Painting and post-treatment of CNC machined metal products is a key process to enhance product appearance, texture, and protective performance. The following provides a detailed explanation from the aspects of pre-treatment, painting process, post-treatment steps, and precautions:

1. Pretreatment: Ensure the surface is clean and roughened

The surface of metal products needs to be pretreated before painting to prevent oil stains, oxides, etc. from affecting the adhesion of the paint film.

Cleaning

Solvent Cleaning: Wipe the surface with alcohol, acetone, or a specialized metal cleaner to remove grease and cutting fluid residues.

Ultrasonic cleaning: For products with complex structures, they can be placed in an ultrasonic cleaning machine and cleaned thoroughly with cleaning agents to remove impurities in the gaps.

Surface roughening

Sandblasting: Use quartz sand, glass beads, or similar materials to sandblast the metal surface, creating a microscopic rough texture (for example, aluminum alloy commonly uses 80–120 mesh abrasive) to enhance paint adhesion.

Chemical Etching: For materials such as stainless steel, the surface can be lightly etched using an acidic solution (e.g., a mixture of hydrofluoric acid and nitric acid) to increase roughness.

Static Removal: Use an ionizing air gun to blow the surface and eliminate dust particles attracted by static electricity.

2. Painting process: choosing suitable paint and spraying method

1. Paint selection

Metal Primer:

Epoxy Primer: Provides strong adhesion and excellent corrosion resistance, suitable for materials such as steel and aluminum alloys.

Phosphate Primer: Forms a phosphate layer to enhance the adhesion of subsequent coatings, commonly used for aluminum and magnesium alloys.

Topcoat:

Polyurethane Paint: Features high gloss and excellent wear resistance, suitable for products with high appearance requirements (such as glossy or matte finishes).

Acrylic Paint: Dries quickly and offers vibrant colors, suitable for fast-drying scenarios (such as urgent prototypes).

Baked Enamel: Requires high-temperature baking (e.g., 120–150℃), providing a hard, scratch-resistant coating, suitable for industrial-grade products.

Special effect paint:

Wrinkle paint: A special process creates a concave and convex texture to increase anti-slip properties.

Fluorescent / Pearl Paint: Meets personalized appearance requirements, such as visibility at night or metallic pearl effects.

2. Spraying method

Air Spraying: The most commonly used method, where paint is atomized through a spray gun and applied evenly to the surface, suitable for large areas and products with complex curves.

Electrostatic Spraying: Utilizes the principle of electrostatic attraction, where paint particles are charged and adhere to the metal surface, resulting in a more uniform coating and reduced paint usage, ideal for mass production.

Dipping / Brushing: Used for small-area touch-ups or special structures (such as deep holes or grooves), though it is less efficient and may result in slightly uneven surface smoothness.

3. Spraying parameter control

Paint Viscosity: Measured using a viscosity cup; for air spraying, viscosity is generally controlled at 15–25 seconds (Ford #4 cup, 25℃).

Spraying Pressure: For air spraying, the pressure is typically 0.3–0.5 MPa; excessive pressure can cause paint overspray, while too low pressure results in poor atomization.

Spraying Distance and Thickness: Keep the spray gun 15–25 cm from the product surface, with a single coating thickness of 5–10 μm, applied in 2–3 passes, allowing 15–30 minutes between each pass for surface drying.

3. Post-processing: Improving appearance and performance

Drying and curing

Air Drying: Suitable for room-temperature curing paints (such as acrylic paint), requiring at least 24 hours to ensure complete drying.

Baking: Baked metal paints require placing the product in an oven, setting the temperature according to the paint specifications (e.g., 80℃ × 30 minutes → 120℃ × 1 hour), gradually increasing the temperature to prevent coating cracks.

Surface modification

Grinding and polishing:

Dry Sanding: Use 600–2000 grit sandpaper to smooth the paint surface, removing particles, runs, and other defects (take care not to sand through the primer).

Wet Sanding: Sand with soapy water to achieve a finer surface, suitable as a pre-treatment for high-gloss finishes.

Mechanical polishingPolishing with cloth wheel and polishing compound on local areas (e.g., curved surfaces, flat surfaces) to enhance glossiness.

Touch-up and RepairSanding the defective areas with fine sandpaper, followed by local touch-up spraying of primer and topcoat, then overall baking or natural drying.

Protective Coating (Optional)

Spraying clear coat (e.g., polyurethane clear coat): Increases surface hardness and scratch resistance while enhancing glossiness (clear coat thickness: 5-10μm).

Electroplating/Electrophoresis: For higher protection, electroplating (e.g., chrome or nickel plating) or electrophoretic coating can be applied after painting (suitable for batch metal parts).

IV. Common Issues and Solutions

Issue Cause Solution
Paint Peeling Inadequate surface cleaning or insufficient roughness Re-sandblasting/chemical etching to ensure the surface is free of oil and dust
Sagging / Paint buildup The spray thickness is too large or the spray gun is too close Reduce the amount of spraying per time and keep the spray gun moving at a uniform speed
Particulate Contamination High environmental dust or unfiltered coating Keep the spraying environment clean and use the paint after sieving
Uneven Gloss Uneven coating thickness or insufficient drying Apply multiple thin coats, ensuring each layer is dry before applying the next
Cracking / Paint bursting Poor compatibility between primer and topcoat or sudden increase in drying temperature Select a compatible coating system and gradually increase temperature during drying

V. Precautions

Environmental Control:

Spraying should be carried out in a dust-free workshop or clean environment (dust particles ≤ 10,000 are recommended) to avoid impurities contaminating the paint film.

Ambient temperature: 15-25°C, humidity ≤70% RH. High humidity may cause whitening of the paint film (can be improved by adding anti-blushing agent).

Safety Protection:

Wear protective masks and gloves to avoid skin contact with paint or inhalation of volatile gases (solvent-based paints contain VOCs).

Ensure good ventilation, keep away from fire sources, and prevent paint vapor explosion.

Balancing Cost and Efficiency:

For small batch products, air spraying + natural drying can be preferred to reduce equipment costs;

For mass production, it is recommended to use electrostatic spraying + assembly line drying to improve efficiency and coating consistency.

Through the above processes, CNC metal products can achieve uniform, aesthetically pleasing, and durable surface effects after painting, meeting requirements for appearance validation, functional testing, or display. Actual operations should flexibly adjust process parameters based on metal material (e.g., aluminum, steel, copper), surface requirements (e.g., matte, mirror, texture), and production conditions.