Defining Responsibility in Duplex System Surface Preparation
Duplex coating systems—combining hot-dip galvanizing with paint or powder coating—deliver exceptional corrosion protection through synergistic performance. However, this enhanced protection depends fundamentally on proper surface preparation of the galvanized substrate before organic coating application. When the galvanizing operation and coating application occur at separate facilities by different contractors, unclear responsibility assignments for surface preparation steps frequently result in coating performance failures.
Surface conditions perfectly acceptable under ASTM A123/A123M for standalone galvanized coatings—including surface roughness, minor dross inclusions, zinc runs, and texture variations—may compromise adhesion or appearance when organic coatings are applied. These conditions require remediation per ASTM D6386 (paint application) or ASTM D7803 (powder coating application) to achieve successful duplex system performance.
ASTM A123/A123M Clarifications: The 2015 Revision
Recognizing industry confusion regarding surface preparation accountability, ASTM revised A123/A123M in 2015 to explicitly define responsibilities between galvanizers and coating applicators. Sections 6.2 and 6.4—addressing painting and powder coating respectively—now contain identical clarifying language:
ASTM A123/A123M, Sections 6.2 and 6.4:
"Surface roughness of articles to be painted or powder coated and the smoothing of the galvanized coating before painting or powder coating shall be mutually determined by the galvanizer and the purchaser. Further preparation of galvanized coatings for painting or powder coating including cleaning, profiling and outgassing shall be in accordance with Practice D6386 for painting and Practice D7803 for powder coating and are the responsibility of the paint or powder coating applicator."
This specification language establishes a framework where certain preparation activities require mutual agreement while others fall explicitly under coating applicator responsibility. Understanding these distinctions prevents gaps in the preparation sequence that compromise coating system integrity.
The Five-Step Surface Preparation Framework
Surface preparation for duplex systems encompasses up to five distinct activities, with execution requirements varying based on the galvanized coating's weathering condition (newly galvanized, partially weathered, or fully weathered). Not all steps apply to every situation—ASTM D6386 and D7803 provide decision matrices correlating surface condition with required preparation activities.
Step 1: Refraining from Post-Galvanizing Treatments
Responsibility: Galvanizer
Critical communication requirement: The customer, general contractor, or party ultimately responsible for paint or powder coating application must inform the galvanizer before galvanizing that the article will receive supplemental coating.
Prohibited treatments when painting/powder coating is planned:
Water quenching: Some galvanizers immerse freshly galvanized articles in water tanks to accelerate cooling, enabling faster handling and inspection. This practice leaves water-soluble zinc salts and hydroxide residues on the coating surface that interfere with paint adhesion even after surface cleaning.
Chromate conversion coatings: Chromate or passivation treatments applied to enhance initial appearance or provide temporary corrosion protection during storage create a chemically passive film that prevents mechanical bonding of subsequently applied coatings. These treatments must be completely removed—a time-consuming process—before painting or powder coating can proceed.
Controlled cooling requirement: For duplex system applications, galvanizers should allow air cooling only, without any quench treatments or chemical post-processing that modifies the zinc surface chemistry.
Documentation: Specify "no post-galvanizing treatments" on purchase orders and fabrication drawings when duplex systems are planned. This simple communication prevents problematic surface conditions requiring extensive remediation.
Step 2: Surface Smoothing
Responsibility: Mutually Agreed Between Galvanizer and Purchaser/Coating Applicator
Surface smoothing addresses galvanized coating irregularities that create cosmetic or functional issues for organic coating application:
Conditions requiring smoothing:
- Heavy zinc runs or drip formations
- Sharp zinc projections or spikes
- Large dross inclusions protruding above the coating plane
- Significant surface roughness from reactive steel galvanizing
- Coating buildup at weld intersections or fabrication details
Smoothing methods:
- Wire brushing (hand or powered rotary brushes)
- Grinding or filing protruding formations
- Abrasive cloth or pad finishing
- Localized abrasive blasting of severe irregularities
Specification considerations defined in ASTM D6386, Section 5.2 and ASTM D7803, Section 5.1.1:
These specification sections describe acceptable surface smoothness levels and provide guidance on smoothing techniques. However, the specifications do not mandate who performs the work—this decision requires agreement between the galvanizer and coating applicator.
Factors influencing responsibility assignment:
Timing efficiency: Smoothing immediately after galvanizing (while articles remain at the galvanizing facility) often proves more efficient than later smoothing at the coating applicator's facility, reducing double-handling and transportation with surface irregularities intact.
Equipment availability: Galvanizers typically maintain powered grinding and wire brushing equipment suitable for smoothing operations, though coating applicators may have equivalent or superior surface preparation tooling.
Cost allocation: Smoothing requirements vary dramatically with steel chemistry and fabrication complexity. Establishing responsibility and pricing before galvanizing prevents disputes when extensive smoothing proves necessary.
Quality control coordination: The party performing smoothing bears responsibility for verifying the surface meets smoothness criteria before advancing to subsequent preparation steps.
Best practice recommendation: Specify smoothing responsibility explicitly in project specifications and purchase orders. Default assumption that "someone will handle it" creates the exact communication gap these specification revisions intended to eliminate.
Step 3: Surface Cleaning
Responsibility: Paint or Powder Coating Applicator
Surface cleaning removes contaminants that interfere with coating adhesion or appearance:
Contaminants requiring removal:
- Dust, dirt, and airborne particulate from storage and transportation
- Oil, grease, and handling residues
- Water-soluble zinc salts (if water quenching occurred despite instructions)
- Chromate conversion coatings (if inadvertently applied)
- Wet storage stain (white rust corrosion product from moisture exposure during storage)
Cleaning method selection per ASTM D6386 and D7803:
For newly galvanized surfaces: Solvent cleaning per SSPC-SP1 typically suffices, removing oils and light contamination without affecting the zinc surface.
For weathered surfaces: Alkaline cleaning solutions (TSP, commercial zinc-safe degreasers) followed by thorough freshwater rinsing effectively remove both organic contamination and zinc corrosion products.
For wet storage stain: Light abrasive cleaning (nylon pads, fine abrasive cloths, or low-pressure water jetting with mild abrasives) removes white corrosion products without excessive zinc removal. Severe wet storage stain may require mechanical or chemical stripping methods detailed in ASTM D6386, Appendix X2.
For chromate conversion coatings: Alkaline stripping solutions or mechanical abrasion removes chromate films. Confirm complete removal through water break testing—properly cleaned zinc surfaces should wet uniformly without water beading.
Critical verification: After cleaning, surfaces must be free from visible contamination and demonstrate proper wetting characteristics. Compressed air cleaning for dust removal should employ oil-free air verified through ASTM D4285 blotter testing.
Step 4: Surface Preparation and Profiling
Responsibility: Paint or Powder Coating Applicator
Surface profiling creates mechanical anchoring sites that enhance coating adhesion beyond chemical bonding alone. Profile requirements vary with weathering condition and coating system type.
For newly galvanized steel (aged less than 48 hours):
Sweep blast cleaning: Light abrasive blasting using fine aluminum oxide, crushed glass, or other zinc-compatible abrasives at reduced pressures (40 to 60 psi) achieves SSPC-SP16 Commercial Blast Cleaning of Coated Surfaces.
Target profile: 0.5 to 1.5 mils (13 to 38 μm)—sufficient for mechanical bonding without excessive zinc removal.
Alternative for powder coating: Some powder coating systems achieve adequate adhesion on newly galvanized steel without profiling, relying on chemical bonding and thermal fusion. Consult powder manufacturer recommendations.
For partially weathered galvanized steel (48 hours to 6 months, some weathering patina present):
Light profiling: Sweep blasting or power tool cleaning per SSPC-SP16 or SSPC-SP11.
Target profile: 0.5 to 1.5 mils (13 to 38 μm).
Surface condition goal: Remove loose zinc corrosion products while preserving the majority of the galvanized coating thickness.
For fully weathered galvanized steel (over 6 months atmospheric exposure, complete weathering patina):
Minimal preparation: Thoroughly cleaned, fully weathered zinc provides an excellent substrate for most coating systems without additional profiling. The stable zinc patina (primarily zinc carbonate) bonds readily with properly formulated coatings.
Optional light cleaning: Power washing or detergent scrubbing removes surface contamination without profiling.
Profile measurement verification: When profiling is performed, verify achievement using ASTM D4417 methods (surface profile micrometers, replica tape, or visual comparators) as described in duplex system specifications.
Step 5: Outgassing (Powder Coating Only)
Responsibility: Powder Coating Applicator
Outgassing represents a powder coating-specific requirement absent from paint system preparation.
Mechanism and necessity:
Zinc coatings contain microscopic porosity and may retain moisture within the zinc crystal structure. During powder coating cure cycles—typically 350°F to 400°F (177°C to 204°C) for 10 to 20 minutes—moisture and absorbed gases within the zinc coating expand and volatilize. If these volatiles escape through the uncured or partially cured powder coating, they create:
- Surface blisters and bubbles
- Pinholes and coating porosity
- Reduced adhesion at gas escape sites
- Unacceptable cosmetic appearance
Outgassing procedure:
Preheat the galvanized substrate to powder coating cure temperature (or slightly higher) for 15 to 30 minutes before powder application. This thermal cycle drives absorbed moisture and gases from the zinc coating while it remains uncoated, preventing subsequent defect formation.
Process integration: Most powder coating operations incorporate outgassing as part of standard pretreatment when coating galvanized steel. However, specifications should explicitly require outgassing verification to ensure applicators recognize its necessity.
Paint system note: Conventional air-dry and baked paints cure at lower temperatures (typically 150°F to 250°F or 66°C to 121°C) insufficient to mobilize zinc-retained moisture. Paint systems therefore do not require dedicated outgassing procedures, though thorough surface drying after cleaning remains essential.
Communication Protocols for Success
The explicit responsibility assignments in ASTM A123/A123M, D6386, and D7803 function only when effective communication occurs between all parties.
Essential communication touchpoints:
During specification development:
- Clearly state duplex system intentions in project specifications
- Specify surface smoothing responsibility explicitly
- Reference ASTM D6386 or D7803 in entirety, not just by title
During galvanizing procurement:
- Indicate "for painting" or "for powder coating" on purchase orders
- Specify "no post-galvanizing treatments"
- Confirm smoothing responsibility and acceptance criteria
Before coating application:
- Coating applicator should verify no prohibited post-treatments occurred
- Assess surface condition (newly galvanized, partially weathered, fully weathered)
- Implement appropriate cleaning, profiling, and outgassing per assessed condition
Documentation and traceability:
- Photograph surface condition at each transfer point
- Document surface preparation activities performed
- Retain surface profile measurements and cleanliness verification records
Consequences of Preparation Gaps
Inadequate surface preparation or unclear responsibility assignment produces predictable failures:
Adhesion defects: Coating delamination during service, often beginning at edges and spreading across surfaces. Cause: Contamination not removed, inadequate profiling, or prohibited post-treatments.
Cosmetic issues: Visible texture irregularities telegraphing through the topcoat, coating thickness variations, or rough appearance. Cause: Insufficient surface smoothing.
Outgassing defects (powder coating): Blistering, pinholes, or poor appearance. Cause: Omitted outgassing procedure.
Warranty disputes: Conflicting claims regarding who should have performed specific preparation steps. Cause: Lack of pre-work responsibility clarification.
These failures necessitate expensive remediation—often complete coating removal, surface re-preparation, and recoating—far exceeding the cost of proper initial surface preparation.
Quality Assurance Best Practices
Organizations regularly producing duplex systems benefit from standardized protocols:
Pre-qualification procedures: Verify coating applicators understand galvanized substrate preparation requirements and maintain appropriate equipment and quality control processes.
Written work instructions: Document surface preparation sequences, acceptance criteria, and inspection hold points specific to duplex systems.
Hold point inspections: Implement mandatory inspection and approval after surface preparation completion, before coating application begins.
Personnel training: Ensure both galvanizing and coating application personnel understand the complete duplex system process and their specific responsibilities within it.
Process audits: Periodically review preparation practices to verify continued specification compliance and identify opportunities for improved efficiency or quality.
Integrated Responsibility Framework
While ASTM specifications define minimum requirements, optimal duplex system quality emerges from collaborative approaches:
Single-source responsibility: Some organizations offer complete duplex coating services—galvanizing, surface preparation, and organic coating application—under unified quality control. This integration eliminates interface coordination issues.
Partnership agreements: Galvanizers and coating applicators developing ongoing working relationships can establish standardized protocols, reducing per-project coordination requirements.
Owner/specifier oversight: Project owners and design professionals should recognize that specifying duplex systems without clarifying preparation responsibilities invites contractor disputes and quality problems. Clear contract language prevents these issues.
Successful duplex systems depend equally on both coating layers and the preparation activities linking them. Understanding and executing the responsibility framework established in ASTM standards transforms duplex coating from a coordination challenge into a reliable, high-performance corrosion protection solution. The original AGA resource article contains more information.
