Deciding whether CNC parts need secondary processes like anodizing or powder coating affects both cost and performance. With decades of experience manufacturing precision components for aerospace, audio, and medical sectors, we’ve learned that the right finishing choice can enhance function and appearance—while unnecessary processes waste time and budget.
Every secondary process should solve a specific engineering problem, not just improve aesthetics. Some aluminum parts work perfectly as-machined with standard tool finishes, while others require anodizing for corrosion resistance or powder coating for electrical isolation. The key is understanding when these processes add real value versus unnecessary cost.
Explore available secondary processes, real costs and lead times, and key design tips—plus how to avoid spec mistakes that delay your project.
Table of Contents
What secondary processes are available for CNC machined parts?
For aluminum parts: anodizing (corrosion + electrical isolation), powder coating (color + durability), or bead blasting (appearance only). For steel: electroplating or passivation. Raw machined aluminum works for internal components, but visible parts usually need at least bead blasting for professional appearance.
The real question isn’t what’s available—it’s what you actually need. Raw machined aluminum with tool marks looks industrial but works fine for internal brackets or hidden components. If your part will be visible or handled, bead blasting removes tool marks and creates a uniform matte finish for $3-8 per part without adding thickness or complexity.
Anodizing makes sense when your aluminum part faces moisture, needs electrical isolation, or requires moderate corrosion protection—like electronics housings or outdoor equipment. Type II anodizing adds 10-25 μm thickness and feels slightly different than raw aluminum. For indoor electronics that won’t see moisture, it’s often overkill.
Powder coating works when you need specific colors, heavy impact resistance, or maximum corrosion protection. It’s thicker (50-100 μm), so sharp edges become slightly rounded and holes may need masking. Think appliance housings or outdoor equipment where durability matters more than precision.
Steel almost always needs something—raw steel rusts quickly. Electroplating or passivation prevents corrosion depending on the application.
Design Takeaway: Start with the minimum: can you use raw machined? If it’s visible, add bead blasting. If it faces moisture or needs isolation, choose anodizing. Only use powder coating when you need colors or heavy-duty protection.
When can CNC parts be left as-machined without finishing?
As-machined aluminum parts work for internal components, hidden brackets, and applications where corrosion isn’t a concern. Raw machined surfaces typically show Ra 1.6-6.3 μm tool marks but provide adequate function for non-cosmetic parts. Steel components almost always need finishing due to rapid oxidation.
We regularly deliver as-machined aluminum parts for internal electronics brackets, mounting hardware, and prototype testing where appearance doesn’t matter. Tool marks from face milling create directional patterns but don’t affect structural performance. 6061-T6 aluminum naturally forms a thin oxide layer that provides basic corrosion resistance indoors.
Raw machined surfaces work well when parts won’t be handled frequently or seen by end users. Think internal chassis components, hidden fastener plates, or test fixtures. The surface feels slightly rough compared to finished parts but machines to precise tolerances without additional processing time or cost.
However, exposed aluminum in humid environments will eventually show water staining or discoloration. Raw steel oxidizes within days unless kept in controlled environments. Any part requiring electrical grounding may need finishing to ensure reliable contact surfaces.
Design Takeaway: Save money by leaving internal, non-critical parts as-machined. Use raw aluminum for prototypes, hidden brackets, or test components. Always finish parts that will be handled, exposed to moisture, or require electrical conductivity.
How much will common finishes actually cost per part?
Basic finishing costs $5-12 for typical aluminum parts: bead blasting ($3-8), anodizing ($5-15), powder coating ($8-25). These prices include setup costs for orders over 25 pieces. Add $50-100 shipping each direction if using external finishers.
For project budgeting, a typical aluminum housing costs $8-10 to anodize including setup. So 50 housings run about $400-500 plus $75 shipping, totaling around $500. Small brackets cost less ($5-6 each), while large faceplates cost more ($15-20 each) due to tank space requirements.
Setup fees hit small orders hardest—under 25 pieces often carry $50-150 in setup costs that get spread across fewer parts. This pushes per-piece costs up 50-100% for prototype quantities. At 100+ pieces, per-piece costs drop 20-30% as setup gets amortized efficiently.
Hidden costs that surprise engineers:
- Threaded holes needing masking add $1-2 per hole
- Custom colors add $2-4 for anodizing, $3-5 for powder coating
- Shipping to external finishers costs $50-100 round trip
- Rush processing under 5 days adds 50% surcharge
You can add finishing later if you start as-machined, but expect 20-30% higher costs due to extra handling and separate shipping. For budget planning, use $10 per aluminum housing plus shipping. Get actual quotes for orders over $500, custom colors, or complex masking requirements.
Design Takeaway: Budget $10 per aluminum housing plus $75 shipping for project planning. These estimates work for standard anodizing with basic masking. Group parts by finish type and get quotes early for custom requirements to avoid budget surprises.
How long do secondary processes add to CNC part lead times?
Plan 7-10 business days total for most secondary processes: anodizing (3-7 days), powder coating (5-10 days), bead blasting (1-3 days), plus 2-3 days shipping each direction. Rush processing available for 50% surcharge but still requires minimum 2-3 days.
Anodizing dominates project timelines due to multiple chemical preparation steps and quality control checkpoints. Standard black or clear finishes in 3-5 days, while custom colors need 5-7 days for color matching and approval. We batch parts by color and alloy type, so timing depends on when your color runs.
Typical process times:
- Bead blasting: 1-3 days (fastest option)
- Anodizing standard colors: 3-5 days
- Anodizing custom colors: 5-7 days
- Powder coating standard: 5-8 days
- Powder coating custom: 7-10 days
Powder coating requires longer lead times due to cleaning, spraying, and controlled curing cycles. External finishers add shipping time both directions plus their current queue time. Rush jobs compress timelines but quality suffers when chemistry gets hurried—most finishers need minimum 2-3 days even with surcharges.
Design Takeaway: Budget 10 business days total including shipping when planning schedules. Choose bead blasting for fastest appearance improvement. Group parts by finish type and color to streamline processing and minimize shipment delays.
What part geometries work best with anodizing and powder coating?
Simple shapes with open, accessible surfaces work best. Avoid holes deeper than 3x their width, knife-sharp internal corners, and walls thinner than credit cards. Complex internal features often show uneven coating or color variation.
The finger test works well—if you can’t reach a surface easily, coating chemicals and powder probably can’t either. Deep threaded holes often come out lighter at the bottom while entrances over-anodize and appear darker. Sharp corners on faceplates concentrate electrical current during anodizing and can burn the aluminum, leaving dark spots.
Geometries that cause problems:
- Holes deeper than 3x their diameter
- Sharp internal corners (knife-edge feel)
- Walls thinner than 1.5mm
- Complex intersecting channels
- Deep pockets without drainage
Audio faceplates with standard knob holes anodize beautifully, but deep holes or intersecting internal channels create issues. Thin parts (smartphone-case thickness around 1mm) risk warping from chemical stress during anodizing.
Simple fixes that work:
- Add small drain holes to deep pockets
- Soften sharp corners with 0.5mm fillets
- Make narrow slots slightly wider
- Break complex parts into simpler assemblies
Design Takeaway: Design parts you can easily reach and inspect by hand. Avoid deep narrow features and knife-sharp corners. Include us in design reviews early—small geometry changes often prevent coating problems entirely.
Do secondary processes change part dimensions and tolerances?
Yes, secondary processes add material thickness that affects dimensions and fits. Anodizing adds 10-25 μm total thickness, powder coating adds 50-100 μm, while bead blasting removes 5-15 μm. Always account for these changes when designing press-fits, threaded holes, and tight assemblies.
Anodizing grows parts uniformly by adding half the coating thickness to each surface. Type II anodizing with 20 μm total thickness means your 10mm shaft becomes 10.02mm after coating. This destroys press-fits and bearing installations that were machined for exact clearances.
We’ve seen perfectly machined assemblies become unusable because engineers forgot about anodizing growth. The coating builds up on all surfaces, so holes shrink while shafts expand. Threaded holes often won’t accept standard bolts without forcing, which strips the soft anodized surface.
Powder coating creates larger changes at 50-100 μm thickness. An M6 threaded hole typically won’t accept bolts after coating unless masked. The thick coating also rounds sharp edges and fills small features. Bead blasting works opposite by removing 5-15 μm, actually opening tight fits slightly.
Design Takeaway: Specify critical dimensions as “after finishing” on drawings. Plan to mask threaded holes, press-fits, and precision surfaces. Build dimensional compensation into your original design rather than discovering fit problems after coating.
How to specify surface finishes on CNC part drawings?
Write “Anodize Type II, clear, mask threads” for aluminum parts or “Powder coat RAL 9005, mask holes” for steel. Always specify what features to mask and which surfaces get finished versus as-machined.
The biggest production problems come from missing masking specifications. When you don’t call out thread masking, they get coated and bolts won’t fit. Press-fit holes that accidentally get anodized won’t accept bearings or inserts. We’ve salvaged production runs by machining out coated threads that should have been masked.
Most aluminum housings just need “Anodize Type II, clear, mask M6 threads” written on the drawing. Steel brackets typically get “Powder coat black (RAL 9005), mask mounting holes.” Audio faceplates work well with “Anodize Type II, black, mask all holes.” For prototypes where appearance isn’t critical, “Bead blast, mask threads and press-fits” handles most situations.
Color specification gets tricky with anodizing since “black” varies between batches and shops. RAL numbers work better for powder coating because they’re standardized. For critical color matching, provide a physical sample and write “match sample” on your drawing.
Design Takeaway: Always specify exactly what features need masking, especially threads, press-fits, and electrical contacts. Use RAL numbers for powder coating colors. Specify “exterior surfaces only” to keep internals as-machined and avoid masking complexity.
Can secondary process defects be fixed or reworked?
Anodizing defects can usually be stripped and redone for 60% additional cost. Powder coating typically requires complete removal and reapplication, doubling costs. Minor color variations are often acceptable—evaluate if rework is truly necessary.
The first decision isn’t whether defects can be fixed, but whether they need fixing. Slight color variation between parts often disappears once assembled. Small scratches or texture issues may not matter if parts aren’t highly visible to end users.
Anodizing offers the best rework options since coating strips off chemically without damaging aluminum. We’ve salvaged parts with color problems or dimensional issues by stripping and re-anodizing. This adds 60% to finishing costs and 3-5 days to delivery.
Powder coating rework gets expensive. Wrong colors or coating runs usually require complete chemical stripping and reapplication, doubling your costs and adding 7-10 days. Small bare spots sometimes get touched up locally without full stripping.
Who pays depends on what caused the problem. If we follow your specifications but you don’t like the result, that’s customer cost. If we mess up color matching or forget specified masking, we cover rework. Unclear specifications create disputes.
Design Takeaway: Build potential rework costs and delays into project planning. Get finish samples approved before production runs. Write specific masking and color requirements to avoid responsibility disputes when problems occur.
Conclusion
Secondary processes should solve specific functional requirements—corrosion protection, electrical isolation, or appearance—rather than being automatic additions. Most aluminum parts work well with standard anodizing or bead blasting, while complex geometries may need design adjustments for optimal coating results. Contact us to explore manufacturing solutions tailored to your product requirements.
Frequently Asked Questions
Use RAL color numbers for consistent results—RAL 9005 for black, RAL 9010 for white, etc. Custom color matching adds $3-5 per part and 2-3 days lead time for sample approval. Provide physical color samples when exact matching is critical, and specify acceptable color variation ranges to avoid rework disputes.
6061-T6 is the most commonly used grade due to excellent anodizing consistency and color uniformity. If your part requires higher strength, 7075 anodizes well but costs more and shows slight color variation between batches. 2024 aluminum has poor anodizing characteristics and should be avoided for visible parts requiring consistent appearance.
Always specify “mask threaded holes” clearly on your drawings. Powder coating adds 50-100 μm thickness that makes standard bolts difficult or impossible to install. Alternative solutions include using thread inserts after coating or specifying oversized holes that accommodate coating thickness, but masking remains the most reliable approach.
Bead blasting offers the best appearance improvement at $3-8 per part with minimal setup costs. It removes tool marks and creates uniform matte texture without adding thickness that affects fits. For aluminum parts requiring corrosion protection, Type II clear anodizing provides both function and improved appearance at reasonable cost.
Yes, but expect 20-30% higher costs due to extra handling, cleaning, and separate shipping to finishing vendors. Parts may also require additional surface preparation if they’ve been handled extensively. We recommend deciding on finishing requirements during initial production planning to optimize both cost and lead times.
For most CNC machined parts, ±0.05 mm is achievable with standard anodizing or powder coating processes. Going tighter than ±0.01 mm often requires specialized masking or post-coating machining, which increases cost significantly. We recommend tolerancing only critical features tightly and keeping others at ISO 2768-m levels for cost efficiency.
