Guide
Should a surface finish callout apply before or after coating?
A surface finish callout is the roughness requirement written on a drawing, usually an Ra value placed on a surface texture symbol and pointed at one face. When that face is later anodized, chem-filmed, plated or painted, the callout also has to say which surface it means: the machined metal underneath, or the finished surface that a seal, a mating part or a customer will actually touch.
By Saola Manufacturing engineering team · Published · Updated
This guide covers why a bare roughness value on a coated part is ambiguous, how common finishes change a machined texture, how to put both requirements on the ASME and ISO symbols, and what to write in an RFQ so the machinist, the finisher and the inspector measure the same thing.
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Does a surface finish callout apply before or after coating by default?
Neither ASME Y14.36 nor the ISO surface texture standard settles the question for you, so a roughness value with no stated condition is ambiguous. The machinist will usually read it as a machining requirement, while a receiving inspector who measures the delivered part will read it as a requirement on the coated surface.
ASME describes Y14.36 as the standard for designating surface texture controls, and says plainly that it does not specify how the texture is to be produced or measured (ASME, Y14.36 Surface Texture Symbols). The symbol tells the reader what texture a surface must have. It does not, by itself, say at which step in the routing the surface is judged.
The closest thing to a default comes from the coating specifications. Paragraph 3.9 of MIL-A-8625F requires anodized parts to meet the dimensional requirements of the drawing after the anodic coating is applied, and its design notes ask designers to show both a machining dimension and a coated dimension. That clause is about size, not roughness, but it reflects the reading most buyers take: every requirement on the drawing applies to the part as delivered.
How does each coating change the roughness of a machined surface?
It depends on how thick the coating is compared with the machining marks and on what the pretreatment does to the metal: thin conversion films largely follow the machined profile, anodic films follow it but can be roughened or smoothed by the pretreatment, and thick organic coatings replace it with a texture of their own.
| Finish | What the specification or practice says | Effect on the machined texture | Which callout to rely on |
|---|---|---|---|
| Chemical conversion coating (chem film) | MIL-DTL-5541F 3.3: applied after machining, forming and welding are complete | Very thin film that follows the machined profile; cleaning and deoxidizing steps can etch the surface slightly | Before-coating value is usually enough; state it anyway |
| Type II sulfuric anodize | MIL-A-8625F 3.13 asks for a continuous, smooth coating; 6.9 notes that chemical brightening smooths the metal before anodizing | Coating grows into and out of the surface and follows the profile; etch pretreatment tends to dull and roughen, brightening tends to smooth | Before-coating value on the machined face; add an after-coating value only where the function needs it |
| Type III hard anodize | MIL-A-8625F 6.7: hard coatings generally have increased surface roughness and may be processed oversize, then lapped or honed | Rougher, less dense outer layer than the base of the coating | After-coating value on sealing and sliding faces, measured after any lap or hone |
| Electroplating (zinc, nickel) | Typical industry guidance: thin deposits follow the substrate; some bright or leveling baths fill fine scratches | Close to the machined texture for thin deposits; can be smoother with leveling baths | Before-plating value, plus an after-plating value on contact or cosmetic faces |
| Powder coating and liquid paint | Typical industry guidance: the film is thick compared with machining marks | Machined texture is hidden; the coated texture comes from the coating’s flow and cure | Before-coating value only where adhesion or flatness matters; judge the coated face to the coating’s appearance standard |
| Abrasive blasting before coating | Typical industry guidance: blasting is used to clean and to give coatings a profile to grip | Replaces the machined texture with a blast texture | Any pre-coat value on a blasted face must say whether it applies before or after blasting |
The thinner the coating relative to the machining marks, the more a before-coating callout already controls the result. The pretreatment line can move the texture either way before any coating is deposited, so ask which cleaning, etching or brightening steps the finisher runs on your part.
How do you show before-coating and after-coating values on the symbol?
Use the fields the symbol already provides: the roughness requirement in the first position, a second requirement below it, and the process or coating in the production-method position, then add a flag note that says which requirement applies at which stage.
The ISO symbol, first standardized in ISO 1302 and now carried by ISO 21920-1:2021, has five text positions around the tick. As summarized in Mitutoyo’s quick guide to EN ISO 1302 (Mitutoyo, Bulletin 2229), position a holds the roughness requirement, position b a further surface requirement, position c the production process with chrome plating given as an example, position d the lay direction and position e the machining allowance. The ASME Y14.36 symbol offers comparable fields for the roughness value, a production method or treatment note, lay and material removal allowance.
Those fields let one symbol carry two requirements and the process that separates them. What the symbol does not do on its own is say which line is the bare-metal value and which is the finished value, so write that in words, either next to each value or in a numbered flag note. The table gives note wording that can be adapted.
| Situation | On the symbol | Flag note wording |
|---|---|---|
| Chem-filmed face, texture set by machining | a: Ra 1.6 · c: CHEM FILM | SURFACE TEXTURE APPLIES AFTER MACHINING, BEFORE CHEMICAL CONVERSION COATING PER MIL-DTL-5541, TYPE II, CLASS 3. |
| Cosmetic Type II anodized face | a: Ra 0.8 · c: ANODIZE | SURFACE TEXTURE APPLIES BEFORE ANODIZE. ANODIZE PER MIL-A-8625, TYPE II, CLASS 2. APPEARANCE TO APPROVED SAMPLE. |
| Hard-anodized seal bore | a: Ra 0.4 · c: HARD ANODIZE, HONE | MACHINE OVERSIZE. SIZE AND SURFACE TEXTURE APPLY AFTER HARD ANODIZE AND HONE. |
| Plated contact face, both stages controlled | a: Ra 0.8 · b: Ra 1.6 · c: PLATE | FIRST VALUE APPLIES BEFORE PLATING; SECOND VALUE APPLIES TO THE PLATED SURFACE. |
| Powder-coated face needing adhesion | a: Ra 3.2 · c: BLAST, POWDER COAT | SURFACE TEXTURE APPLIES AFTER MACHINING, BEFORE BLAST. COATED SURFACE TO COATING SPECIFICATION. |
| Masked face left bare | a: Ra 0.8 | MASK THIS SURFACE; NO COATING PERMITTED. SURFACE TEXTURE APPLIES AS MACHINED. |
Keep one convention across the drawing set. If most coated faces are controlled before coating, a general note such as “surface texture values apply before coating unless marked FINISHED” is shorter than a flag on every symbol, and it leaves the exceptions easy to see.
Which surfaces need an after-coating roughness value?
Only the surfaces where the coated face itself does the functional work: O-ring and gasket faces, sliding and bearing surfaces, electrical contact faces, and cosmetic faces a customer will judge by eye or touch.
For a static seal, the elastomer sits on the coating, not on the metal, so the finished texture decides whether the joint leaks. For a sliding fit in a hard-anodized bore, the coating is the wear surface, which is why MIL-A-8625F expects such coatings may be finished by lapping or honing after anodizing. For electrical contact, the notes of MIL-DTL-5541F point out that surface roughness and contact pressure influence measured resistance on Class 3 coatings, sometimes more than the coating thickness does.
Why is a before-coating value easier to control and inspect?
Because it is measured at the machine, on bare metal, with the instrument and settings the machinist already uses, and it controls the surface the finisher starts from. An after-coating value can only be checked once the part is back from the finishing line, when correcting it means stripping the coating.
Measuring coated surfaces brings its own questions. A stylus trace on a soft paint film can mark it, while a hard anodic layer measures cleanly, and a contact-resistance face may need to stay untouched. Roughness readings also scatter by nature: Mitutoyo’s guide notes that vertical parameters such as Ra vary in an approximate range of −20% to +30% from one reading to the next (Mitutoyo, 2016), which is why the 16% rule of ISO 4288 accepts a limited share of readings over the limit.
Does coating thickness change the size tolerance as well as the finish?
Yes, and size and texture should be stated for the same stage: MIL-A-8625F expects about half of an anodic coating’s thickness to build up on each coated surface, and it asks designers to show both the machining dimension and the coated dimension.
The worked example in paragraph 6.10.1 of MIL-A-8625F is a 0.004 inch hard coating, for which a 0.002 inch per-surface allowance is made before coating. Per MIL-A-8625F paragraph 6.10.2, the pitch diameter of a 60-degree thread grows by four times the surface growth, which is why threads are usually masked or cut oversize.
The practical rule is to pair them: if a bore is toleranced after hard anodize and hone, its texture should be too. If size and texture are stated for different stages, say so in the same flag note. For how the finishes compare on corrosion, wear and cost, see the finish comparison guide.
What should an RFQ to an overseas supplier say about finish and coating?
For each controlled surface, state whether the roughness value applies before or after coating, name the coating specification with its type and class, and say at which stage the inspection report should record the reading.
The question matters more when parts are sourced abroad. Machining and finishing are often split between two plants or two purchase orders, and each side can then meet its own reading of an ambiguous callout while nobody owns the finished surface. When moving a part from one country to another, for example from China to Vietnam, the new supplier also inherits whatever unwritten understanding the old one had with the finisher. Writing the stage onto the drawing replaces that understanding with a requirement.
- Per-surface stage. Before coating, after coating, or both, on every symbol or through one general note.
- Coating by specification. Specification, type and class, plus any masking, so the finisher can say how the pretreatment will affect the surface.
- Measurement point. Which stage each first-article reading is taken at, and on which faces, so the report matches the drawing.
- Cosmetic faces. An approved sample or appearance standard, since roughness alone does not describe how a coated face looks.
At Saola, CNC milling and CNC turning run at our lead CNC plant and surface finishing is done by partner plants we manage, so a drawing that names the stage can be reviewed for both steps in one quote, and the quote names where each step is done. The RFQ package guide covers the rest of what to send, and the quote form takes the STEP file and drawing.
Frequently asked
Does chem film change surface roughness?
Only slightly in most cases. The conversion film is very thin and follows the machined profile; any change usually comes from the cleaning and deoxidizing steps before it. A before-coating value is normally enough, but write that it applies before coating.
Can I write “surface texture applies after all processing”?
Yes, and on thin finishes it is workable. On hard anodize, blasting or powder coating it asks the machinist to predict what the finisher will do to the surface, so expect questions or a cautious quote unless the finished value is realistic for that coating.
My old drawing says RMS. Is that the same as Ra?
No. RMS corresponds to Rq in ASME B46.1, which is a different parameter from Ra. Ask the designer which one is meant before the part is quoted, and state the stage for whichever value is kept.
What about surfaces that are masked from the coating?
A masked face stays bare, so its callout is a machined value. Show the masking boundary on the drawing and say the face is uncoated, otherwise the inspector may look for a finished value that was never meant to exist.
Sources
- ASME Y14.36-2018, Surface Texture Symbols, ASME, 2018 (reaffirmed 2024)
- ASME B46.1-2019, Surface Texture (Surface Roughness, Waviness, and Lay), ASME, 2019 (reaffirmed 2026)
- ISO 21920-1:2021, Surface texture: Profile — Part 1: Indication of surface texture, ISO, 2021
- Quick Guide to Surface Roughness Measurement, Bulletin 2229 (drawing symbols per EN ISO 1302; evaluation per EN ISO 4288), Mitutoyo America, 2016
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 1993, with Amendment 1 of 2003 (paragraphs 3.9, 3.13, 6.7, 6.9, 6.10.1 and 6.10.2)
- MIL-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, US Department of Defense, 2006 (paragraphs 3.3 and 6.1.2.1)