Surface Finishing Guide
Choosing the right finish for your CNC machined prototype affects more than just appearance. It changes corrosion resistance, wear behaviour, dimensional tolerances, and how well mating parts slide together. At OpusFab, we run every finish in-house on our direct-factory 3-axis and 5-axis CNC equipment. This guide covers the four finishes we apply most often to aluminum prototypes and other machined parts.
Photo: As-Machined finish
As-Machined
As-machined means the part comes off the CNC mill exactly as the cutter leaves it. No secondary blasting, coating, or chemical treatment. You will see faint tool marks, slight surface variation between facing passes, and the natural colour of the raw stock, whether that is 6061-T6 aluminum, 7075, or an engineering plastic like PEEK.
For functional prototypes, test fixtures, and tight-tolerance components, this is often the smartest choice because it preserves the precise dimensions programmed into the toolpath and avoids the thickness variation that coatings introduce. It is also the fastest and most economical route through the shop.
- • Tightest tolerances: no added coating layer
- • Fastest turnaround: one-and-done machining
- • Visible tool marks are normal and expected
- • Immediate fit testing for holes and threads
- • Works on aluminum, steel, brass, and plastics
Photo: Bead Blast finish
Bead Blast
Bead blasting bombards the part with fine glass beads under controlled air pressure, producing a smooth, matte, uniform surface that hides minor machining marks without adding measurable thickness. The result is a soft satin grey on aluminum prototypes, or a muted tone on steel and titanium.
Because the process is purely mechanical, it does not alter the underlying alloy chemistry or risk seizing threaded holes the way aggressive chemical etches can. This finish is popular for consumer electronics enclosures, handheld medical devices, and any prototype where you want a clean, modern look without the gloss of paint or the colour commitment of anodizing.
- • Uniform matte texture across all faces
- • Minimal dimensional change (less than 0.025 mm)
- • Excellent prep layer for paint or adhesive
- • Fingerprint and smudge resistant
- • Compatible with aluminum, steel, and titanium
Photo: Anodized finish
Anodized (Type II)
Type II sulfuric acid anodizing builds a hard, porous oxide layer on aluminum that can be dyed black, clear, blue, red, gold, or other standard colours. The coating is electrically non-conductive, increases surface hardness, and provides genuine corrosion resistance for parts that will see moisture, salt spray, or repeated handling.
For CNC machining prototypes that need to look like production units—camera housings, drone chassis, automotive brackets—Type II anodizing is the most requested finish in our shop. Because the oxide layer grows partially into the base metal and partially outward, you should expect a small shift in dimensions on tight features. We adjust our toolpaths accordingly when anodizing is specified upfront.
- • Hard, wear-resistant oxide skin
- • Corrosion and oxidation protection
- • Dyed colours: black, clear, blue, red, gold
- • Electrically insulating layer
- • Best on 6061 and 7075 aluminum alloys
Photo: Powder Coat finish
Powder Coat
Powder coat sprays a dry thermoplastic or thermoset powder onto the part, then bakes it in an oven to form a tough, even skin that is typically 0.05 mm to 0.15 mm thick. The finish is far more durable than liquid paint, resists chipping and scratching, and is available in hundreds of colours and textures from smooth gloss to wrinkle and texture.
For machined prototypes that will spend time on a factory floor, outdoors, or in the hands of investors at a trade show, powder coat gives a production-grade appearance with serious protection. The added thickness does mean you need to design for it. We usually machine features slightly undersize when powder coat is planned, especially for press fits, threaded holes, and bearing bores.
- • Thick, durable barrier against impact
- • Wide colour and texture range
- • Good chemical and solvent resistance
- • Plan for roughly 0.1 mm build-up per surface
- • Ideal for prototype runs and small batches
Quick Comparison
As-Machined
Fastest. No cost adder. Raw metal only. Best for fit checks.
Bead Blast
+1 day. Low cost. Grey matte. Good for cosmetic prototypes.
Anodized (Type II)
+2–3 days. Medium cost. Black, clear, colours. Best on aluminum.
Powder Coat
+2–3 days. Medium cost. Wide colour range. Best for rugged parts.
Questions we get asked
Which finish is best for a functional prototype that just needs to fit?
As-machined is usually the right call. You get the part faster, you avoid coating thickness, and you can verify your CNC machining tolerances immediately.
Will bead blasting change my hole diameters?
Not in any practical sense. Bead blast removes less than 0.025 mm of material, so standard tolerances are unaffected.
Can you anodize steel or titanium parts?
Type II sulfuric anodizing is specific to aluminum. Titanium can be anodized in a different electrolytic process that produces interference colours; steel is typically plated or painted instead.
How do I account for powder coat in my CAD model?
For tight fits, design the raw machined part roughly 0.1 mm undersize per surface. If you prefer, send us the nominal model and we will apply the offset in our CAM software.
Ready to quote your part?
Upload your STEP file and select your finish. We will include the finishing cost in your quote up front.
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