How to Specify Electropolishing on a Drawing

Reviewed by Tom Harrison, member of the ASME BPE Surface Finish Subcommittee · Updated October 2026

The short answer: A complete electropolishing callout states five things: the standard (ASTM B912, or ASME BPE with an SF designation), the surface finish requirement (a maximum Ra with units, or the SF number), which surfaces are to be electropolished and which are to be masked, the allowance for material removal on any toleranced feature, and how the result will be verified. A callout that says only "electropolish" leaves every one of those to the finisher's judgment, and the part you get back may not be the part you meant.

Electropolishing removes metal. That single fact is why it needs a better drawing note than a coating or a cleaning process does. The rest of this page walks through what to write and why.

1. Name the standard

There are two you'll use.

ASTM B912, Standard Specification for Passivation of Stainless Steels Using Electropolishing, is the general-purpose standard. It covers the process, the resulting passive surface, and the verification tests, and it applies to any stainless part where the goal is a passivated, smoothed, bright surface. It does not itself set a required Ra; you add that.

ASME BPE applies to bioprocessing and pharmaceutical equipment. Its Part SF defines surface finish designations SF0 through SF6, with SF4, SF5, and SF6 being electropolished finishes with maximum Ra values of 15, 20, and 25 µin. If the part is product-contact in a pharma or biotech process, specify the SF designation and you get the Ra limit and the surface-condition acceptance criteria along with it. See our ASME BPE surface finish guide for the full table.

Other documents you may encounter: SEMI F19 for semiconductor UHP components, customer-specific specs from pharma and aerospace primes, and older references to ASTM A967 (which is chemical passivation, not electropolishing; if the drawing says A967 but the intent is a bright, smooth finish, it needs correcting).

Example callouts:

ELECTROPOLISH PER ASTM B912. Ra 15 µin MAX ON SURFACES MARKED "EP".

PRODUCT CONTACT SURFACES: ASME BPE SF4. NON-PRODUCT CONTACT: SF0.

2. State the surface finish requirement

Write the Ra as a maximum, with units. "Ra 15" is ambiguous; "Ra 15 µin max" or "Ra 0.38 µm max" is not. If both units appear, make sure they agree (15 µin ≈ 0.38 µm; 20 µin ≈ 0.51 µm; 25 µin ≈ 0.64 µm; 32 µin ≈ 0.8 µm). Our Ra and RMS conversion page has the full table.

If you need something other than Ra (Rz, Rmax, RMS), say so explicitly and be aware that most finishing shops measure and report Ra by default. If the surface will be measured a particular way, for example a specific cutoff length or a minimum number of readings, state it or reference the standard that defines it.

Be realistic about what electropolishing can do. It typically reduces Ra by 30–50% from the starting surface. If you specify Ra 10 µin max on a part that arrives at 60 µin as-machined, no electropolisher can deliver it without a mechanical polish first. Either specify the pre-finish ("mechanically polish to Ra 25 µin max prior to electropolishing") or let the finisher quote the mechanical work as part of the job.

3. Identify the surfaces

Electropolishing every surface of a part is sometimes what you want and sometimes not. Threads, press-fit diameters, sealing faces with a ground finish, and features with tight tolerances may need to be masked. Conversely, internal passages and bores in a manifold are often the surfaces you care most about, and they need to be called out so the finisher fixtures for them.

Use a surface-finish symbol on the feature, a note referencing labeled surfaces ("SURFACES MARKED EP"), or a plain statement ("ELECTROPOLISH ALL SURFACES EXCEPT THREADS AND DATUM A"). If internal passages are included, say so and give the finisher the geometry: internal surfaces need cathode access, and a 0.125-inch bore 8 inches long is a different job from a 2-inch bore.

4. Allow for material removal

This is the item most often missing from drawings. Electropolishing removes roughly 0.0003 to 0.001 inch per surface, which means a diameter loses twice that. On a ±0.005 feature that's noise; on a ±0.0005 feature it's the whole tolerance.

Handle it one of three ways:

• Machine oversize/undersize by the expected removal and note "DIMENSIONS APPLY AFTER ELECTROPOLISH." This is the cleanest approach for tight features.
• Mask the critical feature so it isn't electropolished. Note "MASK DIA. X.XXX."
• Specify the removal ("ELECTROPOLISH, 0.0005 IN. MAX REMOVAL PER SURFACE") when you can tolerate a defined loss and want it controlled.

Whichever you choose, say which dimensions apply before and which apply after finishing. If the drawing is silent, most shops assume dimensions apply after finishing and will ask.

Edges also change. Electropolishing preferentially removes material at sharp edges and light burrs, which is usually welcome (it's a deburring method), but a knife-edge or a sharp thread crest will round. If an edge must stay sharp, mask it or say so.

5. Specify verification and documentation

State what you want measured and what you want to receive. Typical requirements:

• Surface finish report — Ra readings, per surface or per part, with the instrument and parameters used.
• Passivation verification — the ASTM A967 test (water immersion, high humidity, copper sulfate, etc.) that confirms the passive surface, if the application requires it. ASTM B912 references these tests.
• Certificate of conformance to the standard and designation specified.
• Material traceability — MTRs or heat numbers where the application requires it (standard for BPE and aerospace work).
• Visual or borescope inspection of internal surfaces where applicable.

If you need a specific sampling plan (every part vs. a sample per lot), say so. Harrison's standard deliverable is a certificate of conformance to the specified standard, with surface finish reports and additional reports available on request.

6. Say what the part is made of and what came before

The alloy determines the electrolyte and parameters. 316L, 304, 17-4 PH, Hastelloy C-276, AL6XN, and titanium each behave differently, and some grades (free-machining 303, some ferritics, and castings with porosity) do not electropolish to a uniform bright finish. List the material and condition on the drawing, and if the part has been welded, heat-treated, or previously plated or coated, tell the finisher. Weld heat tint and scale come off in electropolishing; heavy oxide from heat treatment may need pickling first.

A complete drawing note

Putting it together, a callout for a pharmaceutical process fitting might read:

MATERIAL: 316L SST PER ASTM A270 / ASME BPE. PRODUCT CONTACT SURFACES (ID AND FACE OF FERRULE): MECHANICALLY POLISH AND ELECTROPOLISH TO ASME BPE SF4, Ra 15 µin (0.38 µm) MAX. EXTERIOR SURFACES: SF0, ELECTROPOLISH FOR APPEARANCE, NO Ra REQUIREMENT. MASK TRI-CLAMP GASKET GROOVE. DIMENSIONS APPLY AFTER ELECTROPOLISH. SUPPLY SURFACE FINISH REPORT AND CERTIFICATE OF CONFORMANCE TO ASME BPE SF4 WITH MTR TRACEABILITY.

And for a general-industry machined part:

ELECTROPOLISH PER ASTM B912 ALL SURFACES EXCEPT 1/4-20 THREADS (MASK). Ra 20 µin MAX ON SURFACES MARKED EP. MAX REMOVAL 0.0005 IN. PER SURFACE. DIMENSIONS APPLY AFTER ELECTROPOLISH. C OF C REQUIRED.

Common mistakes we see on drawings

"Electropolish" with nothing else. No Ra, no standard, no surface identification. The finisher has to guess or call.

Ra without units, or Ra in one unit and a converted value in the other that doesn't match.

An Ra that can't be reached from the as-machined surface without a mechanical polish nobody quoted.

Tight-tolerance features with no removal allowance and no masking instruction.

"Passivate per ASTM A967" on a part where the customer actually wants a bright, smooth electropolished finish. A967 is chemical passivation; it won't change the surface.

Internal passages that need finishing but aren't called out, so the part is fixtured for exterior surfaces only.

Not sure what to write? Send us the drawing

The fastest way to get a specification right is to have the finisher look at it before it's released. Harrison reviews drawings at no charge, will tell you what's achievable from your starting surface, and can quote the mechanical polish, electropolish, and verification as one job. Our team includes a member of the ASME BPE Surface Finish Subcommittee, so if the question is what BPE actually requires, you're asking the right shop.

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