Finishing Services — Complete Guide
Surface finishing is the final step in manufacturing — and often the difference between a part that looks "cheap" and one that commands a premium price. Whether you need corrosion protection, wear resistance, or a specific appearance, choosing the right finish is essential.
This guide covers all major finishing processes for metal and plastic parts, including process descriptions, material suitability, cost factors, and application guidance.
Finishing Classification Overview
| Category | Processes | Primary purpose |
|---|---|---|
| Mechanical | Bead blasting, brushing, polishing, tumbling | Appearance, surface prep |
| Chemical | Passivation, chromate, black oxide, phosphating | Corrosion resistance |
| Electrochemical | Anodizing, electro-polishing, electroplating | Corrosion + appearance |
| Coating | Powder coating, wet painting, PVD, CVD | Appearance + durability |
| Thermal | Hot-dip galvanizing, thermal spray, nitriding | Corrosion + wear |
Mechanical Finishes
Bead Blasting (Abrasive Blasting)
Fine glass beads or aluminum oxide particles are propelled at the part surface to create a uniform matte finish.
| Parameter | Value |
|---|---|
| Surface finish (Ra) | 0.5-2.0 µm |
| Substrate removal | None (cleaning only) |
| Suitable for | Metals, plastics, glass |
| Typical cost | $0.50-3.00 per part (small) |
Best for: Removing tool marks, uniform cosmetic appearance, surface prep for coating
Brushed Finish (Linear Grain)
An abrasive belt or wheel creates a directional grain pattern on the surface.
- Brushed #4: 120-180 grit — most common, slight grain visible
- Brushed #6: 240-320 grit — finer, more refined appearance
- Hairline: Long, continuous parallel lines — premium appearance
Best for: Decorative panels, architectural metal, kitchen equipment, appliances
Tumbling / Vibratory Finishing
Parts are placed in a vibrating bowl or barrel with abrasive media and compound. Removes sharp edges, burrs, and improves surface finish.
| Abrasive media | Material | Best for | Result |
|---|---|---|---|
| Ceramic triangles | Hard metals | Deburring, edge rounding | Fast stock removal |
| Plastic cones | Soft metals, plastics | Smoothing, polishing | Gentle finish |
| Steel balls | All metals | Burnishing (bright finish) | Mirror-like surface |
| Corn cob / walnut shell | Soft, delicate parts | Drying, fine polishing | Non-abrasive |
Best for: Batch deburring, edge breaking, reducing surface roughness
Chemical Finishes
Passivation (Stainless Steel)
An acid bath removes free iron from the surface of stainless steel, forming a thin, transparent chromium oxide layer that naturally resists corrosion.
| Standard | Typical use |
|---|---|
| ASTM A967 | General corrosion resistance |
| ASTM A380 | Cleaning and passivation |
Best for: Medical devices, food processing, marine, pharmaceutical equipment
Black Oxide (Steel)
A chemical conversion coating that produces a black finish (Fe3O4) on steel surfaces.
| Type | Process temp | Corrosion resistance | Cost |
|---|---|---|---|
| Hot black oxide | 135-145°C | Moderate (needs oil/sealer) | Low |
| Cold black oxide | Room temp | Poor (needs sealer) | Very low |
Best for: Tooling, fasteners, automotive under-hood, optical parts (reduces glare)
Chromate Conversion (Alodine / Chem Film)
A chromate coating on aluminum that provides corrosion resistance and electrical conductivity. Required for aerospace.
| Type | Color | Corrosion resistance | Electrical conductivity |
|---|---|---|---|
| Class 1A | Clear/golden | Excellent | Yes |
| Class 3 | Yellow/gold | Good | Yes |
Best for: Aerospace parts, electronics enclosures, structural aluminum
Electrochemical Finishes
Anodizing (Aluminum Only)
An electrochemical process that converts the aluminum surface into a hard, porous aluminum oxide layer. The pores can be sealed (natural finish) or dyed (colored).
| Type | Thickness | Hardness | Color options | Typical use |
|---|---|---|---|---|
| Type II (Decorative) | 5-15 µm | Moderate | Clear, black, many colors | Consumer products, electronics |
| Type III (Hard coat) | 25-100 µm | Very hard (60-70 HRC) | Dark gray only | Aerospace, military, wear parts |
| Sulfuric acid anodize | 15-25 µm | Good | All colors | General purpose |
Sealing options:
- Hot water seal: Most corrosion resistant, no color change
- Nickel acetate seal: Standard, some color change
- Cold seal: Lower cost, slight haze
Best for: Aluminum parts needing wear resistance, corrosion protection, or colored appearance
Electroplating
Using electric current to deposit a thin metal layer onto a part.
| Plating type | Substrate | Thickness | Properties | Typical use |
|---|---|---|---|---|
| Zinc plating | Steel | 5-15 µm | Corrosion protection (sacrificial) | Fasteners, brackets |
| Nickel plating | Steel, brass, zinc | 10-50 µm | Decorative, wear resistant | Automotive trim, hardware |
| Chrome plating | Nickel-plated parts | 0.5-2 µm (decorative) up to 500 µm (hard) | Decorative or hard wearing | Automotive, tools, cylinders |
| Tin plating | Copper, steel | 3-10 µm | Solderable, food safe | Electronics, food equipment |
| Silver plating | Copper, brass | 5-20 µm | Electrical conductivity | Connectors, switch contacts |
| Gold plating | Nickel undercoat | 0.5-5 µm | Corrosion + conductivity | Electronics, connectors |
Electro-polishing
The reverse of electroplating — the part is made anodic, removing surface material to create a smooth, bright, passivated surface. Primarily for stainless steel.
- Result: Mirror-like finish, Ra 0.1-0.4 µm
- Benefits: Removes surface contaminants, improves corrosion resistance
- Best for: Medical instruments, pharmaceutical equipment, food processing, decorative
Coating Processes
Powder Coating
Electrostatically charged dry powder is sprayed onto the part, then cured in an oven. More durable, thicker, and more uniform than wet paint.
| Parameter | Value |
|---|---|
| Thickness | 50-150 µm |
| Hardness | Excellent (3H-6H pencil) |
| UV resistance | Good to very good (formulation dependent) |
| Chemical resistance | Good to excellent |
| Colors | Any RAL color + custom + textures |
Cure temperature: 180-200°C for 10-20 minutes
Best for: Outdoor equipment, heavy machinery, automotive, consumer products
Wet Painting / Liquid Coating
Traditional spray painting using liquid paint. Lower film build, smoother appearance.
- Air spray: Best quality finish, highest transfer efficiency
- HVLP: High transfer efficiency, medium quality
- Dip coating: Even coverage on complex geometry
Best for: Large parts, low-volume custom colors, high-gloss automotive finishes
PVD (Physical Vapor Deposition)
A thin film coating applied in a vacuum chamber. Used for both decorative and functional applications.
| PVD coating | Color | Hardness | Wear resistance |
|---|---|---|---|
| Titanium Nitride (TiN) | Gold | 2,300 HV | Excellent |
| Chrome Nitride (CrN) | Silver-gray | 2,000 HV | Excellent |
| Titanium Aluminum Nitride (TiAlN) | Violet-gray | 3,200 HV | Very high temp resistance |
| Diamond-like Carbon (DLC) | Black | 2,000-5,000 HV | Exceptional (low friction) |
Best for: Cutting tools, molds, dies, decorative hardware, watch cases
Thermal Finishes
Hot-Dip Galvanizing (HDG)
Parts are dipped in molten zinc (450°C). Creates a thick, durable zinc-iron alloy coating.
| Parameter | Value |
|---|---|
| Coating thickness | 50-150 µm |
| Corrosion life | 20-50+ years (varies by environment) |
| Suitable for | Steel, iron |
Best for: Structural steel, outdoor enclosures, utility poles, fencing
Nitriding (Case Hardening)
Nitrogen diffuses into the steel surface at 500-550°C, creating a hard, wear-resistant surface without quenching.
- Case depth: 0.1-0.5mm
- Surface hardness: 900-1,200 HV
- Distortion: Minimal (no quenching)
Best for: Gears, shafts, injection mold cores, extrusion screws
Finish Selection Guide
| Requirement | Recommended finish | Alternative |
|---|---|---|
| Corrosion protection (indoor) | Black oxide + oil | Zinc plating |
| Corrosion protection (outdoor) | Hot-dip galvanizing | Powder coating |
| Wear resistance | Hard anodize (Al) or electroless nickel | Nitriding or DLC |
| Decorative appearance | Powder coating or anodizing | Plating or PVD |
| Food contact | Passivation (SS) or electroless nickel | Tin plating |
| Electrical conductivity | Chromate (Al) or gold plating (precision) | Silver plate |
| Anti-galling (screw threads) | Phosphate coating | Nitriding |
| Low friction / wear | DLC coating | PTFE (Teflon) coating |
| High temperature (250°C+) | PVD (TiAlN) or nitriding | Hard chrome |
Cost Comparison
| Finish | Relative cost (per part) | Setup cost | Best volume |
|---|---|---|---|
| Vibratory tumbling | Very low | None | 100+ |
| Bead blast | Low | Low | 50+ |
| Passivation | Low | Very low | 1+ |
| Black oxide | Low | Low | 100+ |
| Zinc plating | Low | Low | 500+ |
| Powder coating | Low-Medium | Low | 50+ |
| Anodizing (Type II) | Medium | Medium | 50+ |
| Hard anodize (Type III) | High | Medium | 50+ |
| Electroless nickel | Medium-high | Medium | 100+ |
| Electro-polishing | Medium | Low | 1+ |
| PVD coating | High | Very high | 500+ |
| Hot-dip galvanizing | Medium | High (by weight) | 100+ |
| Hard chrome plating | High | Medium | 50+ |
Why MFGABC for Finishing?
- In-house finishing: Bead blasting, vibratory tumbling, passivation, black oxide, powder coating
- Managed partners: Anodizing, plating, PVD, galvanizing — vetting quality with established partners
- One-stop: Part manufacturing + finishing in the same workflow — no shipping between vendors
- Quality control: Finish thickness measurement, adhesion testing, salt spray testing available
- Experience: Automotive, medical, aerospace, and consumer products — we know what finish works where
- Color matching: RAL, Pantone, and custom color matching for powder coating and painting
*Add finishing to your next part order — upload your files for a quote including all surface treatments.*
*→ Back to: [Capabilities Overview](/)*
*→ Related: [CNC Surface Finish Guide](/blog/cnc-surface-finish-guide/)*
*→ Related: [Sheet Metal Fabrication Guide](/capabilities/sheet-metal/)*
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