Surface Finish Specification for Metal Parts: What Buyers Should Put in an RFQ

A useful finish specification links the substrate, service environment and functional surfaces to a standard, appearance criteria, masking map, thickness, inspection, documents and packaging. Missing inputs can create different quotation assumptions under the same finish name.

Black finish or corrosion resistant alone is not an acceptance criterion.

1. Function and environment

State whether the finish supports corrosion protection, electrical contact, sealing, friction control, glare reduction or appearance. Describe exposure, temperature, chemicals, humidity, abrasion, cleaning agents and mating materials. If a laboratory test is required, name its method, specimen condition, acceptance threshold and report. Do not treat test duration as a proven service life.

2. Substrate and incoming condition

Preparation can differ for steel, stainless steel, aluminum, zinc alloy and cast surfaces. Specify grade, heat-treatment condition where relevant, and whether parts arrive cast, forged, machined, welded, polished or blasted. ASTM B633 concerns electrodeposited zinc on iron and steel, including preparation and acceptance topics such as thickness, adhesion, corrosion resistance and hydrogen embrittlement. It is not a general specification for every substrate or zinc-containing finish.

3. Standard, type and class

Include the standard designation and edition, coating type, class, color and supplementary treatment. Give the controlled revision of customer specifications. For acceptable equivalents, define the approval authority and evidence needed. Anodized, plated, passivated, powder coated and painted name process families; they do not fully define preparation, thickness, color tolerance, gloss or sealing. Ask suppliers to identify their assumptions.

4. Map appearance and no-coat zones

Mark threads, contacts, bearing seats, sealing faces, press fits, grounding points and weld areas that need masking or controlled buildup. State whether dimensions apply before or after finishing. Name cosmetic viewing zones, permitted rack marks and unacceptable defects. Use agreed viewing conditions, controlled color references and approved limit samples when practical. Words such as perfect or uniform leave room for different interpretations.

5. Thickness and measurement locations

Specify a minimum, maximum or range and the locations to be measured. Recesses, edges and internal features may not be represented by one unspecified reading. Agree the method, sampling frequency and treatment of inaccessible areas. Coordinate buildup with final tolerances for bores, threads and sealing faces. The custom metal parts RFQ checklist covers the wider drawing and inspection package.

6. Separate texture from appearance

Surface texture is geometrical, not a synonym for color or gloss. ISO 21920-1:2021 covers profile-texture indication in technical documentation; ISO 21920-2:2021 defines profile-method terms and parameters. Follow the buyer's adopted convention and edition. NIST describes stylus and optical profiling among measurement methods. For critical texture, agree the parameter, filter or cutoff rules, direction, method and location.

7. Define suitable verification

Specify each required check and acceptance value: appearance, color, thickness, adhesion, texture, corrosion exposure, final dimensions or documents. Not every test suits every finish. ASTM D3359 covers tape-test methods for rating adhesion of relatively ductile coating films on metallic substrates. It indicates whether a minimum level was met, not an absolute bond force. If selected, state the method, rating and sampling requirement.

8. Evidence and drawing revision

List shipment documents: certificate of conformance, coating certificate, lot reference, thickness results, test report or first-article evidence. Define whether they apply to the first lot, every lot or an approved sample. Reference the released drawing and purchase order. The drawing revision control guide explains why supplier, manufacturing and quality teams need one governing revision.

9. Protect surfaces through delivery

Part-to-part contact, trapped moisture, incompatible packaging and handling can damage an acceptable finish. Define cleanliness, drying, separation, wrapping, labeling and pack quantity, plus corrosion protection when needed. Evaluate packaging marks against appearance criteria. Threads, sealing areas and electrical contacts may require plugs, caps or separators.

10. Resolve quotation assumptions

Ask the supplier to flag material compatibility, access, drainage, rack points, masking, dimensional buildup, test capability, subcontracted processes and packaging risks. Record exceptions in the quotation. Where finishing affects critical dimensions, connect them to a ballooned drawing and reporting plan. The first article inspection guide explains how reports should map to the released definition.

Surface-finish RFQ checklist

Released drawing, revision, model and document precedence.
Material grade, condition and incoming surface.
Standard, edition, type, class, color and treatment.
Service environment and functional purpose.
Cosmetic zones, reference sample and viewing conditions.
Masking and no-coat, threaded, contact and sealing areas.
Thickness limits and measurement locations.
Texture parameters and measurement method where required.
Inspection methods, limits, sampling and reports.
Packaging, labeling and delivery destination.
Written DFM exceptions and quotation assumptions.

Frequently asked questions

Is a finish name enough for quotation?

No. It can hide different preparation, thickness, color, sealing, testing and masking assumptions. Include the governing standard, classification and acceptance details.

Should thickness be measured everywhere?

Not automatically. Define representative and function-critical locations. Confirm that the method suits the substrate, geometry and finish.

How should cosmetic quality be specified?

Define viewing zones, defects, viewing conditions and color or gloss references. Use an approved limit sample when possible.

Does salt-spray duration prove service life?

No. It can serve an agreed comparison or acceptance purpose, but cannot directly predict field life without application-specific evidence.

When should finish enter first-article inspection?

Include finish-related dimensions, appearance, thickness, adhesion, texture or documents when they form part of the released acceptance criteria. Specify the evidence in the RFQ.

Request a specification review

Email the drawing, model, material, finish standard, quantities, environment, critical surfaces and documentation requirements to info@castleintl.com. Castle Mechanical can review open assumptions before quotation. Final finish selection and acceptance remain project-specific.

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