work Best

Notched wet film gauges work best on relatively smooth, accessible surfaces where the support points can reach the substrate.

Readings can be difficult or misleading on:

• Deeply profiled steel
• Small pipes or tightly curved surfaces
• Edges and corners
• Welds and bolt assemblies
• Very fast-curing coatings
• Textured or aggregate-filled coatings

When direct wet-film measurement is impractical, the contractor may need to rely on controlled application trials, material consumption, spray settings, coverage calculations, and another approved method.

Wet Film Thickness Is the Painter's Control Tool

Wet film measurements should begin early in application. They should be taken often enough to identify changes in technique, equipment, material condition, temperature, or geometry.

If the wet film is too thin, the painter may need to reduce travel speed, correct gun distance, change overlap, adjust material delivery, or use additional controlled passes.

If the wet film is too heavy, the painter may need to increase travel speed, reduce excessive overlap, correct gun distance, or adjust fluid delivery.

Dry Film Thickness Measurement

Dry film thickness is commonly measured nondestructively with a magnetic or electromagnetic instrument. The correct instrument and probe must match the substrate and coating.

On carbon steel and other ferrous substrates, instruments use magnetic principles to determine the distance between the probe and the metal.

On nonferrous conductive substrates such as aluminum, a suitable instrument may use an eddy-current principle to measure nonconductive coating thickness.

A gauge designed only for ferrous steel should not be assumed suitable for aluminum, stainless steel, galvanized surfaces, thermal spray metal, or another substrate.

Types of Dry Film Thickness Instruments

Type 1 Magnetic Pull-Off Instruments

Type 1 instruments measure the force required to overcome magnetic attraction between a magnet and the substrate. These instruments are mechanical and may be useful in harsh field conditions.

The operator must place and operate the instrument consistently. Vibration, orientation, surface shape, and operator technique can affect the reading.

Type 2 Electronic Instruments

Type 2 instruments use electronic probes and display the measured thickness digitally. Many can calculate statistics and store readings.

Electronic convenience does not eliminate the need for verification, adjustment, proper probe placement, and representative sampling.

Calibration, Verification, and Adjustment

Calibration, verification, and adjustment are related but different activities. Using these terms correctly improves the reliability of inspection records.

Calibration

Calibration is a controlled process performed by the instrument manufacturer, qualified laboratory, or authorized facility using traceable standards. It establishes the instrument's relationship to known values.

Verification of Accuracy

Verification checks whether the instrument reads known reference standards within the required tolerance. It should be performed at the frequency required by the governing procedure and whenever instrument performance is questioned.

Adjustment

Adjustment aligns the instrument with the substrate, surface condition, expected coating range, and probe configuration used for the work.

A current calibration certificate does not mean that the gauge is automatically ready for every surface. Field verification and adjustment are still required.

Reference Standards and Shims

Certified coated standards and plastic or metal shims may be used to verify gauge performance. Their use depends on the instrument, substrate, project procedure, and applicable standard.

Shims should be clean, flat, undamaged, and within the expected measurement range. A creased, worn, dirty, or permanently compressed shim may introduce error.

Measurements taken on a smooth reference block do not automatically account for the effect of a rough, abrasive-blasted surface.

The Effect of Surface Profile

Abrasive-blasted steel contains peaks and valleys. A magnetic dry-film gauge responds to the distance between its probe and the magnetic substrate, but its result can be influenced by the surface profile beneath the coating.

An instrument placed on uncoated blast-cleaned steel may display a positive value even though no coating is present. This is commonly called a base-metal reading or surface-profile effect.

The required procedure may call for adjustment on representative uncoated prepared steel, use of a measured base-metal reading, or another approved method. Follow the referenced standard and project specification.

Do not automatically subtract the nominal surface-profile depth from every coating reading. Profile depth and magnetic gauge response are not necessarily the same value.

Measure the Prepared Substrate When Possible

The best time to establish the influence of the substrate is before coating application. Representative uncoated prepared areas can be used to verify or adjust the instrument according to the governing procedure.

If all steel has already been coated, obtaining a representative substrate reading may require an approved alternative or a small prepared reference area.

Probe Placement

The probe should be placed squarely and firmly on the coated surface. It should not be dragged across the coating.

Avoid unreliable measurement locations such as:

• Edges where the probe does not sit properly
• Weld peaks and severe irregularities
• Areas with loose dirt or debris
• Wet, soft, or uncured coating
• Locations too small for the probe footprint
• Areas influenced by nearby edges, corners, or geometry beyond the probe's capability

If edges, small parts, pipes, or curved surfaces must be measured, use a suitable probe and follow the instrument manufacturer's instructions and project procedure.

Effects of Curvature and Thin Steel

Curved surfaces can affect magnetic and eddy-current readings. A gauge adjusted on a flat plate may not read the same way on a small-diameter pipe.

Thin steel can also affect gauge response. The substrate must meet the instrument's minimum thickness and geometry requirements.

Verify and adjust the instrument on a representative uncoated substrate with the same material, thickness, curvature, and surface preparation whenever practical.

One Gauge Reading Is Not an Inspection

Coating thickness naturally varies across a structure. A single gauge reading represents only a very small location.

Formal dry-film thickness procedures use multiple individual gauge readings to create spot measurements and multiple spots to evaluate larger inspection areas.

The exact number of readings, spot measurements, inspection areas, frequency, and acceptance limits must come from the project specification and referenced standard.

Contractors should not replace the required sampling procedure with a few readings taken from convenient or favorable locations.

Gauge Reading, Spot Measurement, and Area Measurement

These terms should not be used as though they mean the same thing.

• Gauge reading: One individual instrument reading at one probe placement.
• Spot measurement: The calculated result from multiple gauge readings taken within a small defined area.
• Area measurement: An evaluation using the required number of spot measurements within a larger inspection area.

The applicable standard defines how these readings are collected, calculated, and compared with the specified thickness range.

Measurement Frequency

Measurement frequency should be established before coating begins. It may be based on surface area, structural units, coated components, production lots, work shifts, or another defined basis.

Additional measurements may be needed when:

• The applicator or spray equipment changes.
• A new coating batch is introduced.
• Material temperature or viscosity changes.
• The structure changes from broad plate to complex steel.
• Wet-film readings become inconsistent.
• Results approach an acceptance limit.
• A repair or recoat area is completed.

Measurements should represent the work rather than only the easiest surfaces to reach.

Individual Coat Thickness

In a multi-coat system, measure and document each coat when required. Once later coats are applied, a conventional magnetic gauge normally reports the combined thickness of all nonmagnetic coating layers above the substrate.

If the primer was not measured before the intermediate coat was applied, its individual thickness may be difficult to determine later without a specialized or destructive method.

The contractor should not rely only on the final total thickness when the specification establishes a range for each layer.

Why Coating That Is Too Thin Fails

Thin coating may not provide the continuous barrier required to separate the steel from the environment.

Insufficient film thickness can cause:

• Incomplete coverage of surface-profile peaks
• Pinholes and holidays
• Early pinpoint rusting
• Reduced moisture and chemical resistance
• Poor hiding or color variation
• Shortened service life

Edges, welds, bolts, corners, and shadowed areas are especially vulnerable to insufficient film thickness.

Why Coating That Is Too Thick Fails

Excessive film can create stresses and curing problems. The risks depend on the coating formulation, number of coats, service environment, and amount of excess thickness.

Excessive coating thickness can contribute to:

• Runs and sags
• Solvent or water entrapment
• Slow or incomplete cure
• Cracking, checking, or mud cracking
• Wrinkling
• Poor adhesion between coats
• Bubbling or pinholing during recoating
• Unnecessary material cost

A project specification may permit limited variation above or below the target under a defined acceptance procedure. Do not invent an unofficial tolerance.

Correcting Coating That Is Too Thin

Low thickness is commonly corrected by cleaning and applying additional compatible coating. However, the repair is not always as simple as spraying another pass.

Before adding material, confirm:

• The coating is within its permitted recoat interval.
• The surface is clean and free of contamination.
• The existing coating is sufficiently cured.
• The additional coat will be compatible.
• The repair will not create excessive total thickness elsewhere.
• The correction procedure has been approved.

Areas outside the recoat window may require cleaning, abrasion, or another surface treatment before additional coating is applied.

Correcting Coating That Is Too Thick

Excessive thickness is usually more difficult to correct. Applying another coat will not solve the problem.

The coating may require additional cure time, engineering or manufacturer evaluation, sanding, grinding, removal, or complete replacement in the affected area.

Do not automatically accept thick coating because it appears hard or looks good. Obtain an approved disposition based on the product, thickness, cure, location, and service requirements.

Inspecting Stripe-Coated Areas

Stripe coating intentionally adds material to edges and irregular details. These areas may be difficult to measure accurately with a standard probe because of curvature and limited access.

Stripe-coat quality may require a combination of visual inspection, process control, coverage verification, wet-film checks where practical, and dry-film measurement at suitable locations.

Do not force a probe onto an unsuitable edge and report the resulting number as reliable.

Common Thickness-Measurement Mistakes

• Taking only one reading: One probe placement does not represent an inspection area.
• Measuring only convenient locations: Difficult areas remain unevaluated.
• Using the wrong substrate mode: The instrument is not configured for the actual metal.
• Ignoring surface profile: The blasted substrate affects instrument response.
• Adjusting on a smooth block only: The setup may not represent the prepared steel.
• Measuring soft coating: The probe can indent or damage the film.
• Taking readings too close to edges: The edge can influence the result.
• Dragging the probe: The coating or probe can be damaged.
• Confusing readings with spot measurements: Acceptance calculations become incorrect.
• Checking only the final system: Individual coat thickness is no longer known.
• Inventing a tolerance: Acceptance must follow the project procedure.

Contractor Quality Control

Thickness control belongs to the contractor's production process. The applicator should receive wet-film feedback while working, and the contractor's quality-control personnel should verify dry film before requesting owner inspection.

Waiting for the owner's inspector to find thin or excessive coating can lead to extensive rework and schedule delay.

A practical control sequence is:

1. Confirm the thickness range for each coat and the total system.
2. Calculate or obtain the target wet film thickness.
3. Verify the dry-film gauge on the representative substrate.
4. Take wet-film readings early during application.
5. Give useful feedback to the applicator.
6. Measure dry film after the coat is sufficiently cured.
7. Map areas outside the acceptance requirements.
8. Complete and verify approved repairs.
9. Document the results before the next coat is applied.

Documenting Thickness Measurements

A thickness report may include:

• Project and structure identification
• Coating product, color, and coat number
• Specified thickness range
• Date, time, and measurement location
• Instrument manufacturer, model, probe, and serial number
• Calibration status
• Verification and adjustment procedure
• Reference standards or shims used
• Base-metal or surface-profile adjustment when required
• Individual readings, spot measurements, and area results
• Nonconforming locations
• Corrective actions and repair verification
• Inspector or quality-control technician identification

Record actual measurements. A report stating only "thickness acceptable" does not show how the conclusion was reached.

Thickness-Control Checklist

• Required thickness is known for each coat and the total system.
• The target wet film thickness has been established.
• Permitted thinning has been included in the wet-film plan.
• Wet-film gauges are clean and undamaged.
• The dry-film instrument is suitable for the substrate.
• Calibration status has been confirmed.
• Gauge accuracy has been verified.
• The instrument has been adjusted for the representative substrate.
• The surface-profile effect has been addressed.
• Required measurement frequency and locations are understood.
• Each coat is measured before the next coat is applied when required.
• Thin and excessive areas are marked and mapped.
• Corrective work follows an approved procedure.
• All results and repairs are documented.

Key Takeaways

• Wet film thickness controls the application process.
• Dry film thickness verifies the coating remaining on the structure.
• Volume solids can be used to estimate a wet-film target.
• Thinning changes the wet-film thickness required to obtain the specified dry film.
• Calibration, verification, and adjustment are different activities.
• Abrasive-blast profile can affect dry-film gauge readings.
• One gauge reading does not represent a structure or inspection area.
• Individual coats should be measured before they are covered when required.
• Both insufficient and excessive coating thickness can produce failure.

Knowledge Check

1. What is the main difference between wet film thickness and dry film thickness?

View Answer

Wet film thickness is measured immediately after application. Dry film thickness is the coating thickness remaining after sufficient drying or curing.

2. A coating has 75-percent volume solids and requires 6 mils DFT. What is the estimated WFT before adjustment for thinner?

View Answer

Six mils divided by 0.75 equals an estimated wet film thickness of 8 mils.

3. Why should wet film thickness be checked early during application?

View Answer

It allows the applicator to adjust technique before a large area cures outside the required thickness range.

4. Why can abrasive-blasted steel affect a dry-film gauge reading?

View Answer

The peaks and valleys of the surface profile influence the distance sensed between the probe and the magnetic substrate.

5. Does one gauge reading establish that an inspection area meets the specification?

View Answer

No. The specified procedure requires multiple readings and spot measurements to evaluate the inspection area.

6. Why is excessive coating thickness not automatically acceptable?

View Answer

Excessive film can cause runs, solvent entrapment, slow cure, cracking, wrinkling, or poor adhesion between coating layers.

Tags: wet film thickness, dry film thickness, coating thickness, ASTM D4414, ASTM D7091, SSPC PA 2, DFT gauge, WFT gauge, industrial coatings, film thickness inspection, corrosion protection, coating quality control