# Choosing a surface finish

Select finishing by function, substrate, dimensional change and inspection rather than appearance alone.

Reviewed: 2026-11-02

## Define what the surface must do

In the invented shaft example, masking a **19.980–20.000 mm journal** for a **20.020–20.040 mm bore** preserves **0.020–0.060 mm diametral clearance**. A uniform **0.060–0.080 mm radial powder film** adds **0.120–0.160 mm** to diameter and causes **at least 0.060 mm interference**.

Keep coating and masking residue off datum face A and the full **30 mm journal**. Start the cosmetic coating at **31.0 ±0.5 mm from A**. Verify fit after cure, cooling and mask removal. Use the [powder-coating guide](finishing/powder-coating.md) to specify the coating system and cure evidence.

## Compare processes by their intended change

State the finish's function, such as corrosion protection, wear resistance or preparation for another treatment. Then identify which surfaces and dimensions it may change.

| Process | Main distinction | Selection question |
| --- | --- | --- |
| [Stainless passivation](finishing/stainless-steel-passivation.md) | Removes contaminant iron from an appropriately prepared stainless surface | Is contamination the issue, or are scale and heat tint still present? |
| [Electropolishing](finishing/electropolishing.md) | Removes metal electrochemically and can smooth fine surface peaks | How much removal and edge change can the part tolerate? |
| [Powder coating](finishing/powder-coating.md) | Adds a cured polymer-based coating system | Are substrate preparation, cure and service exposure specified? |
| [Hard anodizing](finishing/hard-anodizing.md) | Produces an anodic surface on suitable aluminum | What alloy, thickness, dimensional and functional requirements apply? |
| [Abrasive blasting](finishing/abrasive-blasting.md) | Changes the surface mechanically and can prepare it for later treatment | What cleanliness, profile and media-contamination limits apply? |

[Carpenter's passivation guidance](https://www.carpentertechnology.com/blog/passivating-stainless-steel-part) addresses embedded iron and preparation. [Euro Inox's electropolishing guide, first edition 2010](https://worldstainless.org/wp-content/uploads/2025/02/Electropolishing_EN.pdf) explains electrochemical removal. They describe different operations; passivation should not be specified as though it removes machining marks or heavy scale.

## Example: a stainless bracket after welding

For an illustrative welded bracket, first identify the alloy, weld condition, heat tint or scale, and required service environment. If contamination removal is required, passivation may belong in the route after appropriate preparation. If smoothing and controlled metal removal are required, electropolishing is a separate candidate with dimensional consequences.

Neither operation changes an unsuitable bulk alloy into a suitable one. Start with [material selection](../materials/grades-tempers-and-conditions.md), then define preparation and final surface acceptance. [Weld porosity](../production/process-effects/porosity-in-welds.md) is a different defect problem; a cosmetic finishing step does not resolve an unknown weld condition.

## Protect functional dimensions and interfaces

Identify threads, fits, sealing surfaces, electrical contacts and datum surfaces before processing. Agree which surfaces receive treatment, which are masked, and which dimensions are accepted before or after finishing. Material-removing and material-adding treatments need different allowance decisions.

## Worked example: a masked shaft journal

Assume an **invented drawing**, with all dimensions in millimetres. A shaft has a 30 mm-long functional journal, measured from datum face A, with a final diameter of **19.980–20.000 mm**. Its mating bore is **20.020–20.040 mm**. Powder coating is required only on the adjacent cosmetic body; the fictional coating specification calls for **0.060–0.080 mm** cured film. These numbers illustrate the calculation, not a recommended fit or powder system.

| Delivered condition | Calculation | Result |
| --- | --- | --- |
| Journal correctly masked | Smallest bore minus largest shaft: 20.020 − 20.000 | **0.020 mm minimum diametral clearance** |
| Journal correctly masked | Largest bore minus smallest shaft: 20.040 − 19.980 | **0.060 mm maximum diametral clearance** |
| Journal coated instead | Added diameter = twice radial film: 2 × 0.060 to 2 × 0.080 | **0.120–0.160 mm diameter growth** |
| Coated journal, best-case fit | Smallest coated shaft 19.980 + 0.120 = 20.100; largest bore 20.040 | **0.060 mm interference**, even in this best case |

Verify coating variation, roughness and mask-edge buildup on the actual part. Follow the drawing's uncoated-surface requirement at any film thickness.

### State the masking boundary on the drawing

For this example, the drawing requires **no coating or masking residue on the journal over its full 30 mm length or on datum face A**. The cosmetic coating begins at axial position **31.0 ±0.5 mm from A**, leaving a nominal 1 mm uncoated band beyond the journal. Even the nearest permitted boundary, 30.5 mm, remains outside the 30 mm engagement. Whether that exposed band is acceptable for corrosion and appearance must be agreed for the actual application.

Protect the journal during blasting and chemical preparation as well as spraying. Identify permitted rack-contact locations away from functional surfaces. If the transition would lie inside a bearing, seal or insertion path, revise the boundary with the designer rather than leaving the applicator to decide.

### Inspect the final condition

Measure the journal and bore after the complete preparation, cure, cooling and mask-removal sequence under the specified inspection conditions. Check the transition for coating ridges and residue. Measure cured film on the designated coated surfaces, and record the part-temperature cure evidence required by the selected powder's data sheet. An oven setting alone does not establish the part's thermal history.

The [powder-coating guide](finishing/powder-coating.md) explains preparation, part-temperature cure and thickness measurement. Use [outside micrometers](../inspection/instruments/outside-micrometers.md) for the external-diameter measurement considerations. The arithmetic above does not by itself define a complete production sampling plan or fit specification.

## Make the finish inspectable

| Requirement | Evidence to define |
| --- | --- |
| Appearance | Agreed sample or explicit color, gloss and defect criteria |
| Texture | Parameter, measurement method, location and direction |
| Coating thickness | Required range, significant surfaces and measurement method |
| Cleanliness or passivity | Applicable method, sampling and acceptance criteria |
| Final geometry | Dimensions and functional checks after the relevant treatment |

Use [surface profilometry](../inspection/methods/surface-profilometry.md) for texture and [gloss measurement](../inspection/methods/gloss-measurement.md) for optical appearance. They are not substitutes for one another. The [powder-coating guide](finishing/powder-coating.md) connects coating chemistry, part-temperature cure and inspection; a color name alone cannot specify the complete system.

## Include finishing in the route and cost

Confirm preparation, masking, processing, final inspection and packaging. For an outsourced service, define its lot and minimum charge in the [cost estimate](../costing/cost-structure-and-cost-bases.md). Use [route comparisons](../production/alternative-processes-and-route-tradeoffs.md) to check whether processing before or after assembly changes access and acceptance.

Reviewed November 2, 2026 PST.