CNC Surface Finishes

Start with as-machined and bead blast

Every part starts as-machined. The surface keeps the lay left by the cutting, the predominant pattern that the production method sets (source 1), so tool marks show. For internal parts, functional brackets, and prototypes where looks don’t matter, bare as-machined metal is often the right answer, and a hidden part has nothing to show off.

Bead blast is a callout a print can use for a cosmetic baseline. Blasting is used to create a uniform surface finish on machined parts (source 2), and one trade article says it can make minor machining lines less noticeable (source 3). The same article says surface preparation has a direct influence on the final anodized appearance (source 3). If tool marks should be less noticeable on a cosmetic or consumer part, the print can name the blast, and on aluminum the anodize after it.

Anodize, powder coat, and passivation

Anodize is quoted on aluminum: an oxide layer grown into and out from the surface (source 15). Type II is the moderate layer, 0.00007 to 0.001 in (1.8 to 25 μm), and it takes dye in colors such as blue, black and red; Type III hardcoat is thicker, and thickness adds wear and corrosion resistance (source 4). The depth is on the dedicated anodizing page.

Powder coat is a dry powder applied electrostatically and heat-cured into a uniform film, thicker than a liquid coating can build without running or sagging (source 5). It is quoted on steel, stainless, and aluminum, covered on the powder coating page. Passivation is the stainless callout: a chemical treatment that removes exogenous iron, cleans the part and creates or restores the passive oxide layer (source 7), covered by ASTM A967/A967M-25 (source 6). Name passivation per ASTM A967 on the print; name citric or nitric only if your spec requires one. See the passivation page for the spec detail.

Cosmetic and EDC looks

When a part is meant to be seen and handled, the print can call out a look. Brushed metal is a unidirectional satin finish with fine lines parallel to the brushing direction (source 8), and polishing makes a smooth, shiny surface (source 9). A print may also name a stonewashed or tumbled look; tumbling runs parts in a barrel with media to deburr, radius and polish small parts (source 10). A print for an EDC piece or a titanium part may name one of these looks.

Plating and conversion coatings are other finishes you will see called out on metal parts. Black oxide is a conversion coating for ferrous metals that adds mild corrosion resistance and a dark look, with a layer about 1 μm thick (source 11). Plated coatings such as nickel can improve corrosion resistance (source 12). Chromate conversion coatings are used where a coating has to preserve electrical conductivity (source 13), which an anodic layer does not, because it is insulative (source 4), so they come up on aluminum that must stay grounded. For cosmetic enclosures and covers, a bead blast followed by anodize is one way to write the callout.

How to choose, and why it goes on the print

Pick the finish by what the part has to do, not by the longest menu. Ask three things: does it need to look finished, does it need corrosion or wear protection, and does it need to stay conductive or grounded. A hidden internal part can stay as-machined; a handled cosmetic part may call for bead blast or tumbling; a wear surface on aluminum may call for Type III; a stainless part may call for passivation.

The reason it matters at quote time: most finishes add to or change thickness, threads, bores, and conductivity, so a press fit or a tapped hole can shift if the coating isn’t planned for. The Aluminum Anodizers Council’s rule of thumb is 2/3 into the surface and 1/3 out for Type II, and half in, half out for hardcoat (source 15), and powder film build varies with the shape of the part, from ±0.5 mil on flat or simple surfaces to as much as 2.5 mils either way on a very complex one (source 14). That’s why the finish goes on the print, and why critical features get masked: sections of an anodized part can be masked, and holes and bores usually without much difficulty (source 15). A finish called out on the print is quoted with the part. Parts come back finished, and every part is inspected against your print before it ships.

Sources

All accessed 4 October 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Surface finish": en.wikipedia.org/wiki/Surface_finish (lay is the direction of the predominant surface pattern, ordinarily determined by the production method; in the United States surface finish is usually specified using ASME Y14.36M).

2. Wikipedia, "Bead blasting": en.wikipedia.org/wiki/Bead_blasting (used in creating a uniform surface finish on machined parts).

3. ALCircle, Aranya Mondal, "Bead Blasting: The Finishing Process Behind High-Quality Aluminium Products", 15 June 2026: alcircle.com/news/bead-blasting-the-finishing-process-behind-high-quality-aluminium-products-119908 (bead blasting can make minor machining lines less noticeable and gives a more uniform texture; the surface preparation stage has a direct influence on the final appearance of anodized parts).

4. Wikipedia, "Anodizing": en.wikipedia.org/wiki/Anodizing (Type II 1.8 μm to 25 μm and Type III thicker than that; porous coatings absorb dyes, with colors including yellow, green, blue, black, orange, purple and red; thickness increases wear and corrosion resistance; the anodic layer is insulative).

5. Wikipedia, "Powder coating": en.wikipedia.org/wiki/Powder_coating (powder applied electrostatically and cured under heat; the object is heated and the powder melts into a uniform film; thicker coatings than conventional liquid coatings without running or sagging; orange-peel texture helps hide metal defects and is less prone to showing fingerprints).

6. ASTM International, “A967/A967M-25, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts”: store.astm.org/a0967_a0967m-25.html (immersion in nitric acid solution, immersion in citric acid solution, and electrochemical treatment; tests intended to confirm the effectiveness of passivation, particularly the removal of contaminant iron and other exogenous matter).

7. Wikipedia, "Passivation (chemistry)": en.wikipedia.org/wiki/Passivation_(chemistry) (removes exogenous iron and creates or restores a passive oxide layer; ASTM A 967 and AMS 2700 are the most prevalent standards).

8. Wikipedia, "Brushed metal": en.wikipedia.org/wiki/Brushed_metal (metal with a unidirectional satin finish, a pattern of very fine lines parallel to the brushing direction).

9. Wikipedia, "Polishing": en.wikipedia.org/wiki/Polishing (the process of creating a smooth and shiny surface by rubbing it or by applying a chemical treatment).

10. Wikipedia, "Tumble finishing": en.wikipedia.org/wiki/Tumble_finishing (a barrel filled with the parts and media; used to deburr, radius, polish and brighten, on relatively small parts).

11. Wikipedia, "Black oxide": en.wikipedia.org/wiki/Black_oxide (a conversion coating for ferrous materials, stainless steel, copper and copper-based alloys, zinc, powdered metals and silver solder; adds mild corrosion resistance, appearance and minimal light reflection; no significant dimensional impact, a layer about 1 μm thick).

12. Wikipedia, "Electroplating": en.wikipedia.org/wiki/Electroplating (coatings such as nickel can improve corrosion resistance).

13. Wikipedia, "Chromate conversion coating": en.wikipedia.org/wiki/Chromate_conversion_coating (the coating serves as a corrosion inhibitor, a primer, a decorative finish, or to preserve electrical conductivity; trade names include Iridite and Bonderite, formerly Alodine).

14. Rodger Talbert, "Film Build Control," Products Finishing, 30 January 2014: pfonline.com/articles/film-build-control (film build can be held to ±0.5 mil on flat or simple surfaces, varies ±1 mil on a medium-complexity shape and by as much as 2.5 mils either way on a very complex part).

15. Aluminum Anodizers Council, "A Conclusive FAQ Guide for Anodized Aluminum": anodizing.org/anodized-aluminum-faq/, no date shown (2/3 in and 1/3 out for Type II and half in, half out for hard coat; sections of a part can be masked; holes and bores can usually be masked without too much difficulty, flat areas can sometimes be more difficult).