Metal polishing is the process of creating a smooth, reflective surface on metal parts by removing material from the surface through mechanical abrasion (grinding, buffing), chemical dissolution (chemical polishing), or electrochemical reaction (electropolishing). The objective ranges from simple deburring and smoothing (Ra 0.4–0.8 μm) to mirror-finish polishing (Ra < 0.05 μm) for decorative or optical applications. Different metals require distinct techniques — stainless steel, aluminum, brass, and titanium each behave differently under polishing compounds and wheels.
China's polishing industry is enormous and supports everything from kitchenware (stainless steel pots and pans) to automotive wheels and consumer electronics enclosures. Quality varies dramatically — from rough hand-polished work by individual craftsmen in small workshops to automated robotic polishing lines in tier-1 precision manufacturing facilities.
| Surface Grade | Typical Ra (μm) | Process Steps | Application |
|---|---|---|---|
| #320 (coarse ground) | 0.8–1.6 | Belt grinding / coarse wheel | Industrial components, hidden surfaces |
| #400 (standard matte) | 0.4–0.8 | Belt + fine grinding | General industrial, structural parts |
| #600 (semi-bright) | 0.2–0.4 | Fine belt + sisal wheel + compound | Architectural hardware, handles |
| #1000 (bright polish) | 0.1–0.2 | Multi-stage + loose grit buffing | Decorative trim, faucets, auto trim |
| #2000 (mirror) | 0.05–0.1 | Multiple buffing stages + compound | Ornamental, lighting, bathroom fixtures |
| Mirror (optical) | < 0.05 | Precision lapping + diamond compound | Medical implants, optical reflectors, dies |
| Parameter | Stainless Steel | Aluminum (bright dip) |
|---|---|---|
| Surface Improvement | Ra 0.4 → 0.1 μm typical | Ra 0.6 → 0.2 μm typical |
| Material Removal | 0.005–0.025 mm per side | 0.010–0.035 mm per side |
| Bath Temperature | 50–70 °C (sulfuric/phosphoric acid) | 85–95 °C (phosphoric/nitric acid) |
| Current Density | 5–15 A/dm² | 8–15 A/dm² |
| Best Results On | 304, 316, 17-4 PH | 6061, 6063 (not 380 die-cast) |
| Limitation | Surface pits/weld defects magnified | Si content > 2% causes staining |
China's precision polishing hub for consumer electronics, stainless steel, and aluminum. Hundreds of shops with automated robotic polishing cells for smartphone frames, watch cases, and laptop enclosures. Surface finish grades from #600 to mirror (Ra 0.05). Price: ¥5–30/piece for small precision parts depending on RA specification. UV-curable masking for selective polishing (brushed + polished on same part) is well-developed here.
Stainless steel kitchenware and sanitary ware polishing center. Large-scale shops with conveyorized polishing lines for pots, pans, faucets, and shower heads. Hand-polishing shops charge ¥8–20/kg ($1.20–3.00/kg). Robotic polishing lines charge ¥15–30/kg but deliver 3× better consistency. Surface quality ranges from #320 to #1000.
Hardware polishing hub — door handles, locks, hinges, cabinet hardware, and metal furniture. Extreme cost competition: as low as ¥1–5/kg ($0.15–0.75/kg) for #320 matte finish. Large-scale hand-polishing operations. Quality inconsistency is the main complaint from foreign buyers. Automated lines are rare; almost exclusively manual buffing work.
The polishing paradox: A hand-polished part by an experienced Chinese craftsman can achieve a mirror finish that looks extraordinary — but the same craftsman will produce parts that vary ±30% in surface quality within the same batch. If consistency matters more than peak quality, pay extra for CNC or robotic polishing. If peak quality is paramount, pay for one skilled polisher to do all your parts personally (and accept the lead time).
We see buyers frequently specifying "mirror finish" on surfaces that will never be seen. Each step from #400 to mirror adds 2–4× the processing cost. Be honest about which surfaces need the premium finish: functional sealing surfaces? Visible decorative surfaces? Hidden mounting flanges? Specify finish by surface (e.g., "visible surfaces: #1000 polished, all others: #400") instead of a blanket requirement. This can reduce polishing cost by 40–60%.
Mechanical polishing generates significant heat. On thin-walled parts (sheet metal < 1.5 mm, tubing < 0.8 mm wall), the localized heating can cause thermal distortion — the part becomes slightly warped. The fix: use lower RPM + lighter pressure + more passes, or use chemical polishing / electropolishing instead of mechanical. Many Chinese shops run their polishing wheels at 2,800–3,600 RPM — for thin parts, specify 1,800–2,400 RPM max.
Polishing over welded joints on stainless steel is a specialized skill. If the weld filler metal doesn't match the base metal composition perfectly, the polished weld area may have a different color, etch differently during passivation, or exhibit "weld shadow." For visible welds that need a continuous polished appearance, specify TIG welding with matching filler rod and request that the polishing process starts with a finer grit than standard (e.g., start at #240 rather than #120) blended across a wide transition zone.
| Process | Cost per Piece (small part, 100 g) | Cost per kg (bulk) | Notes |
|---|---|---|---|
| #320 matte finishing | $0.10–0.30 | $0.50–1.50 | Single-step grinding, deburring only |
| #600 semi-bright | $0.30–1.00 | $1.50–4.00 | Two-stage (grind + buff) |
| #1000 bright polish | $0.80–3.00 | $3.00–8.00 | Three-stage, skilled polisher |
| Mirror finish (hand) | $2.00–10.00 | $8.00–20.00 | 5+ stages, premium skill |
| Electropolishing | $0.50–5.00 | $3.00–12.00 | Depends on part size vs tank volume |
| Robotic polishing | $0.50–3.00 | $2.00–8.00 | High initial setup, consistent per unit |
Typical MOQ: 100–1,000 pieces for hand polishing, 500–5,000 pieces for robotic. Minimum batch charge: ¥200–500 ($30–75).
This is a common misconception — polishing does not inherently provide corrosion protection. In fact, aggressive mechanical polishing can smear the surface, embedding abrasive particles and iron contamination, which actually reduces corrosion resistance. The key step for stainless steel corrosion protection is passivation (ASTM A967 / AMS 2700) — a nitric or citric acid bath that removes surface iron and restores the chromium oxide layer. Always specify final passivation after mechanical polishing for stainless steel parts. Electropolishing, however, both polishes and passivates simultaneously because it preferentially removes iron from the surface.
Orange peel on polished aluminum is caused by uneven grain structure in the base metal. Extruded 6063 aluminum has an elongated grain structure that polishes differently depending on whether you go with or against the extrusion direction. Solution: use a finer starting grit (#600 instead of #320), apply even cross-hatch polishing, and finish with a buffing compound specifically formulated for aluminum (not stainless steel compound — those contain silica that scratches aluminum). Pre-machining the surface to a finer initial finish also helps.
Yes, but the heat tint zone (the oxidized layer around a weld) extends deeper into the metal than visible discoloration. If you simply polish the discoloration away without removing the underlying chromium-depleted layer (typically 0.02–0.10 mm deep), the polished area will still have reduced corrosion resistance. For food-grade or pharmaceutical applications, specify pickling (acid treatment) before final polishing to restore the corrosion-resistant surface.
Brushed finish creates a unidirectional satin texture using abrasive belts or pads (typically #240 to #600 grit), producing a linear grain pattern with low reflectivity. Polished finish (mirror) uses progressively finer compounds and soft buffing wheels to create a mirror-like reflective surface with no visible grain direction. Brushed is cheaper (>50% savings), hides fingerprints better, and is preferred for most architectural and consumer products. Mirror finish is dramatically more expensive and shows every fingerprint and scratch, but is required for decorative trim, jewelry, and medical lighting.
Yes — this is "selective finishing" or "two-tone finishing." The typical process: brush the entire part, then mask the areas that should stay brushed with UV-curable or laser-cut masking, then buff/polish the exposed areas. Common on faucet handles, automotive trim, and mobile phone frames. Chinese shops in Shenzhen do this routinely. Expect 30–50% cost premium over a single surface finish.