In the world of precision manufacturing, CNC machining and injection molding produce high-accuracy parts, but the final surface finish plays a crucial role in functionality, durability, and aesthetics.
A well-executed surface treatment can enhance corrosion resistance, improve wear performance, reduce friction, and even influence electrical conductivity.
This in-depth guide explores the most common surface finishing techniques, their benefits, applications, and how to choose the best option for your project.
Why Surface Finishing Matters in CNC Machining & Injection Molding
Before diving into specific surface treatments, it’s essential to understand why surface finishing is critical:
Improved Aesthetics – A polished or coated surface enhances visual appeal, which is vital for consumer products, automotive parts, and luxury goods.
Enhanced Durability – Many finishes protect against wear, corrosion, and environmental factors, extending part lifespan.
Better Functionality – Some finishes reduce friction (e.g., for moving parts) or improve electrical properties (e.g., conductive coatings).
Regulatory Compliance – Medical and food-grade components often require specific finishes (e.g., passivation for stainless steel).
Improved Adhesion – Certain finishes prepare surfaces for painting, bonding, or further coatings.
Now, let’s explore the three main categories of surface finishing:
Physical Surface Treatments
Chemical Surface Treatments
Surface Spraying & Coating
Physical Surface Treatment
Physical surface treatments modify the part’s exterior through mechanical processes, improving texture, smoothness, or appearance without altering the material’s chemical composition.
#1. Polishing
Polishing is a surface finishing process that smooths and refines a material’s exterior to achieve a high-gloss, mirror-like, or satin finish. It removes microscopic imperfections, burrs, and scratches, enhancing both aesthetics and functionality.
This method is ideal for aesthetic applications, such as consumer products, jewelry, and medical devices.
Benefits:
- Enhances visual appeal
- Reduces surface roughness
- Improves cleanliness (easier to clean)
#2. Brushing
Brushing process that creates fine linear textures on metal workpieces through precision abrasion. This technique enhances aesthetic appeal while maintaining material integrity.
Brushing (also called satin finishing or directional polishing) is a mechanical surface treatment that creates a uniform, linear grain pattern on metal surfaces. This finish is achieved by abrading the material with abrasive belts, brushes, or pads in a single direction.
It’s commonly used for stainless steel and aluminum parts in appliances, automotive trim, and architectural components.
Benefits:
Conceals minor scratches
Provides a matte, satin-like finish
Improves resistance to fingerprints
#3. Sandblasting
Sandblasting (also called abrasive blasting) is a versatile surface treatment that propels abrasive media at high velocity to clean, texture, or prepare metal and plastic surfaces. This process creates a uniform matte finish while removing contaminants, oxidation, and imperfections.
Benefits:
Removes burrs and imperfections
Creates a uniform matte texture
Enhances adhesion for subsequent coatings
Polishing vs. Brushing vs. Sandblasting
Finish Type
Texture
Best For
Polishing
Mirror-smooth
Aesthetic & hygienic parts
Brushing
Directional grain
Scratch-resistant surfaces
Sandblasting
Matte/textured
Paint adhesion & grip
Chemical Surface Treatment
Chemical treatments alter the surface layer of a part through chemical reactions, improving corrosion resistance, hardness, or aesthetic properties.
#4. Black Oxidation
Black oxidation (also called blackening or black oxide coating) is a chemical conversion coating that creates a dark, matte-black finish on ferrous metals (steel, stainless steel, cast iron) and some non-ferrous metals (copper, brass). Unlike plating, it doesn’t add material but chemically alters the surface for corrosion resistance, aesthetics, and reduced light reflection.
Benefits:
Minimal dimensional change
Improves corrosion resistance
Reduces light reflection
Types of Black Oxide Finishes
Finish Type
Process
Best For
Hot Black Oxide
Traditional alkaline bath
Steel, stainless steel
Mid-Temp Black Oxide
Operates at ~200°F (93°C)
More environmentally friendly
Cold Black Oxide
Room-temperature selenium-based solution
Low-cost touch-ups (less durable)
#5. Anodizing
Anodizing is an electrochemical process that forms a protective Al₂O₃ (aluminum oxide) layer on the surface of aluminum and its alloys.
This oxide coating possesses enhances the natural oxide layer on metals (primarily aluminum, but also titanium, magnesium, and zinc). This process creates a durable, corrosion-resistant, and often decorative finish while maintaining the metal’s lightweight properties.
Standard anodized finishes typically include black anodizing and natural clear anodizing, while custom colors can be produced to meet specific requirements.
Benefits:
Excellent corrosion resistance
Increased surface hardness
Available in various colors
Types of Anodizing Finishes
Type
Process
Thickness
Best For
Type I (Chromate Anodizing)
Uses chromic acid
0.5–5 µm
Aerospace (lightweight, non-conductive)
Type II (Sulfuric Acid Anodizing)
Most common method
5–25 µm
Industrial, automotive, consumer goods
Type III (Hardcoat Anodizing)
Thicker, more wear-resistant
25–150 µm
Military, firearms, high-wear parts
Color Anodizing
Dye added after anodizing
Varies
Aesthetic applications (electronics, jewelry)
#6. Phosphating
Phosphating is a chemical conversion coating process that creates a crystalline or amorphous phosphate layer on metal surfaces (typically steel, zinc, or aluminum). This treatment enhances corrosion resistance, paint adhesion, and wear properties, while also providing lubrication for metal forming operations.
Benefits:
Enhances lubricity
Provides a good base for painting
Extends part lifespan
#7. Passivation
Passivation is a chemical treatment (typically using nitric or citric acid) that removes free iron and contaminants from stainless steel or other metals, enhancing corrosion resistance by forming a protective oxide layer without altering the part’s dimensions.
Benefits:
Prevents rust and oxidation
Maintains biocompatibility
No dimensional changes
Surface Spraying
#8. Painting
Painting is a surface coating process that applies liquid or powder formulations to substrates for protection, decoration, or functional performance.
Benefits:
Wide range of colors and finishes
Protects against UV and chemical exposure
Customizable for branding
#9. Electroplating
Electroplating is an electrochemical process that deposits a thin, adherent metal coating onto a conductive substrate to enhance corrosion resistance, wear properties, aesthetics, or electrical conductivity.
Benefits:
Enhances corrosion resistance
Improves electrical conductivity
Adds decorative appeal
#10. Screen printing
Screen printing is a stencil-based printing technique where ink is forced through a fine mesh screen onto a substrate to create precise, durable designs or functional coatings.
Benefits:
Enhances corrosion resistance
Improves electrical conductivity
Adds decorative appeal
#11. Powder Coating
Powder coating is a dry finishing process where electrostatically charged powder particles are sprayed onto a substrate and then cured under heat to form a durable, high-quality protective or decorative layer.
Benefits:
Superior scratch and chip resistance
Environmentally friendly (no solvents)
Wide range of colors and textures