Stripe Coating, Edges, Welds, Penetrations, and Difficult Areas

Why Difficult Areas Fail First

Spray application works best on open, flat surfaces. Edges, welds, corners, bolts, pits, nozzles, attachments, and penetrations interrupt the spray pattern and change how liquid lining flows and cures.

At a sharp edge, surface tension may pull the wet lining away from the edge. Behind an attachment, the spray fan may not reach the surface. In a deep pit or rough weld, air can remain trapped beneath the lining.

These areas may look coated while containing thin film, voids, pinholes, bridging, or holidays.

What Is Stripe Coating?

Stripe coating is the application of an additional coat or controlled pass to areas that are difficult to cover uniformly during general spray application.

Stripe coating may be applied by brush, roller, spray gun, dauber, or another approved method. The method, material, sequence, width, thickness, and recoat time should be established by the specification and lining manufacturer.

Typical stripe-coat locations include:

• Plate edges and corners
• Welds and weld transitions
• Bolt heads, nuts, and fasteners
• Pits, depressions, and irregular steel
• Stiffeners, brackets, and attachments
• Nozzles, manways, and pipe penetrations
• Drains and wall-to-floor transitions
• Lining terminations and cutback areas

Stripe Coating Is Not a Repair for Bad Fabrication

Stripe coating improves coverage. It does not correct sharp edges, weld spatter, undercut, pinholes in welds, slag, laminations, open seams, or inaccessible crevices.

Fabrication defects must be corrected before surface preparation and lining. Depending on the specification, work may include:

• Rounding or grinding sharp edges
• Removing weld spatter and slag
• Smoothing rough weld starts and stops
• Repairing weld porosity, undercut, and pinholes
• Closing open seams or skip welds
• Grinding sharp projections
• Filling or fairing approved pits and depressions

Edges and Corners

Sharp edges are difficult to coat because wet lining tends to pull away as it cures. The dry-film thickness directly over the edge may be much lower than the thickness on the adjoining flat surface.

Inspect:

• Cut plate edges
• Stiffener edges
• Structural angles
• Access openings
• Support brackets
• Flange edges
• Internal and external corners

Edges should be rounded or treated to the specified condition before blasting. The stripe coat must wet the edge and extend onto both adjoining surfaces.

Welds

Welds create irregular shapes that can trap air and interrupt spray coverage. A weld may also contain spatter, slag, undercut, porosity, sharp ridges, and rough transitions.

After blasting, inspect the weld again. Blasting can reveal defects that were hidden by rust, mill scale, or previous coating.

Apply stripe material so it reaches:

• The weld face
• Both weld toes
• Starts and stops
• Ground or repaired areas
• Small pits and irregularities surrounding the weld

Avoid leaving a thick ridge along the sides of the stripe coat. The general lining coat should transition smoothly over the striped area.

Pits and Corroded Areas

Deep pits are difficult to blast clean, inspect, fill, and coat. They can retain salts, dust, abrasive, air, and moisture after the surrounding steel appears ready.

Clean and inspect pits closely. Determine whether section loss requires engineering evaluation or steel repair. Approved filling material may be used when the specification allows it, but filler does not restore structural thickness.

Stripe material should be worked into approved pits without trapping air. Simply spraying across the opening can bridge the pit.

Bolts, Nuts, and Fasteners

Fasteners contain sharp edges, recesses, threads, and shielded surfaces. General spray application may leave thin film behind bolt heads and around washers and nuts.

Stripe coating should cover all accessible sides without leaving runs, air pockets, or thick rings around the fastener. Bolted connections that cannot be cleaned and coated continuously may require a special detail.

Attachments and Crevices

Brackets, clips, supports, ladders, stiffeners, and reinforcement pads create shadowed areas. Closely spaced components may prevent the blast nozzle, applicator, wet-film gauge, or holiday detector from reaching the surface.

Inspect access before application. If an area cannot be properly prepared, coated, and inspected, report it before the lining is applied.

Nozzles, Manways, and Penetrations

Penetrations combine curved surfaces, welds, sharp transitions, and difficult spray angles. These areas are especially vulnerable to thin film and holidays.

Confirm:

• How far the lining extends into the nozzle or pipe
• Where the lining terminates
• Whether flange faces are included or excluded
• How gasket-contact surfaces are protected
• Whether reinforcement is required
• How the completed area will be inspected and holiday tested

Use controlled passes from different directions. Avoid excessive buildup at the nozzle-to-shell transition.

Concrete Transitions and Penetrations

Concrete linings require detailing around drains, pipe sleeves, embedded steel, wall-to-floor transitions, equipment bases, cracks, and joints.

Inside corners may require a cove or fillet to eliminate a sharp angle. Penetrations may require compatible sealant, mortar, reinforcement, or flexible detailing.

Do not use lining material to fill large voids unless the system specifically permits it. Repair and transition materials must be compatible with the primer, lining, substrate, chemical service, and expected movement.

Inside and Outside Corners

Inside corners can collect excessive material and trap air. Outside corners tend to receive insufficient thickness directly over the edge.

Brush, Roller, or Spray?

Brush application

Brushing can work material into welds, pits, edges, and irregular surfaces. The applicator must avoid heavy ridges, brush contamination, loose bristles, and overworking fast-reacting material.

Roller application

Small rollers may provide consistent coverage on certain details but can bridge recesses or introduce bubbles. Use only the roller type approved for the material.

Spray application

Spray stripe coating can cover complex shapes rapidly but may reproduce the same shadowing and thin-film problems as the general spray coat. Use controlled gun angles and multiple directions.

The project specification should establish the permitted method. The applicator should not select a method solely because it is faster.

Stripe-Coat Sequence

A stripe coat may be applied before or after a general coat, depending on the approved system. Some projects require stripe coating at more than one stage.

The sequence must account for:

• Primer cure and recoat window
• Stripe-coat cure and recoat window
• Compatibility between coats
• Required total system thickness
• Visibility of the stripe coat during inspection
• Potential contamination between operations

Do not apply the general coat over uncured stripe material unless the manufacturer's instructions allow wet-on-wet application.

Stripe-Coat Thickness

Stripe coating is intended to improve coverage, not create an uncontrolled mound of material. Excessive thickness can sag, crack, trap solvent, cure unevenly, or interfere with application of the general coat.

Thin the stripe material only when expressly permitted. Excess thinner may reduce protective thickness and increase runs, emissions, and cure time.

Apply enough material to wet and protect the detail while maintaining a smooth transition to adjacent surfaces.

Avoid Bridging

Bridging occurs when lining spans across a gap, pit, crevice, crack, or inside corner without contacting the surface beneath it.

A bridge may look complete from the exposed side but can contain an air pocket or open path underneath. Pressure, temperature changes, impact, or chemical exposure can rupture the unsupported film.

Use proper repair, filling, coving, reinforcement, or fabrication correction instead of relying on the lining to span an unsupported opening.

Reinforcement at Difficult Details

Some lining systems require fiberglass mat, fabric, mesh, scrim, or flexible reinforcement at corners, joints, cracks, penetrations, and transitions.

Reinforcement must be:

• The type approved for the lining system
• Cut and fitted without wrinkles or bridging
• Properly overlapped
• Fully wetted with resin
• Rolled or consolidated to remove trapped air
• Covered with the specified corrosion barrier

Dry fibers, exposed reinforcement, lifted edges, wrinkles, and air pockets are lining defects.

Inspect Before General Application

Stripe-coated areas should be inspected before they are hidden beneath the general lining coat. Check for:

• Complete coverage of specified areas
• Missed edges, weld toes, pits, and back sides
• Runs, sags, ridges, and excessive buildup
• Pinholes, bubbles, voids, and bridging
• Dry spray or poor wetting
• Contamination and embedded debris
• Proper cure and acceptable recoat condition

Correct defects using the approved procedure before applying the general coat.

Holiday Testing Difficult Areas

Holidays frequently occur at edges, welds, penetrations, pits, fasteners, and terminations. These locations require careful testing when holiday detection is specified.

Test voltage and equipment must match the lining thickness, substrate, and applicable procedure. Excessive voltage can damage the lining. Insufficient voltage or poor probe contact can miss defects.

Probes must contact the lining without skipping over geometry. Test difficult areas from several directions where necessary.

Repairing Defects

1. Mark and document the defect.
2. Determine whether the defect extends to the substrate.
3. Remove weak, uncured, contaminated, or damaged material.
4. Prepare the repair edges and exposed substrate as required.
5. Clean the area without leaving dust or residue.
6. Apply compatible repair material within the approved recoat procedure.
7. Allow the repair to cure.
8. Reinspect and retest the completed repair.

Dabbing material over a detected holiday without preparing the defect may leave contamination, poor adhesion, or an unsealed pathway beneath the patch.

When Work Should Stop

Contractor Field Checklist

Knowledge Check

Key Takeaway