1. Hot-Rolled Mill Scale Finish (As-Rolled)
Core characteristics: Unprocessed surface with a thick, brittle layer of iron oxide mill scale (FeO/Fe₂O₃/Fe₃O₄); irregular rough texture, minor embedded mill dirt/oil, uneven color (grey/black/brown).
Production: The default finish from hot rolling, no post-processing after the mill.
Key applications: Temporary structural use, raw material for further processing (blasting/pickling), non-aesthetic load-bearing components (e.g., steel frame substructures).
Patina note: Poor for patina formation (scale delaminates easily); requires full prep for any protective patina development.
2. Pickled & Oiled (P&O) Finish
Core characteristics: Scale-free, smooth surface (Ra 0.8–1.5 μm); a thin, protective mineral oil film is applied after pickling to prevent flash rust; uniform metallic grey appearance, low surface roughness.
Production: Descaled via acid pickling (removes mill scale), then coated with a light rust-inhibiting oil.
Key applications: Cold forming, machining, welding, aesthetic non-patina structural parts, components requiring smooth surface contact (e.g., brackets, connectors).
Patina note: Oil must be fully removed for patina formation; smooth texture hinders oxide adhesion (poor for coastal patina development).
3. SA2.5/SA3 Sandblasted (Abrasive Blasted) Finish
Core characteristics: Ultra-clean, uniform micro-rough texture (Ra 3.0–5.0 μm); 95%+ removal of mill scale, rust, oil and contaminants (SA2.5 = commercial blast, SA3 = white metal blast); matte metallic grey surface with open porous texture for coating/oxide adhesion.
Production: Abrasive blasting with sand, steel grit or aluminum oxide; the most common prep finish for weathering steel patina.
Key applications: Patina formation (inland/coastal), sealer/coating application, architectural weathering steel (sculptures, facades), coastal structural components.
Patina note: The optimal finish for ASTM A242 patina development (enables dense, bonded oxide formation even in coastal areas).
4. Wire-Brushed Finish
Core characteristics: Lightly textured surface (Ra 1.5–3.0 μm); removes loose mill scale, rust and surface dirt (no full scale removal); retains a thin, tight base oxide layer; uniform brushed metallic appearance.
Production: Mechanical wire brushing (hand or automated) to de-scale and smooth the surface lightly.
Key applications: Mild inland patina formation, aesthetic architectural parts (minimal texture), on-site surface touch-up, non-critical outdoor components.
Patina note: Suitable for mild inland environments only; insufficient for coastal patina formation (limited oxide adhesion).
5. Ground/Polished Finish
Core characteristics: Extremely smooth, low-roughness surface (Ra 0.2–0.8 μm); machined finish with precise surface flatness; mirror (polished) or satin (ground) metallic appearance; no surface texture for oxide bonding.
Production: Precision grinding (satin) or mechanical polishing (mirror) for tight surface tolerance.
Key applications: Machined components, precision structural parts, aesthetic non-patina steel, parts requiring smooth surface finish (e.g., bearing surfaces).
Patina note: Poor for patina formation (smooth texture blocks iron-alloy oxide adhesion; only forms loose, non-protective rust).
