The Critical Role of Coating Repair
Hot-dip galvanized coatings demonstrate exceptional mechanical durability and abrasion resistance compared to organic coating systems, yet no protective coating proves entirely immune to damage under all circumstances. Coating discontinuities can arise from multiple sources: minor imperfections during the galvanizing process, rough handling during transportation and storage, aggressive installation techniques, field welding operations, or in-service mechanical damage. While such coating breaches may represent small percentages of total surface area, unprotected steel at damage sites initiates corrosion that progressively compromises structural integrity and reduces effective service life.
Timely, properly executed coating repair restores both the barrier protection preventing corrosive agents from contacting steel substrate and the galvanic (cathodic) protection that zinc provides to exposed steel at coating discontinuities. Understanding the performance characteristics, advantages, limitations, and appropriate application contexts for ASTM A780-approved repair materials enables informed repair method selection optimizing durability, cost-effectiveness, and aesthetic outcomes for specific project requirements.
Regulatory Framework: ASTM A780
ASTM A780, "Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings," establishes industry consensus guidance for repairing coating damage occurring during galvanizing, post-galvanizing handling, or field service:
Repair Material Categories
The standard recognizes three acceptable repair material categories:
1. Zinc-Rich Paints
High concentrations of metallic zinc particles (typically 65-95% zinc by weight) suspended in organic or inorganic binders providing both barrier and galvanic protection
2. Zinc-Based Solders
Metallic zinc alloy sticks applied molten to damaged areas, solidifying to form essentially pure zinc repair coating metallurgically bonded to substrate
3. Zinc-Spray Metallizing (Thermal Spray)
Molten or semi-molten zinc particles pneumatically deposited onto prepared surfaces through arc-spray or flame-spray processes, creating thick metallic zinc coatings
Repair Size Limitations
At Galvanizing Plant (Pre-Acceptance):
ASTM A123 (general products) and A153 (hardware/fasteners) establish maximum repairable area criteria:
Dimensional Limit: Individual bare areas must not exceed 25 mm (1 inch) in narrowest dimension
Areal Limit:
- ASTM A123: 0.5% of accessible surface area OR 36 square inches per short ton (256 cm² per metric ton), whichever is less
- ASTM A153: 0.5% of surface area to be coated
Damage exceeding these limits requires re-galvanizing or customer acceptance of non-conforming material.
Field Repairs (Post-Delivery):
ASTM A123 explicitly states: "Once the parts have left the galvanizing facility there shall be no limit on the size of the area subject to renovation."
Field repairs can address damage of any size using ASTM A780 materials and methods, though practical and economic considerations may favor alternative approaches for extensive damage.
Comparative Evaluation Framework
Selecting optimal repair materials requires evaluating multiple performance criteria:
Essential Performance Attributes
Corrosion Protection Effectiveness:
- Barrier protection preventing moisture and oxygen access
- Galvanic (cathodic) protection at coating defects
- Long-term protection duration in service environment
Mechanical Durability:
- Adhesion strength to substrate and surrounding coating
- Abrasion resistance under handling and service conditions
- Impact resistance and mechanical robustness
Application Characteristics:
- Ease of application in diverse conditions and geometries
- Required equipment and operator skill level
- Surface preparation demands
- Environmental application constraints
Aesthetic Compatibility:
- Initial appearance match to galvanized base coating
- Long-term weathering behavior
- Uniformity after natural patina development
Economic Factors:
- Material costs per unit area
- Application labor requirements
- Equipment investment and availability
- Total cost of ownership
Material Category 1: Zinc-Rich Paints
Zinc-rich paints represent the most widely used repair material category due to versatility, ease of application, and availability:
Composition and Types
Organic Zinc-Rich Paints:
Binder Systems:
- Epoxy resins (two-component systems)
- Polyurethane binders
- Alkyd resins (air-dry formulations)
Characteristics:
- Air-dry or catalyzed cure
- Easier application than inorganics
- Better flexibility and adhesion
- Slightly lower corrosion protection than inorganics
Inorganic Zinc-Rich Paints:
Binder Systems:
- Ethyl silicate (alcohol-based)
- Water-based silicate formulations
Cure Mechanism: Silicate polymerization creating ceramic-like matrix
Characteristics:
- Superior corrosion protection approaching metallic zinc performance
- More demanding application requirements
- Sensitive to temperature and humidity during application
- Lower flexibility than organic systems
Zinc Content:
Both types contain high metallic zinc concentrations:
- Standard formulations: 65-75% zinc by weight
- Premium formulations: 85-95% zinc by weight
Higher zinc content generally improves galvanic protection effectiveness but may increase application difficulty and cost.
Protection Mechanisms
Barrier Protection:
The paint film creates physical barrier between steel substrate and corrosive environment:
- Prevents moisture contact
- Excludes oxygen necessary for corrosion
- Restricts chloride and pollutant access
Galvanic Protection:
High zinc content enables cathodic protection at coating defects:
- Metallic zinc particles provide electrical contact through conductive film
- Zinc corrodes sacrificially, protecting exposed steel
- Protection extends beyond paint film boundaries to small defects
- Effectiveness depends on zinc particle contact and concentration
Application Methodology
Surface Preparation:
Minimum Requirement: Remove loose coating, rust, oil, grease, and contaminants. Roughen surface through wire brushing, light grinding, or sweep blast cleaning.
Optimal Preparation: SSPC-SP 7 (brush-off blast cleaning) or SP 11 (power tool cleaning to bare metal) maximizes adhesion
Application Methods:
Brush Application:
- Most common field repair method
- Simple, requires minimal equipment
- Enables precise application to small areas
- Easy to achieve adequate film build (3-5 mils dry film thickness typical)
- Suitable for all surface orientations
Spray Application:
- Efficient for larger repair areas
- Requires HVLP spray equipment or airless sprayer
- Produces thinner film per pass
- Multiple coats often necessary to achieve minimum thickness
- Overspray control and containment considerations
Roller Application:
- Practical for flat surfaces and larger areas
- Moderate equipment requirements
- Adequate film build achievable
- Less common for repair (more suited to painting large areas)
Thickness Achievement:
ASTM A780 and primary galvanizing specifications (A123, A153) require repair coating thickness matching original galvanizing minimums:
- Typical requirement: 3.0-5.0 mils (75-125 micrometers) dry film thickness
- Brush application: Easily achieves adequate thickness in 1-2 coats
- Spray application: May require 3-4 coats for adequate build
Cure/Drying:
Air-Dry Organics:
- Handling time: 2-4 hours
- Full cure: 24-72 hours depending on temperature
Catalyzed Organics:
- Pot life: 2-8 hours typical
- Handling time: 4-8 hours
- Full cure: 24-48 hours
Inorganics:
- Handling time: 4-8 hours
- Full cure: 24-72 hours
- Moisture cure benefits performance
Performance Characteristics
Corrosion Protection:
Barrier Effectiveness: Very good—dense paint film excludes moisture and oxygen when intact
Galvanic Protection: Good to very good depending on zinc content and film quality. Lower than metallic zinc repair methods due to:
- Binder material reducing zinc-to-zinc particle contact
- Organic resin acting as electrical insulator
- Reduced zinc content compared to 100% metallic zinc methods
Protection Duration:
- Severe exposure: 10-20 years typical
- Moderate exposure: 20-40 years
- Mild exposure: 30-60 years
Adhesion:
Very good adhesion to properly prepared galvanized surfaces and steel substrate. Paint films demonstrate:
- Adequate bond strength for handling and service loads
- Flexibility accommodating minor substrate movement
- Good adhesion retention during wet/dry cycling
Abrasion Resistance:
Low to moderate abrasion resistance compared to metallic coatings:
- Paint film susceptible to mechanical wear
- Organic binders provide limited scratch resistance
- Handling damage during installation can compromise film
- Not suitable for applications experiencing severe abrasion
Advantages
Ease of Application:
Zinc-rich paints provide the easiest application among ASTM A780 repair materials:
- Minimal equipment requirements (brush and container)
- Simple surface preparation adequate (though blast cleaning optimal)
- No specialized training necessary for basic competence
- Suitable for field application in diverse environments
Versatility:
Application flexibility accommodates diverse repair scenarios:
- Small or large repair areas
- Any surface orientation (vertical, overhead, horizontal)
- Complex geometries including recesses, corners, and internal surfaces
- Variable access conditions from ideal to constrained
Cost-Effectiveness:
Material Cost: Lowest among repair options
- Typical cost: $40-100 per gallon
- Coverage: 100-400 square feet per gallon depending on application method and desired thickness
- Cost per square inch: $0.10-1.00 typical for small repairs
Application Cost: Minimal labor requirements reduce total cost
No Limit on Coverage Area:
Unlike solders with practical size limitations, zinc-rich paints readily address repairs of any size from square inches to square feet or larger.
Color Options:
Various formulations available:
- Bright/shiny silver matching fresh galvanizing
- Weathered gray/matte matching aged galvanizing
- Custom tints for special applications
This variety enables aesthetic coordination with base coating appearance.
Limitations
Lower Cathodic Protection:
Compared to pure metallic zinc repair methods (solder, metallizing), zinc-rich paints provide reduced galvanic protection due to binder materials interrupting zinc particle contact.
Abrasion Vulnerability:
Paint films show low abrasion resistance. Applications involving:
- Frequent handling or contact
- Sliding or rubbing contact
- Impact from falling objects
- Mechanical wear
may quickly damage paint repairs requiring re-application.
Appearance Matching Challenges:
Achieving perfect appearance match proves difficult:
- Shiny formulations maintain bright appearance as galvanizing weathers gray (permanent contrast)
- Weathered gray formulations initially contrast with bright fresh galvanizing
- Color matching varies by product and galvanizing appearance
- Complete uniformity difficult to achieve
Natural weathering eventually reduces appearance differences as all surfaces converge toward matte gray patina.
Application Sensitivity:
Inorganic zinc-rich formulations demonstrate sensitivity to:
- Temperature (below 40°F or above 100°F reduces quality)
- Humidity (relative humidity >85% causes moisture contamination)
- Surface contamination (oils or salts reduce adhesion)
- Mixing and application procedures (improper technique degrades performance)
Organic formulations prove less sensitive but still require appropriate conditions.
Optimal Applications
Zinc-rich paints suit:
- Small to moderate damage areas (under 100 square inches typical)
- Field repairs with limited equipment access
- Budget-conscious projects
- Interior applications or mild exterior exposure
- Situations where appearance is secondary concern
- Repairs requiring rapid completion
Material Category 2: Zinc-Based Solders
Zinc solders produce metallic zinc repairs providing excellent appearance matching and authentic galvanic protection:
Composition and Characteristics
Material Composition:
Zinc solder sticks typically contain:
- Primary constituent: High-purity zinc (>95%)
- Alloying elements: Small amounts of aluminum, tin, or other elements modifying melting point and flow characteristics
Melting Point:
Pure zinc melts at 787°F (419°C). Solder alloys may have slightly modified melting points (750-800°F typical) improving application characteristics.
Physical Form:
Solid sticks or rods approximately 12-18 inches long, 3/8 to 1/2 inch diameter for hand-held application.
Application Methodology
Surface Preparation:
Requirements:
- Remove loose coating, rust, and contaminants
- Wire brush or grind damaged area
- Clean bare steel should be exposed
- Surrounding galvanized coating should be sound
Heating Equipment:
Propane Torch: Most common heat source for field application
- Readily available
- Adequate heat output
- Portable and economical
Oxy-Fuel Torch: Provides higher heat for larger repairs or challenging geometries
Application Technique:
- Heat Substrate: Apply torch flame to damaged steel area, heating to approximately 600-750°F (steel shows dull red color at higher end)
- Apply Solder: Touch solid zinc solder stick to heated steel surface
- Solder Melting: Heat conducted from steel melts solder stick tip
- Zinc Flow: Molten zinc flows onto hot steel surface
- Coating Building: Continue applying solder, allowing molten zinc to build coating thickness
- Solidification: As steel cools or torch is removed, zinc solidifies forming repair coating
Skill Requirements:
Effective zinc solder application requires:
- Heat control maintaining adequate temperature without overheating
- Technique ensuring complete wetting and bonding
- Speed balancing zinc flow with controlled buildup
- Practice developing proficiency
Novice applicators often experience:
- Insufficient heating causing poor zinc flow and bonding
- Overheating causing excessive zinc oxidation
- Uneven coating thickness
- Difficulty on vertical or overhead surfaces
Performance Characteristics
Corrosion Protection:
Barrier Protection: Good—metallic zinc creates continuous barrier over repaired area
Galvanic Protection: Excellent—essentially pure metallic zinc provides authentic cathodic protection equivalent to hot-dip galvanizing
Protection Duration: Similar to hot-dip galvanizing based on coating thickness achieved:
- Conservative estimate: 0.5-1.0 mils (12-25 micrometers) typical thickness from solder application
- Corrosion rate: 1-2 micrometers/year moderate exposure
- Service life estimate: 6-15 years depending on thickness achieved and exposure severity
Adhesion:
Very good to excellent adhesion:
- Metallurgical bonding to steel substrate through zinc-iron interdiffusion at interface
- Superior adhesion compared to mechanically bonded coatings
- Minimal risk of coating delamination or peeling
Abrasion Resistance:
Low to moderate:
- Pure zinc is relatively soft metal (Brinell hardness ~30-40)
- More abrasion-resistant than paint but less than hardened metallizing
- Handling wear can occur but coating resists catastrophic failure
- Suitable for most service conditions without severe abrasion
Appearance:
Initial: Bright metallic zinc appearance closely matching fresh hot-dip galvanizing
Long-Term: Identical weathering behavior to galvanizing
- Atmospheric exposure forms zinc hydroxides and carbonates
- Patina development proceeds at same rate as base galvanizing
- After 6-24 months, repair becomes essentially indistinguishable from surrounding coating
Appearance Match: Best among all repair materials for long-term uniformity
Advantages
Excellent Appearance Compatibility:
Zinc solder repairs provide best long-term appearance matching because:
- Metallic zinc composition identical to galvanizing
- Weathering behavior matches base coating
- Natural patina development creates seamless appearance integration
Authentic Galvanic Protection:
Pure metallic zinc delivers genuine cathodic protection mechanism equivalent to hot-dip galvanizing
Good Adhesion:
Metallurgical bonding ensures durable adhesion resistant to coating delamination
Affordability:
Material Cost: Moderate
- Zinc solder sticks: $15-30 per pound
- Coverage: Approximately 50-100 square inches per pound (highly variable based on thickness)
- Cost per square inch: $0.15-0.60 typical
Equipment Cost: Minimal—propane torch ($30-80) plus fuel
Single-Application Adequacy:
Unlike paints requiring multiple coats, single solder application typically achieves adequate thickness (though thin in absolute terms).
Limitations
Application Difficulty on Complex Geometries:
Zinc solder application proves challenging on:
Vertical Surfaces: Molten zinc tends to sag or drip before solidifying, creating:
- Uneven coating thickness
- Zinc accumulation at bottom of repair area
- Thin coating at top of vertical surfaces
Overhead Surfaces: Extremely difficult application with gravity working against zinc adhesion:
- Safety concerns from falling molten zinc droplets
- Very thin coatings achievable
- High skill requirement
Curved Surfaces: Complex three-dimensional geometries challenge uniform zinc application
Internal Corners and Recesses: Torch access limitations and heat dissipation complicate application
Large Area Challenges:
Zinc solder proves impractical for extensive damage:
- Labor-intensive application for areas beyond several square inches
- Time-consuming compared to paint or spray metallizing
- Difficult to achieve uniform thickness over large areas
Practical Size Limit: Repairs under 10-25 square inches prove most economical and practical
Thin Coating Production:
Typical zinc solder application produces relatively thin coatings:
- Common thickness: 0.5-1.5 mils (12-38 micrometers)
- Substantially less than hot-dip galvanizing: 3-6 mils (75-150 micrometers) typical
Thin repairs provide proportionally shorter protection duration than thick galvanized base coating.
Heat Source Requirement:
Torch use creates practical concerns:
Safety:
- Open flame hazards
- Burn risks from torch and hot substrate
- Fire risk in combustible environments
Site Restrictions:
- Many facilities prohibit open flames (refineries, chemical plants)
- Hot work permits and fire watch requirements
- Confined space complications
Weather Sensitivity:
- Wind interferes with torch flame and cooling
- Rain prevents adequate heating
- Cold temperatures require extended heating
Optimal Applications
Zinc solder suits:
- Small damage areas (under 25 square inches)
- Flat or gently sloped surfaces
- Situations prioritizing appearance matching
- Field repairs where torch use is permitted
- Applications valuing authentic metallic zinc protection
- Moderate skill level available
Material Category 3: Zinc-Spray Metallizing (Thermal Spray)
Thermal spray metallizing deposits thick, durable metallic zinc coatings through pneumatic atomization of molten zinc:
Process Technologies
Arc-Spray Process:
Mechanism:
- Two electrically conductive zinc wires fed through spray gun
- Electric arc generated between wire tips melts zinc
- Compressed air atomizes molten zinc into fine droplets
- Droplets accelerate toward substrate, depositing as coating
Advantages:
- High deposition rates (10-40 lbs/hour)
- Thick coatings achievable (4-15+ mils)
- Relatively clean process (minimal fumes)
- Good coating quality
Equipment: Arc-spray gun, power supply (typically 30-40 volts DC), compressed air source, zinc wire spools
Flame-Spray Process:
Mechanism:
- Zinc wire or powder fed through oxy-fuel combustion flame
- Combustion heat melts zinc
- Compressed air atomizes and propels molten particles
Advantages:
- Lower equipment cost than arc-spray
- Wire or powder feedstock flexibility
- Portable equipment
Disadvantages:
- Lower deposition rates than arc-spray
- Greater overspray and material waste
- More fumes and oxidation
Coating Formation:
Molten zinc droplets impact substrate at high velocity:
- Mechanical interlocking with surface roughness
- Some metallurgical bonding at particle-substrate interfaces
- Particle-on-particle building creates layered structure
- Slightly porous coating (1-10% porosity typical)
Application Methodology
Surface Preparation:
Critical Requirement:
Thermal spray zinc requires aggressive surface preparation for adequate adhesion:
White Metal Blast Cleaning (SSPC-SP 5):
- Remove all coating, rust, and mill scale
- Expose clean, bare steel substrate
- Create angular surface profile (2-4 mils peak-to-valley)
Near-White Blast Cleaning (SSPC-SP 10):
- Minimum acceptable preparation
- 95% removal of visible contaminants
- Adequate surface profile
Blast Media:
- Aluminum oxide (most common)
- Steel grit
- Garnet
- Silica sand (where permitted)
Profile Requirements: Angular surface texture 2-4 mils peak-to-valley depth essential for mechanical interlocking
Equipment Requirements:
Arc-Spray System Components:
- Arc-spray gun: $3,000-8,000
- Power supply: $2,000-6,000
- Compressed air supply: 15-30 CFM at 60-90 PSI
- Zinc wire: 1/8 inch diameter typical, $3-8 per pound
- Personal protective equipment
Blast Cleaning Equipment:
- Portable blast pot or blast cabinet
- Compressed air compressor (100+ CFM for production work)
- Blast media and recovery system
- Dust collection and containment
Total Equipment Investment: $10,000-30,000+ for complete capable system
Application Technique:
- Blast Surface: Thoroughly blast repair area plus 2-3 inch perimeter creating proper profile
- Immediate Application: Apply zinc spray within minutes to hours preventing surface oxidation
- Gun Distance: Maintain 4-8 inch standoff distance from surface
- Spray Angle: Perpendicular to surface (90°) for optimal deposition
- Spray Pattern: Overlap passes 50% building uniform thickness
- Coating Build: Continue until specified thickness achieved (typically 4-8 mils)
- Cooling: Allow coating to cool before handling
Operator Skill:
Quality metallized repairs require trained, experienced operators:
- Spray technique maintaining consistent distance and speed
- Pattern control achieving uniform coverage
- Thickness verification using magnetic gauge
- Recognition of application problems (overheating, poor adhesion, excessive porosity)
Novice operators typically produce inferior coatings with:
- Non-uniform thickness
- Excessive porosity
- Poor adhesion from improper surface preparation
- Overspray waste
Performance Characteristics
Corrosion Protection:
Barrier Protection: Very good—thick metallic zinc coating (4-12+ mils typical) provides substantial physical barrier
Galvanic Protection: Excellent—100% metallic zinc composition provides authentic cathodic protection equivalent to hot-dip galvanizing
Protection Duration:
Metallized zinc coatings provide long service life due to thickness:
- Typical application: 6-8 mils (150-200 micrometers)
- Corrosion rate: 1-2 micrometers/year moderate exposure
- Service life estimate: 75-150 years in moderate atmospheric exposure
Performance rivals or exceeds hot-dip galvanizing base coating.
Adhesion:
Very good adhesion achieved through:
- Mechanical interlocking with blast-prepared surface profile
- Some metallurgical bonding at particle interfaces
- High particle impact velocity enhancing contact
Adhesion Dependency: Critically dependent on proper surface preparation. Inadequate blast cleaning produces poor adhesion and premature coating failure.
Abrasion Resistance:
Good to very good:
- Metallurgically bonded coating resists mechanical wear
- Zinc-on-zinc particle layering creates moderately durable structure
- Better abrasion resistance than zinc-rich paints
- Lower than hardened alloys but adequate for most applications
Appearance:
Initial: Matte gray metallic appearance similar to zinc intermetallic layers in hot-dip galvanizing (delta/zeta phase appearance)
Long-Term: Excellent matching to galvanized base coating
- Metallic zinc weathers identically to hot-dip galvanizing
- Patina development matches base coating
- After full weathering (12-24 months), metallized repairs blend seamlessly
Appearance Quality: Second only to zinc solder for long-term appearance uniformity
Advantages
No Coverage Limit:
Unlike zinc solder with practical size constraints, thermal spray readily addresses repairs of any size from square inches to hundreds of square feet.
Thick Coating Capability:
Single-pass application achieves 4-15+ mil thickness—meeting or exceeding hot-dip galvanizing minimums and providing extended service life.
Excellent Protection:
Combination of thickness and pure metallic zinc composition delivers optimal corrosion protection among repair options:
- Superior to zinc-rich paints (thicker, higher zinc content)
- Superior to zinc solders (thicker coating achieved)
- Equivalent or better than original galvanizing in many cases
Good Appearance Matching:
Metallic zinc composition ensures identical weathering behavior and long-term appearance uniformity with base galvanizing.
Good Abrasion Resistance:
Metallurgically bonded zinc coating provides mechanical durability suitable for handling and service loads.
Limitations
Highest Cost:
Material Cost: Moderate to high
- Zinc wire: $3-8 per pound
- Coverage: 15-30 square feet per pound at 6 mils thickness
- Material cost per square inch: $0.02-0.05
Equipment Cost: Substantial
- Complete system: $10,000-30,000+
- Prohibitive for occasional repair work
- Rental equipment option: $200-800 per day
Application Cost: High labor rates for skilled operators
Total Cost per Square Inch: $1-10 typical depending on repair size and mobilization
Equipment Intensive:
Required equipment suite:
- Arc-spray or flame-spray system
- Blast cleaning equipment
- Compressed air supply (high CFM requirement)
- Power supply
- Material handling and storage
Large, heavy equipment limits portability and field application flexibility.
Skilled Operator Requirement:
Quality metallizing demands:
- Formal training on equipment operation
- Practical experience developing technique
- Understanding of surface preparation criticality
- Troubleshooting skills
Operator availability may be limited in some regions.
Surface Preparation Criticality:
Blast cleaning to white metal or near-white metal is mandatory:
- Labor-intensive and time-consuming
- Generates substantial waste media and debris
- Requires containment and cleanup
- Environmental concerns (dust, media disposal)
- May damage adjacent coatings if area not protected
Application Limitations:
Challenging application scenarios:
Internal Surfaces: Spray gun access to internal corners, recesses, or confined spaces difficult or impossible
Hidden Areas: Blind holes, back sides of flanges, or unexposed surfaces inaccessible
Complex Geometries: Maintaining consistent spray distance and angle on irregular shapes proves difficult
Overspray Concerns: Masking and containment necessary protecting adjacent surfaces and equipment
Optimal Applications
Zinc metallizing suits:
- Large repair areas (>50-100 square inches) justifying equipment and labor costs
- Critical applications requiring maximum protection
- Situations where long-term performance justifies high initial investment
- New construction projects with multiple structures requiring repair
- Facilities with in-house metallizing capability
- Applications where appearance matching is important
- Projects with adequate budget for premium repair quality
Comparative Performance Summary
Protection Effectiveness Ranking
Cathodic (Galvanic) Protection:
- Zinc-Spray Metallizing (Excellent)
- Zinc-Based Solder (Excellent)
- Zinc-Rich Paint (Good to Very Good)
Metallic zinc methods provide superior galvanic protection due to 100% zinc content versus zinc particles in binder for paints.
Barrier Protection:
- Zinc-Spray Metallizing (Very Good—thickness advantage)
- Zinc-Rich Paint (Very Good—dense film)
- Zinc-Based Solder (Good—typically thinner applications)
Overall Corrosion Protection:
- Zinc-Spray Metallizing
- Zinc-Rich Paint (inorganic formulations)
- Zinc-Based Solder (thickness limitation)
Mechanical Performance Ranking
Adhesion:
- Zinc-Based Solder (Excellent—metallurgical bond)
- Zinc-Spray Metallizing (Very Good—mechanical interlock)
- Zinc-Rich Paint (Very Good—adhesive bond)
Abrasion Resistance:
- Zinc-Spray Metallizing (Good)
- Zinc-Based Solder (Moderate)
- Zinc-Rich Paint (Low)
Application Characteristics Ranking
Ease of Application:
- Zinc-Rich Paint (Easiest)
- Zinc-Based Solder (Moderate skill required)
- Zinc-Spray Metallizing (Highest skill and equipment requirement)
Versatility (Geometries and Conditions):
- Zinc-Rich Paint (All surfaces, any orientation)
- Zinc-Based Solder (Flat/gentle slopes best)
- Zinc-Spray Metallizing (Limited by access and containment)
Coverage Area Capability:
- Zinc-Spray Metallizing (Unlimited, efficient for large areas)
- Zinc-Rich Paint (Unlimited, practical for moderate areas)
- Zinc-Based Solder (Small areas only)
Aesthetic Performance Ranking
Long-Term Appearance Match:
- Zinc-Based Solder (Best—identical material and weathering)
- Zinc-Spray Metallizing (Excellent—metallic zinc weathering)
- Zinc-Rich Paint (Variable—depends on formulation selection)
Economic Ranking
Material Cost (Small Repairs):
- Zinc-Rich Paint (Lowest)
- Zinc-Based Solder (Moderate)
- Zinc-Spray Metallizing (Highest considering equipment)
Total Cost per Square Inch:
- Zinc-Rich Paint: $0.10-1.00
- Zinc-Based Solder: $0.15-0.60
- Zinc-Spray Metallizing: $1.00-10.00
Selection Decision Matrix
Select Zinc-Rich Paint When:
Project Characteristics:
- Small to moderate damage areas (under 100 square inches typical)
- Field repairs with limited equipment access
- Budget constraints prioritize low cost
- Rapid repair completion required
- Multiple small repairs distributed across structure
Performance Requirements:
- Moderate exposure environment (not extreme corrosivity)
- Appearance is secondary consideration or weathering time is acceptable
- Abrasion is minimal concern
Application Context:
- Minimal repair experience or equipment available
- Variable surface orientations including vertical/overhead
- Complex geometries with internal surfaces
Select Zinc-Based Solder When:
Project Characteristics:
- Small damage areas (under 25 square inches)
- Appearance matching is high priority
- Authentic metallic zinc protection desired
- Moderate budget available
Performance Requirements:
- Moderate exposure environment
- Good aesthetic outcome required
- Moderate durability acceptable
Application Context:
- Flat or gently sloped surfaces
- Open flame use is permitted
- Moderate skill level available
- Individual repairs rather than production volumes
Select Zinc-Spray Metallizing When:
Project Characteristics:
- Large repair areas (over 50-100 square inches) justifying equipment costs
- Multiple structures or production-scale repair programs
- Long-term critical application requiring maximum protection
- Premium quality specification
Performance Requirements:
- Severe exposure environment or extended design life
- Thick coating needed matching or exceeding original galvanizing
- Appearance matching important
- High abrasion resistance beneficial
Application Context:
- Equipment and skilled operators available
- Adequate budget for premium repair method
- Surface blast cleaning feasible
- Access and overspray containment manageable
Quality Assurance and Verification
Regardless of repair material selected, verification ensures adequate protection:
Thickness Verification
Magnetic Thickness Gauge: Measure repair coating thickness using calibrated gauge per ASTM E376
Minimum Requirements: Repair thickness must meet minimum values in primary galvanizing specification:
- ASTM A123 Table 1 minimums (typically 2.0-3.5 mils depending on material category and thickness)
- ASTM A153 minimums (1.7-3.4 mils depending on fastener size)
Documentation: Record coating thickness measurements for quality records
Adhesion Verification
Knife Test: Use stout knife attempting to pry repair coating from substrate. Coating should resist removal without flaking or peeling.
Visual Verification: No evidence of coating delamination, cracking, or poor adhesion
Appearance Assessment
While appearance is subjective, evaluate:
- Repair coating coverage completeness
- Uniformity of application
- Obvious defects or discontinuities
- Reasonable appearance compatibility (recognizing initial differences may resolve through weathering)
Pre-Repair Coordination
Successful repairs require communication:
At Galvanizing Plant
Galvanizer-Customer Discussion:
Before galvanizing, establish:
- Repair material preferences or requirements
- Acceptance criteria for repair quality
- Responsibility for repair costs
- Documentation requirements
Standard Practice: Most galvanizers have established repair material preferences based on experience, inventory, and quality considerations. If customer prefers different material, discuss and agree before processing.
Field Repairs
Contractor-Owner Coordination:
For field repair specifications:
- Identify acceptable repair materials
- Establish qualification requirements for applicators
- Define quality acceptance criteria
- Specify documentation requirements
- Clarify cost allocation
ASTM A780 recognizes three acceptable repair material categories—zinc-rich paints, zinc-based solders, and zinc-spray metallizing—each offering distinct advantages suited to specific repair scenarios and performance requirements. Zinc-rich paints provide the most versatile and economical repair solution with easiest application using minimal equipment, making them optimal for small to moderate field repairs where budget constraints or limited equipment access apply, though they deliver lower cathodic protection and abrasion resistance compared to metallic zinc methods. Zinc-based solders produce excellent long-term appearance matching through pure metallic zinc application that weathers identically to hot-dip galvanizing, providing authentic galvanic protection suitable for small repairs on flat or gently sloped surfaces where appearance is prioritized and torch use is permitted, though application difficulty on complex geometries and thin coating production limit versatility. Zinc-spray metallizing delivers superior corrosion protection through thick metallic zinc coatings (4-15+ mils) providing century-scale service life, good abrasion resistance, and excellent appearance compatibility, justifying substantial equipment investment and high application costs for large repair areas, critical applications, or production-scale programs where maximum protection quality is specified. Material selection requires systematic evaluation balancing corrosion protection effectiveness (metallizing > paint/solder), application ease (paint > solder > metallizing), economic factors (paint lowest cost, metallizing highest), coverage area capacity (metallizing/paint unlimited, solder limited), aesthetic outcomes (solder best match, metallizing excellent, paint variable), and mechanical durability (metallizing superior abrasion resistance). Successful repairs regardless of material demand proper surface preparation removing contaminants and creating appropriate surface condition, adequate coating thickness meeting primary galvanizing specification minimums, application following manufacturer recommendations and ASTM A780 guidance, and quality verification through thickness measurement and adhesion testing. The optimal repair material emerges from matching method characteristics to specific project requirements considering damage extent, performance expectations, budget constraints, equipment availability, applicator skill, and aesthetic priorities, recognizing that all three ASTM A780 materials provide effective corrosion protection when properly applied. To read more, see the original AGA resource on HDG Repair Materials.
