CNC Part Finishes: Anodising, Plating and Powder Coating

Choose the finish around what the part must do

Start with the material, exposure, mating surfaces and appearance requirements. Specify the function of each important surface, including any sliding surfaces or electrical contacts.

For an individual buying a replacement part, provide the mating component and photographs where available. Small businesses and larger procurement teams should also supply controlled drawings, acceptance criteria and any customer requirements.

Confirm the exact substrate before choosing a process. Use the CNC materials guide to support material selection, and confirm the proposed finish is suitable for the selected alloy or resin.

Anodising, plating and powder coating: what changes?

Anodising forms an oxide layer on aluminium. Specify whether the requirement is decorative and protective anodising or hard anodising for engineering use. ISO's decorative and protective anodising specification addresses alloy suitability and pretreatment; its scope excludes hard anodic coatings used mainly for abrasion and wear resistance.

Metal plating deposits a metallic coating. Name the complete coating system, including undercoats and supplementary treatments, rather than requesting only 'silver finish' or 'chrome'. Distinguish electroplating from electroless deposition: ISO publishes a separate specification for autocatalytic nickel-phosphorus alloy coatings.

Powder coating provides an organic coating. Specify substrate preparation, exposure conditions, colour, texture and performance requirements together. ISO's aluminium powder-coating specification relates performance requirements to the application and environment.

Specify mechanical polishing, chemical passivation and electropolishing separately. Abrasive mechanical polishing removes material to alter the surface finish. ISO describes electropolishing as a process for smoothing and passivating stainless steel; it should not be specified as a deposited decorative coating.

Specify coating thickness and tolerances after finishing

State on the drawing which dimensions apply after the complete finishing sequence. Identify bearing seats, bores, shafts, threads, sealing faces and other mating features. Require final dimensional inspection after finishing for the features that determine fit.

A deposited coating on a shaft increases its outside size, while coating inside a bore reduces the opening. Anodising also converts substrate into oxide, so total oxide thickness must not be treated as an identical amount of outward growth.

Require the supplier to propose machining allowances for the selected process and alloy. NASA's anodising specification recommends defining both final and machining dimensions for the hard-anodised geometries discussed in its design guidance. Treat this as an engineering reference, rather than a universal commercial requirement.

Agree the permitted thickness range, measurement locations and measurement method. Specify local requirements for recesses and holes: NASA's hard-anodising guidance explains that internal and external coating thicknesses can differ. If the proposed finish cannot hold the required fit, revise the process or masking plan before production.

Approve colour, gloss and texture on a finished sample

Use a physical reference sample and agree acceptable variation and viewing conditions. Record any colour reference, but do not make photographs or screen images the sole acceptance standard.

For anodising, approve a sample made from the intended alloy with the intended pretreatment. NASA's guidance notes that hard-anodised colour varies with alloy composition. Approve the appearance of the actual proposed process rather than assuming it will match a different anodised finish.

For powder coating, record colour, gloss, texture, coating product and the intended indoor or outdoor environment. For plated parts, record whether the target is bright, satin or another agreed appearance.

If parts must match across an assembly or repeat orders, ask the supplier to explain batch control and retain labelled reference samples. Make prior approval of changes to the alloy, preparation, finishing subcontractor or coating system a purchase requirement.

Check plating chemistry: RoHS and REACH are different questions

Ask for the coating stack and the chemistry of any passivation, conversion coating, sealer or topcoat. Do not infer substance compliance from appearance or the word 'chrome'.

Great Britain example, official guidance checked on 18 October 2026: RoHS restricts hazardous substances, including hexavalent chromium, in covered electrical and electronic equipment, cables and spare parts, subject to applicable exclusions and exemptions. It is not a blanket rule applying to every machined metal part.

EU REACH information checked on 18 October 2026: ECHA lists chromium trioxide on the Authorisation List and separately explains obligations concerning Candidate List substances in articles. Assess process-chemical authorisation and finished-article obligations separately; one does not establish compliance with the other.

Request declarations that identify the supplied part, material, finish and applicable requirements. Where necessary, agree testing with a competent laboratory for the relevant substance and coating. Require evidence for the specified system rather than relying solely on a generic supplier certificate or a claim of 'trivalent' treatment.

Get a finishing schedule, not an assumed turnaround

Ask for separate milestones for machining, sample finishing, approval, production finishing, inspection and packing. Confirm whether finishing is subcontracted and who controls the drawing revision and approved sample.

Have the supplier identify any scheduling constraints involving colour approval, masking, coating availability, batch scheduling, testing and required post-treatment. Obtain an order-specific commitment.

Agree what happens if a batch fails colour, thickness or fit checks. Before approving stripping and refinishing, require an assessment of the proposed rework's effects on the substrate, dimensions and acceptance requirements.

For quote comparisons, use the CNC machining cost drivers guide. Compare the same finish, masking, inspection and packing requirements.

If you sell in the US or EU

US, official guidance checked on 18 October 2026: CBP states that the importer of record remains responsible for entry-document accuracy and applicable duties, taxes and fees even when using a broker. Give the broker the part's material, function and finish. USTR publishes China Section 301 tariff actions and exclusion information; ask the broker to check classification, origin, entry date and all applicable tariff measures before pricing an order.

EU, official guidance checked on 18 October 2026: CE marking applies only where the relevant EU product legislation requires it. Assess the product and intended use rather than deciding from its surface finish. ECHA explains that Candidate List substances in articles can trigger information and notification obligations under specified conditions; obtain composition information for the finished part.

UK import valuation: account for finishing

HMRC guidance checked on 18 October 2026: where Method 1 applies, customs valuation starts from the price paid or payable for goods sold for export to the UK, with applicable additions and exclusions. For finished components, give the customs agent the full commercial arrangement, including separately invoiced finishing, so the agent can assess the complete imported goods rather than only the machining charge.

HMRC's import VAT guidance starts with the customs value and requires applicable import duties and charges, plus relevant incidental expenses to the first UK destination and any further UK destination known at importation. Give the agent a breakdown of packing, transport and insurance costs, identifying amounts already included. Have the agent calculate the value for the actual transaction without double counting; no universal duty or VAT rate is assumed here.

What to send before requesting a finished-part quote

Send the drawing revision, exact material, quantity, intended use and destination market. Add the finish system, thickness requirements where applicable, final tolerances, masking drawing, cosmetic reference, test requirements and packing instructions.

Ask for a finished sample and evidence of the agreed dimensional and coating checks before approving production. Define sample approval and production-batch acceptance separately.

Agree the supplier's deliverables, inspection records and acceptance responsibilities before commissioning work.

Sources

The official sources below support the process descriptions and regulatory guidance. Check each standard's scope before specifying it. A reference to a standard is not evidence that a supplier or delivered batch complies. Regulatory guidance was checked on 18 October 2026; confirm the requirements applicable when placing the order and importing the goods.