AirSprayTech Academy Certificate Program
Secondary Containment Coating Systems for Industrial Contractors
Article 18 of 24
Film Thickness, Coverage, and Lining Continuity
A containment lining must do more than cover the substrate. It must achieve the
specified thickness across the entire area, remain continuous through difficult
details, and provide no direct pathway for chemicals to reach the concrete or steel.
Learning Objectives
After completing this article, you should be able to:
- Explain why specified film thickness is part of the containment-system design.
- Distinguish wet-film thickness, dry-film thickness, laminate thickness, and system thickness.
- Use theoretical coverage and actual material usage as production-control tools.
- Recognize where low film thickness commonly occurs.
- Understand the limitations of thickness measurements on concrete and rough surfaces.
- Explain why acceptable thickness does not, by itself, prove lining continuity.
Thickness Is an Engineering Requirement
Film thickness influences chemical permeation, pinhole resistance, durability,
abrasion resistance, crack distribution, reinforcement coverage, and service life.
The specified thickness is therefore not an appearance preference or a general target.
The project may specify minimum, maximum, nominal, or range requirements for
individual coats and the complete system. These terms are not interchangeable.
The specification should also identify the number of readings, measurement
frequency, acceptable variation, and procedure for correcting deficient areas.
A contractor should resolve unclear thickness requirements before estimating
material or beginning application. A specification stating only “apply two coats”
does not establish the thickness or performance of those coats.
Average Thickness Can Conceal Failure
A satisfactory average does not prove that every location meets the required
minimum. Excessively thick areas can mathematically hide thin areas when all
readings are averaged together.
Chemicals will find the thin spot, pinhole, exposed edge, unsealed penetration,
or missed corner. Thickness acceptance must therefore consider individual readings,
spot measurements, area averages, specified restriction levels, and critical details.
Do not use heavy application in accessible areas to compensate for inadequate
coverage in difficult areas.
Know Which Thickness Is Being Specified
-
Wet-film thickness:
The thickness immediately after a liquid coating is applied and before
volatile loss or cure.
-
Dry-film thickness:
The thickness of a cured coating layer remaining on the substrate.
-
Laminate thickness:
The total thickness of resin and reinforcement in a constructed laminate.
-
Mortar thickness:
The depth of a filled resin or resin-mortar layer.
-
Total system thickness:
The combined thickness of all specified primers, repair layers, membranes,
laminates, coatings, and topcoats when the specification defines them as part
of the measured system.
Confirm which layers are included in the requirement. A primer absorbed into
concrete may not contribute measurable thickness in the same way as a high-build lining.
Wet-Film Thickness
Wet-film measurement gives the applicator immediate information while the coating
can still be adjusted. A notched gauge is pressed into the wet coating and removed.
The wetted teeth indicate an approximate thickness range.
The gauge must contact the underlying substrate or established reference plane.
On very rough concrete, deeply profiled steel, aggregate-filled coatings, or rapidly
gelling materials, readings may be difficult to obtain or interpret.
Take measurements promptly before solvent loss, leveling, reaction, or cure changes
the film. Position readings throughout the work rather than selecting only areas
that are easy to reach.
Repair the small gauge marks when required by the lining manufacturer. Do not leave
a measurement mark that becomes a discontinuity in the finished system.
Estimating Wet Film from Volume Solids
For a conventional liquid coating, an approximate wet-film target may be calculated
from the desired dry-film thickness and the material’s volume-solids percentage:
Required Wet-Film Thickness = Desired Dry-Film Thickness ÷ Volume Solids as a Decimal
For example, a coating with 80-percent volume solids theoretically requires
approximately 25 wet mils to produce 20 dry mils:
20 mils ÷ 0.80 = 25 wet mils
This is a theoretical relationship. Surface profile, porosity, roughness, thinning,
application loss, overspray, retained solvent, and reaction behavior can affect
the actual result. Use the manufacturer’s published guidance for the specific product.
Effect of Thinning
When approved thinner is added, additional wet material may be required to produce
the same dry thickness because a greater portion of the applied volume will evaporate.
Unauthorized or undocumented thinning makes thickness calculations unreliable and
can affect sag resistance, solvent release, cure, volatile emissions, and chemical resistance.
Record the thinner name, amount, batch, area, and reason for use. Never add thinner
to restore material that has exceeded its pot life.
Dry-Film Thickness on Steel
Nonmagnetic coatings on ferrous steel can commonly be measured with magnetic or
electronic dry-film-thickness gauges. Instruments must be appropriate for the
substrate, thickness range, geometry, surface profile, and project procedure.
Before use, the inspector should perform the required instrument verification and
adjustment using suitable standards and the prepared substrate or a representative
reference surface.
Abrasive-blast profile affects gauge response. The project procedure should address
how the instrument will be adjusted and how readings will be interpreted over the
prepared surface.
Record individual gauge readings, spot measurements, area measurements, instrument
identification, verification results, and locations. Do not record only a final average.
Thickness Measurement on Concrete
Concrete is nonuniform, porous, and often too rough for the same nondestructive
instruments used on steel. Primers may penetrate the surface, while high-build
layers follow the texture of peaks, valleys, pores, patches, and aggregate.
Approved thickness-control methods may include:
- Wet-film measurements when practical
- Notched squeegee or screed control
- Material usage compared with measured area
- Witness panels prepared with the production work
- Depth gauges or controlled reference points
- Destructive measurements at agreed locations
- Core samples when specifically required
- Ultrasonic or specialized instruments approved for the system
Any destructive measurement location must be repaired using the approved procedure
and reinspected before acceptance.
Laminate and Mortar Thickness
Fiberglass-reinforced laminates and resin mortars may be much thicker than ordinary
coatings. Their thickness can be controlled by reinforcement schedule, aggregate
size, screed rails, gauge pins, depth measurements, sample panels, or destructive checks.
A laminate with the specified number of reinforcement layers can still be too thin
if the glass is compressed improperly or the resin content is insufficient.
It can also become excessively thick where laps are stacked or resin pools in corners.
Mortar thickness should be checked at high areas, low areas, slopes, drains, coves,
transitions, and repaired regions—not only in the middle of the floor.
Theoretical Coverage
One U.S. gallon theoretically covers approximately 1,604 square feet at one mil
of wet-film thickness on a perfectly smooth, nonporous surface with no waste.
Theoretical Coverage in Square Feet per Gallon = 1,604 × Volume Solids ÷ Desired Dry Mils
If a 100-percent-solids material is applied at 40 dry mils, theoretical coverage
is approximately:
1,604 ÷ 40 = Approximately 40 Square Feet per Gallon
This calculation does not include surface profile, porosity, texture, overspray,
hose and equipment hold-up, container residue, waste, spills, detail work, or
application loss. Actual estimating coverage must include realistic loss factors.
Material Usage Is a Warning System
Compare material used with the measured area completed during each shift or work zone.
Do not wait until the entire project is finished.
Unusually low usage may indicate:
- Thin application
- Excessive spreading
- Missed or skipped areas
- Incorrect area calculation
- Unauthorized thinning
- Incomplete material-transfer records
Unusually high usage may indicate:
- Excessive film thickness
- Overspray or application waste
- Leaks or spills
- Material remaining in hoses or equipment
- Greater-than-expected porosity or surface roughness
- Incorrect area measurement
Material usage does not replace physical inspection, but it helps identify where
additional investigation is needed.
Where Thin Areas Commonly Occur
- Sharp edges and outside corners
- Inside corners without proper coves
- Welds, bolts, nuts, brackets, and stiffeners
- Behind pipes and equipment
- Around drains, sumps, and penetrations
- At wall-to-floor transitions
- At spray-pass edges
- Near the beginning and end of roller or squeegee runs
- On rough or highly porous concrete
- Over deep blast profiles and corrosion pits
- At vertical terminations and tie-ins
- Where access or lighting is poor
Stripe Coating and Detail Coats
A stripe coat places additional material on edges, welds, corners, bolts, pits,
penetrations, and other locations that are difficult to cover uniformly during
general application.
Stripe coats may be brushed, rolled, or sprayed as permitted. The material,
color, sequence, recoat interval, and extent of stripe coating should be defined
by the specification.
A stripe coat does not correct contamination, a sharp untreated edge, defective
weld, active crack, or improper detail. The substrate condition must be acceptable first.
Thickness Does Not Prove Continuity
A lining can meet its thickness requirement and still contain a pinhole, holiday,
crack, cut, exposed fiber, unsealed termination, or missed area. Thickness and
continuity are separate acceptance questions.
Continuity inspection may include close visual examination, holiday testing,
water testing where appropriate, detail inspection, seam examination, or another
project-approved method.
The completed system must satisfy both the thickness requirement and the continuity
requirement before it is accepted.
Holiday Testing
Holiday testing uses electrical equipment to locate discontinuities in a nonconductive
lining applied over a conductive substrate or another suitable conductive reference.
Low-voltage wet-sponge methods and high-voltage spark methods serve different
thickness ranges and system conditions. The specification should identify the
method, voltage, surface condition, timing, and acceptance procedure.
Excessive voltage can damage a sound lining. Insufficient voltage may fail to
detect a discontinuity. Test settings should follow the applicable standard,
lining thickness, manufacturer’s recommendation, and project specification.
Mark each holiday without damaging the surrounding film. Repair it using the
approved procedure, allow the repair to cure, and retest it before acceptance.
Excessive Thickness Can Also Cause Failure
More material is not automatically safer. Application above the permitted maximum
can cause solvent retention, excessive reaction heat, sagging, wrinkling, cracking,
incomplete cure, internal stress, or poor intercoat adhesion.
Excessive thickness often develops at overlaps, corners, coves, drains, low areas,
end points, and locations repeatedly crossed by the spray pattern.
Do not grind or remove an excessively thick area without an approved repair procedure.
Surface correction can expose reinforcement or reduce the chemical-barrier layer.
Correcting Low Thickness
Before adding material, determine whether the surface remains within its recoat
window. If the window has been exceeded, the existing coating may require cleaning,
abrasion, solvent treatment when expressly approved, or a tie coat.
The repair should extend beyond the deficient area onto sound, properly prepared
coating. Feathered repair edges should follow the manufacturer’s procedure and
should not leave exposed reinforcement or an unsealed termination.
After cure, repeat the applicable thickness and continuity testing. A repair is
not complete merely because additional coating was applied.
Thickness-Control Plan
Before production application begins, the contractor should establish:
- Thickness requirement for every system layer
- Whether requirements are minimum, maximum, nominal, or a range
- Approved wet- and dry-thickness measurement methods
- Instrument verification and adjustment procedures
- Measurement frequency and locations
- Treatment of rough concrete and abrasive-blasted steel
- Theoretical coverage and expected field-loss factor
- Material-usage tracking by area
- Holiday-testing method and voltage
- Acceptance criteria for individual and area measurements
- Repair procedure for low or excessive thickness
- Retesting and final documentation requirements
Thickness and Coverage Records
Records should identify:
- Project, area, elevation, grid, or equipment identification
- Product, coat number, color, and batch numbers
- Specified thickness for the layer and complete system
- Wet-film readings and locations
- Dry-film readings, spots, area averages, and locations
- Instrument manufacturer, model, and serial number
- Instrument verification and adjustment results
- Measured area and quantity of material used
- Approved thinner and amount added
- Deficient locations and corrective actions
- Repair thickness and retest results
- Holiday-test results and repaired discontinuities
- Inspector or responsible applicator identification
Field Quality-Control Checklist
- Thickness requirement for each layer is understood.
- Approved measurement methods are available.
- Instruments are suitable and properly verified.
- Wet-film measurements are taken while correction is practical.
- Dry-film readings are distributed across the work.
- Critical details receive separate thickness evaluation.
- Material usage is compared with actual area completed.
- Stripe coats and detail coats are completed as specified.
- Individual low readings are not hidden by high averages.
- Excessive thickness is evaluated and corrected properly.
- Continuity is inspected separately from thickness.
- Holiday-test equipment and voltage are appropriate.
- All repairs are cured and retested.
- Final measurements and locations are documented.
Technical References
Use the editions identified in the contract documents and verify current
designations before incorporating standards into a proposal, submittal,
inspection plan, or acceptance procedure.
Key Takeaways
- Film thickness is a system-design requirement, not an appearance preference.
- Confirm whether the specification addresses each coat, the complete system, or both.
- Wet-film readings allow correction while material is still workable.
- Dry-film instruments must be appropriate and properly verified.
- Concrete may require different thickness-control methods than steel.
- Track material usage by measured area as an early warning system.
- Individual thin areas must not be hidden by acceptable averages.
- Excessive thickness can be as damaging as insufficient thickness.
- Acceptable thickness does not prove that the lining is holiday free.
- Repairs must be reinspected and retested before final acceptance.
Professional responsibility:
This article provides foundational educational information and is not a substitute
for the project specification, engineering direction, inspection qualification,
or the coating manufacturer’s current written instructions. Always confirm the
required thickness, measurement method, frequency, acceptance limits, instrument
procedure, holiday-test method, voltage, repair procedure, and documentation
requirements before application begins.
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