1. Abrasive Blasting (The Gold Standard)
This is the most highly recommended and effective method for preparing Q355NH.
Process: Propelling abrasive media at high velocity against the steel surface.
Standard: Blast to a Sa 2½ (Near-White Metal) cleanliness, as defined by international standards (e.g., ISO 8501-1). This level requires the removal of all mill scale, rust, paint, and foreign matter, with only very faint discolorations remaining.
Abrasive Media: Use of non-contaminating media such as:
Metallic Abrasives: Steel grit or shot.
Non-Metallic Abrasives: Copper slag, garnet, or aluminum oxide (corundum).
Why it's best:
Perfect Uniformity: Creates an ideal, consistent surface profile (anchor pattern) for the patina to nucleate and adhere to evenly.
Removes Mill Scale: The hot-rolled steel comes with a brittle, non-uniform mill scale that will flake off at different rates, causing ugly, uneven rusting if not removed.
Efficiency: It is the fastest and most thorough method for large-scale projects.
2. Power Tool Cleaning (For Repair or Small Areas)
This method is suitable for small projects, on-site repairs, or touch-ups where abrasive blasting is not feasible.
Process: Using power tools equipped with wire brushes (cup or wheel), grinding discs (flap discs), or needle scalers.
Standard: Aim for a St 3 standard, which is "Very Thorough Power Tool Cleaning." It should remove all mill scale and rust until only a faint metallic shine remains.
Limitations:
It is more labor-intensive and time-consuming for large surfaces.
It may not achieve the same level of uniformity and surface profile as abrasive blasting, potentially leading to slight variations in the initial patina color.
3. Hand Tool Cleaning (Least Effective)
This is generally not recommended for primary preparation but may be used for very minor touch-ups.
Process: Using hand-held wire brushes, scrapers, and abrasive pads.
Standard: Results in a St 2 standard ("Thorough Hand Tool Cleaning"), which leaves more residual mill scale and rust than power tools.
Drawback: The surface will almost certainly rust unevenly if this is the only method used, as it is impossible to achieve a perfectly uniform base.
Step-by-Step Pretreatment Protocol:
For a perfect result, follow this sequence:
Degreasing: Before any mechanical cleaning, remove all oil, grease, and dirt using a suitable industrial solvent or emulsifying cleaner. This prevents contaminants from being ground into the surface.
Mechanical Cleaning: Perform the primary cleaning using one of the methods above (Abrasive Blasting to Sa 2½ is strongly preferred).
Remove Dust: After blasting or grinding, thoroughly remove all dust from the surface using clean, dry, oil-free compressed air or a brush.
Immediate Next Steps:
For Natural Weathering: Move the cleaned steel to its final location. The clean surface will immediately begin to form a uniform layer of light rust upon exposure to moisture.
For Accelerated Patination: Immediately apply the chemical patination solution to the clean, bare metal surface to induce a uniform rust color quickly.
