Article 13: Coating Application Methods and Spray Technique

Turning Properly Prepared Steel and Mixed Coating into a Continuous Protective Film

Application Is More Than Moving Paint

The purpose of application equipment is not merely to move coating from a container to the steel. It must deliver the material at a controlled rate and allow the applicator to produce the required film.

A successful application should provide:

• Complete coverage of the prepared substrate
• Proper wetting of the surface profile
• The specified film thickness
• A continuous film without holidays or pinholes
• Uniform appearance where appearance is required
• Acceptable runs, sags, overspray, and surface texture
• Coverage of edges, welds, bolts, corners, and shadowed areas

The application method must be approved for the coating and suitable for the structure, project conditions, and required production rate.

Selecting the Application Method

Industrial coatings may be applied by brush, roller, conventional air spray, HVLP, airless spray, air-assisted airless spray, plural-component spray, or another approved method.

Selection should consider:

• Coating viscosity and solids content
• Required wet and dry film thickness
• Pot life and cure speed
• Size and geometry of the structure
• Accessibility of the work
• Required finish quality
• Overspray and containment limitations
• Ventilation and environmental conditions
• Production requirements
• Equipment and applicator capability

Brush Application

Brush application is slower than most spray methods, but it gives the applicator direct control over material placement. Brushing can work coating into pits, welds, seams, bolt assemblies, corners, and irregular surfaces.

Brush application is commonly used for:

• Stripe coating
• Small or complex components
• Touch-up and repair areas
• Locations where overspray is unacceptable
• Working coating into irregular surfaces

The brush should be compatible with the coating and suitable for the surface. The applicator should work the coating into the substrate and then level it without overbrushing.

Excessive brushing after the coating begins to set can create brush marks, pull material away from edges, reduce thickness, or disturb film formation.

Roller Application

Roller application can be productive on broad surfaces where spray is impractical. It can also reduce overspray and simplify application in occupied or restricted areas.

Roller cover material, nap length, solvent resistance, and texture should match the coating and required finish. A roller that absorbs solvent, sheds fibers, introduces bubbles, or cannot deliver the required film thickness is unsuitable.

Common roller problems include:

• Thin or uneven film
• Roller stipple
• Air bubbles
• Fiber contamination
• Missed pits and surface irregularities
• Pulling coating away from edges

Rolling is not automatically acceptable for every industrial coating. The product data and specification must permit the method.

Conventional Air Spray

Conventional air spray uses compressed air to atomize the coating at the spray gun. It can provide fine control and a high-quality finish when equipment, coating viscosity, atomizing air, fluid delivery, and technique are properly balanced.

Excessive atomizing air can increase overspray, bounce-back, material waste, and dry spray. Insufficient atomizing air can produce large droplets, heavy texture, poor breakup, and an uneven film.

The goal is not to use the highest possible air pressure. The goal is to use enough air to produce the required atomization and pattern at the desired production rate.

HVLP Spray

High-volume, low-pressure spray equipment uses a high volume of air at relatively low air-cap pressure. It can provide good finish control and improved transfer efficiency in suitable applications.

HVLP is not automatically the best method for every industrial coating. Heavy, high-solids, fast-curing, or high-build materials may require specialized equipment, fluid heating, pressure feed, or another application technology.

Air and fluid delivery must be balanced. Increasing fluid flow without sufficient atomization can create coarse droplets and heavy texture. Excessive air can create overspray without improving the finished film.

Airless Spray

Airless equipment uses high fluid pressure to force coating through a small spray tip. The pressure drop at the tip atomizes the material without adding compressed air at the gun.

Airless spray is widely used for industrial coatings because it can apply high-viscosity materials at high production rates and produce substantial film build.

Airless-system performance depends on:

• Pump capacity and pressure capability
• Correct hose diameter and length
• Filter and strainer selection
• Spray-tip orifice and fan width
• Coating viscosity and material temperature
• Pressure available while spraying
• Tip wear and equipment condition

Excessive pressure increases wear, overspray, bounce-back, and injection risk. Pressure should be only high enough to maintain an acceptable spray pattern under operating conditions.

Air-Assisted Airless Spray

Air-assisted airless equipment uses fluid pressure for most of the atomization and adds a controlled amount of compressed air at the gun to shape and refine the pattern.

This method can provide a finer finish than standard airless spray while maintaining higher production than many conventional air-spray systems. Fluid and air pressures must be balanced rather than adjusted independently without a plan.

Plural-Component Spray

Plural-component equipment proportions separate coating components and combines them at or near the spray system before application. It is commonly used for fast-reacting, high-solids, or heated protective coatings and linings.

Ratio accuracy, component temperatures, pressure balance, flow, mixing quality, hose temperature, shutdown procedures, and flushing are critical.

A coating may appear acceptable at the gun while being off ratio. Required ratio checks, alarms, data monitoring, and quality-control procedures must be followed.

Understanding the Spray Pattern

The spray pattern tells the applicator how the equipment, coating, and settings are working together. The pattern should be evaluated before coating the structure and whenever conditions or equipment change.

An unacceptable pattern may show:

• Heavy material at the top and bottom of the fan
• An uneven or one-sided fan
• Pulsation or surging
• Large droplets or poor atomization
• A narrow or unstable fan
• Spitting when the gun is triggered or released
• Excessive fog or overspray

Possible causes include incorrect pressure, improper viscosity, material that is too cold, a worn or blocked tip, insufficient fluid delivery, dirty air-cap passages, restricted filters, or unsuitable equipment.

Gun Distance

The spray gun should be held at the distance needed to produce the intended pattern and film. The correct distance depends on the spray method, gun, tip or nozzle, material, pressure, geometry, and manufacturer instructions.

Holding the gun too far away can produce dry spray, overspray, poor transfer efficiency, rough texture, and thin film. Holding it too close can create runs, sags, excessive film build, a narrow pattern, or uneven distribution.

The distance should remain consistent throughout the stroke. Swinging the gun in an arc changes the distance at both ends of the pass and produces uneven film thickness.

Gun Angle

On a flat surface, the gun should normally be held approximately perpendicular to the work so the spray pattern strikes the surface evenly.

Excessive angling directs more coating to one side of the pattern and increases overspray. Complex shapes require the applicator to reposition the gun and body so surfaces can be coated directly rather than from one convenient position.

Spray passes should be directed into corners, behind flanges, around stiffeners, under structural members, and across all sides of bolts and connections.

Overlap

Spray passes must overlap so adjacent patterns combine into a uniform film. A 50-percent overlap is commonly used as a starting technique, but the correct overlap depends on the actual pattern and coating.

Too little overlap can leave light bands between passes. Too much overlap can create excessive thickness, runs, and material waste.

The applicator should watch the wet film being formed rather than follow a percentage mechanically when the spray pattern or geometry changes.

Triggering the Spray Gun

On open surfaces, the gun should normally begin moving before it is triggered and continue moving until after the trigger is released. This helps prevent heavy spots at the beginning and end of each pass.

Trigger technique must be adapted for structural details, restricted spaces, and specialized equipment. The goal is controlled film build without dumping material into one location.

Travel Speed

Travel speed controls how much material is deposited. Moving too quickly can leave thin film, skips, and incomplete coverage. Moving too slowly can produce excessive film, runs, sags, and solvent entrapment.

The required speed changes with fluid delivery, fan width, distance, overlap, coating solids, surface geometry, and the target wet film thickness.

A trained applicator adjusts speed deliberately while maintaining gun distance and angle.

Cross-Coating

Some coating systems and surfaces benefit from passes applied in different directions. For example, one set of passes may be vertical and another horizontal.

Cross-coating can help improve coverage and uniformity, but it can also produce excessive thickness if the applicator treats each direction as a complete independent coat.

The combined passes must produce the specified wet and dry film thickness.

Coating Complex Steel

Complex structural steel cannot be coated successfully from one direction. Flanges, angles, stiffeners, bolts, back-to-back members, and connections create shadowed areas that block the spray pattern.

A practical sequence may include:

1. Complete the required stripe coating.
2. Coat difficult and recessed areas first.
3. Spray structural details from multiple directions.
4. Coat broad surfaces using controlled overlapping passes.
5. Inspect back sides, lower surfaces, and shadowed areas.
6. Correct missed or thin areas within the permitted application period.

The applicator should plan body position, hose routing, access, and work sequence before beginning. Poor access often becomes poor coating coverage.

Maintaining a Wet Edge

Each spray pass should merge into coating that is still wet enough to form a continuous film. If the previous section has already set, the overlap may remain visible or develop poor texture.

Large surfaces should be divided into manageable work areas using natural breaks, seams, edges, or structural boundaries whenever possible.

Temperature, wind, ventilation, coating cure speed, and production rate all affect how long the wet edge remains workable.

Wet Film Thickness as a Process Control

Wet film thickness should be checked while the coating is still wet enough for the applicator to adjust technique. Waiting for dry film inspection may allow a large nonconforming area to be completed.

A wet film thickness gauge is pressed into the wet coating, and the wetted teeth are used to estimate the film thickness at that location. The gauge must be used promptly and cleaned after each reading.

The target wet film thickness is related to the required dry film thickness and the coating's volume solids. Permitted thinning must be considered.

This calculation is a practical estimate. Use the manufacturer's instructions and project procedure when they provide a specific target or calculation method.

Control the Material Temperature

Coating temperature affects viscosity, atomization, pressure requirements, pot life, flow, leveling, and cure.

Cold material may require excessive pressure and may atomize poorly. Hot material may have a shortened pot life, reduced sag resistance, and rapid solvent release.

Condition the material within the manufacturer's approved range. Do not use unapproved thinner to compensate for poor material-temperature control.

Monitor Equipment During Production

Equipment conditions change during the shift. Filters load, spray tips wear, coatings react, temperatures change, and components can loosen or leak.

Monitor for:

• Changing spray-pattern shape
• Pressure fluctuations or pump surging
• Material leaks
• Blocked or damaged filters
• Worn spray tips
• Settling of heavy pigments
• Changes in viscosity or material temperature
• Material approaching the end of its pot life

Stop and correct equipment problems before they become coating defects.

Common Application Defects

These are possible causes rather than automatic conclusions. Defect investigation should include the coating, surface, environment, equipment, mixing records, and application history.

Good Lighting Is a Quality-Control Tool

Applicators cannot control what they cannot see. Lighting should allow the painter to observe the wet edge, spray pattern, coverage, surface texture, runs, dry spray, and missed areas.

Complex structures may require portable or repositioned lighting. Bright light from one direction can create deep shadows, so the lighting arrangement should be evaluated as the work progresses.

Housekeeping and Contamination Control

Dust, spent abrasive, fibers, insects, grinding debris, and overspray can become embedded in wet coating. Cleaning only the steel is not enough if surrounding surfaces continue releasing contamination.

Coordinate blasting, grinding, cleaning, coating, and other trades so contamination is not directed toward prepared or freshly coated surfaces.

Spray Safety

Industrial spray application can expose workers to coating vapors, aerosols, high pressure, static electricity, moving equipment, confined spaces, and flammable materials.

High-pressure fluid can penetrate skin and cause a medical emergency. Never point an airless gun at any person, place a hand near a spray tip, or attempt to stop a leak with a body part.

Before cleaning, changing tips, opening filters, or servicing the system, shut down the equipment and relieve stored pressure according to the equipment manufacturer's procedure.

Use the required respiratory protection, protective clothing, gloves, eye protection, ventilation, grounding, bonding, and ignition controls. Follow the project safety plan, safety data sheet, equipment instructions, and applicable regulations.

Contractor's Application Checklist

• The approved coating and application method have been confirmed.
• The prepared surface has been accepted.
• Environmental conditions are acceptable and stable.
• The coating is within its approved temperature range.
• Mixing, induction, thinning, agitation, and pot-life requirements are being followed.
• The pump, hoses, gun, filters, tip, nozzle, and air supply are suitable.
• The spray pattern has been tested away from the production surface.
• Stripe coating and difficult-area work have been completed as required.
• Gun distance, angle, overlap, and travel speed are controlled.
• Wet film thickness is being checked during production.
• Lighting allows applicators to see coverage and defects.
• Adjacent work is not contaminating the wet coating.
• Application and quality-control results are documented.

Key Takeaways

• Application equipment must match the coating, structure, and required finish.
• Brushes and rollers remain valuable for stripe coating, repair work, and restricted areas.
• Conventional air, HVLP, airless, air-assisted airless, and plural-component systems perform different jobs.
• The spray pattern should be tested before coating the structure.
• Gun distance, angle, overlap, triggering, and travel speed control film quality.
• Complex steel must be coated from multiple directions.
• Wet film measurements provide immediate application feedback.
• Equipment changes and material condition must be monitored throughout production.

Knowledge Check

1. Why should the spray pattern be tested before coating the structure?

View Answer

The test confirms that the equipment, coating, pressure, and settings are producing an acceptable pattern before production begins.

2. What can happen when the spray gun is held too far from the surface?

View Answer

It can cause dry spray, overspray, rough texture, reduced transfer efficiency, and insufficient film thickness.

3. Why should the gun remain approximately perpendicular to a flat surface?

View Answer

A perpendicular angle distributes the spray pattern more evenly and reduces heavy coating on one side and overspray on the other.

4. What is the purpose of overlapping spray passes?

View Answer

Overlap combines adjacent spray patterns to produce a more uniform, continuous film without light bands.

5. Why should wet film thickness be measured during application?

View Answer

It provides immediate feedback so the applicator can adjust technique before a large area cures outside the required thickness range.

6. Why must complex structural steel be sprayed from multiple directions?

View Answer

Flanges, stiffeners, bolts, connections, and other members create shadowed surfaces that cannot be properly covered from one direction.

Tags: coating application, spray technique, airless spray, air-assisted airless, HVLP spray, industrial painting, wet film thickness, spray pattern, protective coatings, corrosion protection, plural-component spray, coating defects