Step 1: Clean the Steel Surface First – Ensure Uniform Patina Growth
First, wipe or spray the surface with a neutral cleaner (e.g., a 5% sodium carbonate solution) to remove oil stains, then rinse thoroughly with clean water. Oil stains will prevent local rusting, leaving "white spots."
Next, remove the mill scale using sandblasting (80–120 mesh sand, pressure 0.5–0.8 MPa) or dilute phosphoric acid (5–8% concentration): Sandblasting creates a rough, uniform surface, making it easier for the rust to "adhere firmly"; soaking in phosphoric acid for 20–40 minutes can also dissolve the mill scale and form a thin film to help the subsequent patina grow evenly.
Finally, make sure to dry the surface completely (leave it in a well-ventilated area for 2–4 hours or blow-dry at low temperature). Do not leave moisture on the surface, otherwise, local rusting will occur too quickly.
Step 2: Accelerate Oxidation Targetedly – Fast for Small Parts, Stable for Large Structures
For small parts (e.g., decorative panels, hardware):
Soak them in a dilute acceleration solution (mix 0.5–1% ferric chloride, 0.3–0.8% sodium chloride, and clean water) for 30–60 minutes at room temperature (20–25°C). Gently agitate the solution every 15 minutes to avoid air bubbles (which cause rust-free "white spots"). After soaking, rinse with slightly acidic water (add 0.1% acetic acid to tap water) to neutralize residues, then place the parts in an environment with 60–80% humidity for 1–3 days. The surface will gradually turn from orange to a uniform reddish-brown.
For large structures (e.g., facades, brackets):
Spray a 1–2% sodium chloride solution (dilute saltwater) evenly on the surface once a day for 3–5 days. After each spray, keep the surface slightly damp (not soaked) – you can cover it with a breathable tarp at night to retain moisture (avoid plastic tarps, which trap condensation and cause pitting). Ensure good ventilation to let all areas (including undersides and crevices) rust evenly. A stable patina will form in 5–7 days.
Step 3: Stabilize the Patina – Keep It Dense and Long-Lasting
Use a soft-bristle brush or lint-free cloth to gently brush off any loose, powdery rust on the surface. Do not use wire brushes or sandpaper, as these will scratch the dense patina and expose fresh steel.
If you used chemical solutions (for small parts), wipe the surface with a damp cloth dipped in a 0.5% baking soda solution to neutralize any remaining acidic/alkaline residues, then rinse with clean water and air-dry.
(Optional but recommended) For long-term protection (especially in humid or coastal areas), apply 1–2 thin coats of a transparent, breathable patina stabilizer (e.g., diluted acrylic resin). This won't change the patina's color but will form a thin porous film to prevent rain from washing away the patina and reduce dust buildup. Allow 24 hours to dry between coats.
