The short answer
On stainless RFQs, **when to specify electropolish after CNC** is not the same decision as [passivation versus electropolishing](https://3dwk.com/resources/passivation-vs-electropolishing-stainless), a [passivation-after-CNC timing note](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless), or a general Ra cloud. Electropolish is a controlled metal-removal finish: it can brighten named faces, lower roughness, and help cleanability, but it also moves dimensions and can wash out sharp edges. Silence on which faces need EP, on allowed stock removal, and on whether passivation still follows, is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the tank time alone.
This guide is for buyers packaging stainless RFQs where CAD or a notes cloud says “electropolish as needed” or “EP all over” with no face list or stock allowance. Illustrations use an **Example part** only—not a customer fitting. Sibling notes live in the passivation-versus-EP comparison and the passivation timing guide; here we focus on **when EP is worth calling on the RFQ** so estimators do not have to guess whether a sealing land may shrink by a few hundredths.
What electropolish means for quoting
**Electropolish** dissolves a thin surface layer in an electrolyte. It is not a coating, and it is not a silent substitute for a named [passivation](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless) step unless you say so. Process pages for aerospace and general stainless ([CNC 3-axis SS 316 aerospace](https://3dwk.com/en/cnc-3axis/ss-316/aerospace.html), [CNC 5-axis SS 316 aerospace](https://3dwk.com/en/cnc-5axis/ss-316/aerospace.html), [CNC 3-axis SS 316](https://3dwk.com/en/cnc-3axis/ss-316.html)) assume face, stock, and sequence notes when lots mix bright cosmetic faces with fitted bores.
Do not bury “EP to a nice finish” inside a notes cloud. Put **ELECTROPOLISH FACE A + BORE C, ALLOW 0.01 PER SIDE ON C, THEN PASSIVATE; LEAVE THREAD D AS-MACHINED** on the PDF and mirror it in RFQ text. Background: [DFM checklist before an online CNC quote](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote), [surface roughness Ra for machined parts](https://3dwk.com/resources/surface-roughness-ra-for-machined-parts).
Faces, stock removal, and tight fits
EP stock is small, but it is not zero. A press or sealing bore that is already at the bottom of a tight tolerance can go undersize if you polish after final size. Name the faces, an allowed removal or a “size after EP” note, and any [mask or plug](https://3dwk.com/resources/paint-mask-and-plug-notes-cnc-anodize) keep-out (the anodize-mask guide is about coatings; the same idea—protect threads and fits—applies here). [Cosmetic A-side vs B-side notes](https://3dwk.com/resources/cosmetic-a-side-vs-b-side-notes-cnc-housings) should match the EP face list so a bright face and a hidden face are not quoted the same way.
Stainless 316 bar $/kg planning bands sit in the mid-commodity stainless range under **rates.v1.1** material tables; material cost is rarely the EP story—masking, allowed shrink, and whether a second passivation step follows are.
When EP is worth calling—and when it is not
Call EP when you need a brighter named face, easier cleanability on a wetted or hygienic surface, or a roughness target that milling alone will not hold without extra time. Skip a blanket EP when the part is hidden, when only [passivation](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless) is required for corrosion, or when edges and laser marks must stay sharp. Grade choice still matters—see [304 vs 316 vs 17-4PH](https://3dwk.com/resources/stainless-304-vs-316-vs-17-4ph)—but it does not replace a face list.
Tolerance and inspection traps
Calling “electropolish all over,” mixing a tight H7 bore with unspoken EP stock, or asking for 100% gloss photos of every pocket, all force **needs_manual**. Default non-critical faces to as-machined plus passivation if needed, and put EP only on named faces with a stock or after-size note. Related: [ISO 2768 general tolerances on CNC RFQs](https://3dwk.com/resources/iso-2768-general-tolerances-cnc-rfq), [Ra callouts that raise quotes](https://3dwk.com/resources/surface-roughness-ra-callouts-that-raise-quotes). If you have a named EP specification, write it; do not assume a shop standard or invent a certificate.
Illustration 1 shows an **Example part** fitting with a labeled EP on face A and bore C versus a thread and a hidden face left as-machined, with a small after-EP stock allowance on the bore.
Example part walkthrough
**Example part:** Stainless 316 housing ~80 × 60 × 25 mm, electropolish on cosmetic face A and sealing bore C with 0.01 mm per side allowed on C, passivation after EP, M6 thread D left as-machined, ISO 2768-m general. With STEP + **EP FACE A + BORE C, ALLOW 0.01 / SIDE ON C, THEN PASSIVATE; THREAD D AS-MACHINED**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “electropolish as needed” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“EP everything, keep all sizes”) versus a labeled face-and-stock pack and the clearer estimate path.
What widens bands fastest
Unlabeled all-over EP on mixed cosmetic and fitted parts, tight bores with no after-size note, missing passivation sequence, and 100% cosmetic inspection of every wall. Quantity breaks help unit minutes but do not erase finish ambiguity—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost) and [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
What to do next
Mark each functional or cosmetic face that truly needs **electropolish**, plus allowed stock and the passivation sequence, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Service pages: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not invoices. If the UI shows **needs_manual**, answer the EP-face questions rather than forcing a fake price.
Common questions
Is electropolish always required on CNC 316 parts?
No. Many 316 parts only need a named [passivation](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless) after machining. EP is worth calling when brightness, cleanability, or a roughness target on named faces justifies the extra removal and masking.
Can I write “EP all over” and keep the same tolerances?
Only if every diameter already has enough remaining stock or a written after-EP size. A tight sealing bore is not rescued by a notes cloud. Update the size or add an allowed removal so the shop and the estimate see the same intent.
Does electropolish replace passivation?
Not as a silent default under **rates.v1.1** estimates. Some EP sequences include a passivating effect; many RFQs still want a named passivation after EP. Write the sequence you actually need instead of assuming a shop default.
How should I protect threads, laser marks, or sharp edges?
Name them as keep-outs or as-machined, and say whether a slight edge break from EP is allowed. Leaving only “electropolish” leaves the estimator guessing whether an M6 class and a laser mark must survive the tank.