Article 12: Intermediate Coats, Finish Coats, and Stripe Coating

Building Film Thickness and Protecting the Areas Most Likely to Fail

Every Coat Has a Job

A multi-coat protective system is designed so that its individual layers work together. The layers are not automatically interchangeable, and each coat may provide different performance characteristics.

A typical system may include:

1. Primer: Adheres to the substrate and begins the corrosion-protection system.
2. Stripe coat: Adds material to edges, welds, bolts, crevices, and other difficult areas.
3. Intermediate coat: Builds thickness and strengthens the protective barrier.
4. Finish coat: Resists weathering and provides the system's final appearance and service surface.

Not every system contains all four stages. Some products combine functions, and some specifications require more than one stripe or intermediate coat. The approved specification and product data control the actual sequence.

The Role of the Intermediate Coat

The intermediate coat is sometimes called the build coat or body coat. Its primary job is often to increase the total film thickness and improve the barrier between the steel and the service environment.

Depending on the system, an intermediate coat may provide:

• Additional resistance to moisture and oxygen penetration
• Increased total dry film thickness
• Improved chemical resistance
• Greater abrasion or impact resistance
• A compatible bridge between primer and finish coat
• Improved coverage of surface irregularities
• A contrasting color that helps verify coverage

High-build epoxies are commonly used as intermediate coats because they can produce substantial film thickness and provide strong barrier protection. The exact product must still be suitable for the service environment and compatible with the primer and finish coat.

Why Multiple Coats Improve Barrier Protection

No field-applied coat is perfectly uniform. One coat may contain a thin spot, pinhole, or minor application irregularity. A properly applied second coat can cover weaknesses in the preceding layer.

Multiple coats also create a more difficult path for moisture, oxygen, and ions attempting to reach the steel. This is especially valuable when coat colors are contrasted and spray passes are arranged to provide complete coverage.

Additional coats do not excuse poor workmanship. Contamination, incomplete cure, missed areas, or poor intercoat adhesion can create failure between layers even when the total thickness appears acceptable.

The Role of the Finish Coat

The finish coat is the outermost layer and receives direct exposure to sunlight, weather, chemicals, abrasion, cleaning, and normal service activity.

A finish coat may be selected to provide:

• Ultraviolet and weathering resistance
• Color and gloss retention
• Chemical or stain resistance
• Abrasion and impact resistance
• Improved cleanability
• Safety, identification, or appearance colors
• Protection of an ultraviolet-sensitive intermediate coat

The finish coat must be compatible with the underlying layer and suitable for the actual exposure. A coating selected mainly for appearance may not provide the chemical, abrasion, or weathering resistance required by an industrial structure.

Common Finish-Coat Technologies

The following are general descriptions. Performance varies by formulation, and the manufacturer's current product data must be used for the specific material.

Polyurethane Finish Coats

Aliphatic polyurethane coatings are commonly used where exterior color and gloss retention are important. They can provide good weathering, abrasion resistance, and cleanability.

Product chemistry and worker-protection requirements must be reviewed carefully. Many polyurethane coatings involve reactive components that require specific respiratory, skin, ventilation, and handling controls.

Polysiloxane Finish Coats

Polysiloxane coatings may provide exterior weathering, color retention, gloss retention, and corrosion resistance. Some systems use a polysiloxane finish to reduce the number of coats while still meeting performance requirements.

Fewer coats do not mean fewer controls. Surface preparation, application thickness, coverage, cure, and compatibility remain critical.

Epoxy Finish Coats

Epoxies can provide strong barrier, chemical, and abrasion resistance. They are often used as final coats in interior, immersion, or other services where ultraviolet appearance is not the primary concern.

Exterior ultraviolet exposure can cause many epoxies to chalk, fade, or lose gloss. Chalking is not always immediate corrosion failure, but it can reduce appearance and gradually erode the coating surface.

Other Specialized Finishes

Acrylic, fluoropolymer, silicone, moisture-cure, polyurea, and other specialized coatings may be selected for particular weathering, heat, chemical, cure-time, or service requirements. Generic coating names alone are not enough to establish suitability.

What Is Stripe Coating?

Stripe coating is the deliberate application of an additional coating layer to edges, corners, welds, fasteners, crevices, pits, and other irregular areas where a normal full coat may not produce adequate coverage.

Stripe coating is not decorative striping. It is a corrosion-control procedure intended to increase film continuity and thickness on vulnerable details.

Why Edges Receive Less Coating

Wet coating tends to pull away from a sharp edge as surface tension causes the film to flow and cure. This behavior can leave the coating thinner on the edge than on the adjacent flat steel.

Abrasive blasting can also make an unprepared edge sharper or create irregular projections. If the edge is not rounded or treated as required, the applied coating may not maintain enough thickness to protect it.

Corrosion that begins along an edge can spread beneath surrounding coating and become an early system failure.

Areas That May Require Stripe Coating

• Plate edges and cut edges
• Inside and outside corners
• Welds and weld transitions
• Bolt heads, nuts, and fasteners
• Crevices, seams, and lap joints
• Pits and irregular corrosion areas
• Stiffeners, brackets, and structural connections
• Drain holes and openings
• Areas around attachments and penetrations
• Locations difficult to cover by spray application

The specification should identify the required areas. The contractor should also report difficult details that may not be adequately addressed by the written requirements.

Surface Preparation Before Stripe Coating

Stripe coating begins with proper steel preparation. Edges, welds, and irregular areas should be examined before abrasive blasting and coating application.

Preparation may include:

• Rounding or easing sharp edges as specified
• Removing weld spatter
• Grinding sharp projections and burrs
• Addressing unacceptable weld irregularities
• Removing slag, flux, soot, and fabrication debris
• Cleaning pits and crevices
• Abrasive blasting difficult areas to the specified standard
• Removing dust and loose abrasive

Brush, Roller, or Spray?

Stripe coats may be applied by brush, roller, spray, or a combination of methods when permitted by the specification and product manufacturer.

Brush Application

Brushing can work coating into pits, weld contours, bolt assemblies, and irregular surfaces. The applicator can see and physically work the material into difficult locations.

Brushing should not leave excessive ridges, thick pockets, missed areas, loose bristles, or a stripe so wide that it interferes with the appearance of the full coat.

Roller Application

A small roller may be effective on certain edges and accessible details. Roller type, nap, solvent resistance, and the coating manufacturer's application instructions must be considered.

Rolling may not work material into deep pits, tight crevices, or complex bolt assemblies as effectively as brushing.

Spray Application

Spray application can be used for stripe coating when approved, but it requires controlled gun angle, distance, pattern, and travel speed. The applicator must direct material onto the vulnerable detail rather than merely fogging the surrounding area.

Spray striping may be productive on long edges and repetitive details, but complex connections may still require brushing.

When Is the Stripe Coat Applied?

The coating specification should define the stripe-coat sequence. Possible sequences include:

• Stripe coat before the full primer coat
• Stripe coat after the full primer coat
• Stripe coat between system coats
• More than one stripe coat at different stages

Applying the stripe coat before a full coat allows the full coat to cover and blend with the stripe. Applying it after a full coat can make coverage easier to inspect. Each sequence has practical advantages and limitations.

The contractor should not change the specified sequence without approval. The stripe coat must also remain within its recoat and curing limits.

Avoid Excessive Stripe-Coat Thickness

Stripe coating is intended to correct thin-film risk, not create uncontrolled buildup. Heavy brush deposits, puddles, and repeated overlapping can produce excessive thickness.

Excessive stripe-coat thickness can cause:

• Runs and sags
• Solvent entrapment
• Slow or incomplete cure
• Cracking or wrinkling
• Poor adhesion of later coats
• Visible ridges in the finished work

Stripe coating requires deliberate coverage, not simply more material.

Using Contrasting Coat Colors

Adjacent coating layers are often supplied in contrasting colors. The color difference helps applicators and inspectors identify skips, thin areas, shadowing, and incomplete coverage.

Colors should be different enough to support inspection without interfering with the final appearance. The finish coat must completely cover the underlying color at the specified film thickness.

Color contrast is a visual aid. It does not replace wet film or dry film thickness measurement.

Preparing the Previous Coat

Before applying an intermediate or finish coat, inspect the preceding layer. It must be sufficiently cured, clean, sound, and suitable for recoating.

Possible intercoat contaminants include:

• Dust and abrasive
• Oil and grease
• Condensation or rainwater
• Chalk and degraded coating
• Amine blush on susceptible epoxy coatings
• Zinc salts on zinc-rich primers
• Overspray from nearby work
• Dirt from handling, scaffolding, or foot traffic

Contamination must be removed using the approved method. Applying another coat over contamination can produce intercoat failure even though each individual product is compatible.

Minimum and Maximum Recoat Times

The minimum recoat time is the earliest point at which the next coat may be applied under the stated conditions. Applying too soon can disturb the previous film, trap solvent, or interfere with cure.

The maximum recoat interval is the longest permitted time before additional preparation may be required. As some coatings cure, their surfaces become harder, smoother, or less chemically receptive to another coat.

If the maximum interval is exceeded, the surface may require cleaning, abrasion, application of a tie coat, or another manufacturer-approved procedure.

Recoat times vary with temperature, humidity, ventilation, film thickness, and product formulation. The crew should use actual conditions rather than relying on a single time printed for a different temperature.

Intercoat Adhesion

Intercoat adhesion is the bond between coating layers. The complete system can separate between coats when the underlying surface is contaminated, overcured, undercured, too smooth, or incompatible with the next material.

Good intercoat adhesion depends on:

• Approved product compatibility
• Correct cure of the previous coat
• Application within the permitted recoat window
• Removal of surface contamination
• Required mechanical abrasion or surface treatment
• Acceptable environmental conditions

When compatibility or recoat condition is uncertain, obtain written technical direction before applying the next coat.

Checking the Previous Coat for Defects

Applying another coat does not make existing defects disappear. The next layer may hide the defect visually while leaving the cause of failure in place.

Before recoating, inspect for:

• Runs and sags
• Dry spray and overspray
• Pinholes, craters, and holidays
• Mud cracking, checking, or wrinkling
• Bubbles, blisters, or trapped debris
• Soft or uncured coating
• Areas below or above the specified thickness
• Mechanical damage
• Missed edges, bolts, welds, or shadowed areas

Defects should be evaluated and corrected using an approved procedure before the next coat is applied.

Controlling Film Thickness by Coat

A total system thickness within the specified range does not prove that every individual coat is acceptable. One coat may be too thin while another is excessively thick.

Specifications may establish thickness requirements for each coat and for the total system. Wet film measurements can help control application, while dry film measurements verify the cured film according to the required inspection procedure.

Contractors should maintain coat-by-coat records rather than waiting until the complete system has been applied.

Managing Overspray and Dry Spray

Dry spray occurs when coating droplets partially dry before reaching or merging into the surface film. The result may be a rough, powdery, or poorly bonded surface.

Contributing conditions include:

• Excessive spray-gun distance
• Incorrect gun angle
• High air movement
• Hot surfaces or high temperatures
• Poorly balanced atomization
• Spraying complex steel from only one direction

Dry spray should not be buried beneath the next coat without evaluation. The approved correction may include removal, sanding, cleaning, or recoating within specified limits.

Maintaining a Wet Edge

On large surfaces, the applicator should plan the work so each pass blends into coating that is still receptive. Allowing one section to dry before the next pass overlaps it can produce lap marks, rough texture, uneven color, and inconsistent thickness.

Wind, heat, complex geometry, fast-drying materials, and poor work sequencing make wet-edge control more difficult. The crew should plan break points around natural structural boundaries whenever possible.

Protecting the Finish Coat

The finish coat must be protected during curing and subsequent construction activity. Scaffolding, welding, rigging, tools, foot traffic, dust, concrete work, and equipment installation can damage a completed coating system.

Final inspection should not occur until the coating is sufficiently cured for access and examination. Damage discovered later should be repaired using the approved system and documented.

Common Multi-Coat System Mistakes

• Applying the next coat over contamination: The system separates between layers.
• Recoating too soon: Solvent becomes trapped or the previous coat is disturbed.
• Exceeding the maximum recoat interval: Intercoat adhesion may be reduced.
• Using similar colors for adjacent coats: Missed areas become difficult to see.
• Skipping stripe coating: Edges, welds, and fasteners remain underprotected.
• Applying stripe coats too heavily: Excessive buildup causes cure and appearance problems.
• Burying defects beneath the next coat: The visible evidence disappears, but the defect remains.
• Checking only total system thickness: An individual coat may be outside its permitted range.
• Ignoring construction damage: Completed coating is damaged before the structure enters service.

Contractor's Intercoat and Stripe-Coat Checklist

• The required coating sequence has been confirmed.
• Stripe-coat locations and application method are understood.
• Edges, welds, bolts, and irregular areas are properly prepared.
• The previous coat is sufficiently cured.
• The surface is within the approved recoat window.
• Dust, moisture, salts, chalk, blush, and other contaminants are absent.
• Defects and mechanical damage have been corrected.
• Environmental conditions are acceptable.
• Adjacent coats have suitable color contrast.
• Wet film thickness will be monitored during application.
• Dry film thickness will be verified for each required coat.
• Completed work will be protected during cure and construction.

Key Takeaways

• Intermediate coats build film thickness and barrier protection.
• Finish coats resist the final service environment and provide appearance.
• Stripe coating adds protection to edges, welds, bolts, crevices, and irregular details.
• Stripe coating does not replace proper steel preparation.
• Contrasting coat colors help reveal missed and thin areas.
• Every preceding coat must be clean, sound, and properly cured before recoating.
• Minimum and maximum recoat times both matter.
• Film thickness should be controlled and documented by coat.

Knowledge Check

1. What is the primary purpose of an intermediate coat?

View Answer

It generally builds system thickness and strengthens the protective barrier between the steel and its environment.

2. What is the purpose of a finish coat?

View Answer

It provides the system's final service surface and may supply weathering, ultraviolet, chemical, abrasion, color, gloss, and cleanability properties.

3. Why are sharp edges vulnerable to early corrosion?

View Answer

Wet coating tends to pull away from a sharp edge, leaving less film thickness than on adjacent flat steel.

4. Does stripe coating correct poor welds or sharp, unprepared edges?

View Answer

No. Required steel and weld preparation must be completed before stripe coating.

5. Why are contrasting colors useful between coating layers?

View Answer

They help applicators and inspectors identify missed areas, thin coverage, and shadowing.

6. What should happen when the maximum recoat interval has been exceeded?

View Answer

The surface should receive the cleaning, abrasion, tie coat, or other treatment required by the specification and coating manufacturer before recoating.

Tags: intermediate coats, finish coats, stripe coating, industrial coatings, corrosion protection, edge coating, weld coating, polyurethane topcoat, polysiloxane coating, intercoat adhesion, recoat window, film thickness