CNC Surface Finish Options: Anodize, Plate, Blast, or Polish? A Complete Selection Guide
Every machined part leaves the spindle with a measurable surface texture. That texture — measured in Ra, Rz, or RMS — determines how the part behaves in service. Surface finish is not cosmetic. It directly controls corrosion initiation, fatigue crack nucleation, wear particle generation, and assembly clearance. Selecting the wrong post-machining treatment can scrap a $5,000 part in under 100 hours of operation.
This guide compares eight common CNC surface finishing processes across four critical axes: cost per kilogram, lead time impact, achievable surface roughness, and material compatibility. You will walk away with a decision matrix that applies to production runs from prototype quantities to 10,000-piece orders.
Anodizing: The Aluminum Standard
Anodizing is an electrochemical conversion process that grows a controlled oxide layer from the base aluminum. It is not a coating — it is a transformation. For 6061-T6 and 7075-T6 alloys, Type II sulfuric anodizing produces a 0.0002 to 0.0004 inch thick layer with a hardness approaching 300 HV. Type III hard coat anodizing pushes thickness to 0.002 inches and hardness above 450 HV.
Cost runs $2.50 to $6.00 per kilogram for Type II, and $5.00 to $12.00 per kilogram for Type III. Lead time for standard colors (clear, black) is 3 to 5 business days. Custom colors add 2 to 3 days for dye validation.
Critical tolerance note: Anodizing builds dimension. A 0.001 inch hard coat layer adds 0.001 inch to each surface. For threaded holes or close-fit bores, you must mask or oversize the pre-machined dimension. Expect a 0.0002 to 0.0005 inch loss of surface roughness — a 16 Ra as-machined surface will measure 20 to 24 Ra after anodizing.
Material compatibility: Aluminum and titanium only. No ferrous alloys, no copper, no brass. For stainless steel, see passivation below.
Electroless Nickel Plating: Uniform Coverage, Hard Surface
Electroless nickel (EN) deposits a nickel-phosphorus alloy via autocatalytic chemical reduction. Unlike electrolytic plating, there is no current density variation — EN covers internal threads, blind holes, and complex internal geometries uniformly. Typical thickness is 0.0005 to 0.002 inches per side.
Cost: $4.00 to $8.00 per kilogram. Lead time: 4 to 7 business days. The as-plated hardness is 48 to 52 HRC, and heat treatment at 400°C for one hour raises it to 62 to 65 HRC.
EN is the go-to finish for steel and aluminum parts requiring wear resistance plus corrosion protection. It works on carbon steel, alloy steel, stainless steel, aluminum, copper alloys, and beryllium copper. Do not use on magnesium or zinc die castings — the bath chemistry attacks these substrates.
Tolerance impact: EN adds thickness uniformly. For a 0.001 inch spec, pre-plate dimensions must be 0.001 inch undersize. Surface roughness typically degrades by 0.0001 to 0.0002 inches Ra.
Media Blasting: Texture and Surface Preparation
Blasting uses compressed air to propel media against the part surface. The media type determines the finish. Glass bead at 60 to 80 psi produces a satin matte finish with 32 to 64 Ra. Aluminum oxide at 80 to 100 psi cuts faster and leaves a rougher surface, 64 to 125 Ra. Ceramic shot at 40 to 60 psi peens the surface, inducing compressive residual stress that improves fatigue life.
Cost is low: $0.50 to $1.50 per kilogram. Lead time is 1 to 2 business days. Blasting is often a preparation step before anodizing or painting, not a final finish.
Critical consideration: Blasting removes material. Glass bead at 60 psi removes approximately 0.0001 to 0.0003 inches per pass. Aluminum oxide at 100 psi can remove 0.0005 to 0.001 inches per pass. For thin-wall parts or tight-tolerance features, this material loss can push dimensions out of spec. Always machine oversize by 0.001 to 0.002 inches if blasting is in the process flow.
Material compatibility: All metals. Avoid on soft plastics and elastomers.
Polishing and Buffing: Mirror Finish
Polishing is an abrasive process that removes surface peaks to achieve low Ra values. Mechanical polishing with progressively finer grits — 120, 240, 400, 600, then 800 and 1200 — can bring a surface from 32 Ra as-machined down to 4 to 8 Ra. Buffing with compound on a soft wheel can push that to 0.5 to 2 Ra, a true mirror finish.
Cost: $3.00 to $10.00 per kilogram, depending on starting roughness and final spec. Lead time: 3 to 7 business days. Complex geometries with deep pockets or internal corners require hand work, which multiplies cost and time.
Polishing is not a coating. It removes base material. For a 4 Ra finish on stainless steel, expect 0.002 to 0.005 inches of material removal. For mirror finish, 0.005 to 0.010 inches. Tolerance-critical parts must be machined oversize accordingly.
Material compatibility: Any metal. Aluminum polishes well but requires immediate anodizing or clear coating to prevent oxidation. Stainless steel 304 and 316 polish to a deep luster and hold it without coating.
Passivation: The Invisible Finish
Passivation removes free iron from stainless steel surfaces using nitric or citric acid. It is not a coating and does not change dimensions or roughness. What it does: eliminates surface contaminants that cause corrosion initiation. For 304 and 316 stainless, passivation raises corrosion resistance by 10x to 50x in salt spray testing.
Cost: $0.30 to $0.80 per kilogram. Lead time: 1 to 2 business days. It is the cheapest finishing option and should be specified on every stainless steel part that will see moisture, chemicals, or food contact.
Material compatibility: Stainless steel grades 304, 316, 17-4 PH, and 400-series. Not for aluminum, carbon steel, or copper alloys.
Black Oxide: Low-Cost Corrosion Resistance
Black oxide converts the surface of steel to magnetite (Fe3O4). The process is a hot alkaline bath at 285 to 310°F. It adds no measurable dimension — thickness is 0.00002 to 0.00005 inches. Cost: $0.50 to $1.50 per kilogram. Lead time: 2 to 3 business days.
The finish is matte black with mild corrosion resistance. In salt spray, black oxide fails at 24 to 48 hours without a supplemental oil or wax. With oil, it reaches 100 to 200 hours.
Material compatibility: Carbon steel, alloy steel, stainless steel (with modified bath), and some cast irons. Not for aluminum, copper, or brass.
Chemical Conversion Coating (Alodine/Chem Film)
For aluminum, chem film (MIL-DTL-5541) provides corrosion resistance and electrical conductivity. It adds no significant dimension — thickness is 0.00002 to 0.0001 inches. Cost: $1.00 to $2.00 per kilogram. Lead time: 2 to 3 business days.
Surface roughness is unchanged. The finish is a thin, iridescent gold or clear layer. It is the preferred finish for aluminum parts that require electrical bonding or will be painted.
Material compatibility: Aluminum and some magnesium alloys.
Vapor Blasting: The Hybrid
Vapor blasting combines fine abrasive media with water at low pressure, typically 30 to 50 psi. It produces a uniform, low-reflectivity satin finish with 16 to 32 Ra. Unlike dry blasting, it does not embed media particles in the surface.
Cost: $2.00 to $4.00 per kilogram. Lead time: 3 to 5 business days. Material removal is minimal — 0.0001 to 0.0003 inches per pass.
Material compatibility: All metals. Excellent for titanium and stainless steel where surface integrity is critical.
Decision Matrix for Engineers
When sending parts to a CNC shop, use this priority list:
- Material: Aluminum, steel, stainless, or copper alloy? That eliminates half the options immediately.
- Corrosion environment: Indoor dry, outdoor, salt spray, chemical immersion?
- Tolerance requirement: Can you afford 0.001 inch growth per side? If not, stick with chem film, passivation, or black oxide.
- Wear requirement: Hard anodize or electroless nickel for sliding contact. Polish for low friction.
- Budget: Under $1/kg? Blast or passivate. Under $5/kg? Anodize or EN. Over $5/kg? Polish or custom hard coat.
At Andas Precision, our Guangdong factory operates under ISO 9001:2015 certification with in-house finishing lines for anodizing, EN plating, blasting, and polishing. Every incoming order runs through our automated project review, which flags tolerance conflicts with your specified finish before a single chip is cut. This eliminates the most common cause of finishing rework: dimensional interference.
Send us a part print with your surface finish callout. We will return a process sheet with pre-finish dimensions, cost per kilogram, and lead time — all verified against your material and tolerance stack.