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

The short answer: ASME BPE (Bioprocessing Equipment) defines seven surface finish designations for metallic product-contact surfaces, SF0 through SF6. SF1, SF2, and SF3 are mechanically polished finishes with maximum Ra values of 20, 25, and 30 µin (0.51, 0.64, and 0.76 µm). SF4, SF5, and SF6 are mechanically polished and electropolished finishes with maximum Ra values of 15, 20, and 25 µin (0.38, 0.51, and 0.64 µm). SF0 means no finish requirement. The most commonly specified finish for product-contact surfaces in pharmaceutical and biotech equipment is SF4.

If you design, specify, fabricate, or qualify equipment for pharmaceutical, biotech, or other high-purity processes, the BPE surface finish table is the one you'll use most. This page explains what each designation requires, how it's measured, and how to write it on a drawing so your finisher delivers what you meant.

Why BPE defines surface finish the way it does

In bioprocessing, the product-contact surface is a hygiene control. A rougher surface has more area for product residue and microorganisms to hold onto, is harder to clean and sterilize, and is more prone to corrosion and rouging. The BPE standard therefore controls two things: the surface roughness, expressed as a maximum Ra, and the surface condition, meaning the absence of defects like pits, scratches, inclusions, and residual weld discoloration beyond stated limits.

Electropolishing earns its place in the higher designations because it does both at once. It reduces Ra by removing the peaks of the surface profile, and it removes the smeared, stressed, iron-contaminated layer that mechanical polishing leaves behind. The resulting surface is chromium-enriched, passive, and easier to clean than a mechanically polished surface of the same Ra. That is why SF4 at 15 µin is a different surface from a mechanically polished surface that happens to measure 15 µin, and why the standard does not treat them as equivalent.

The surface finish table

Values per ASME BPE, Part SF. Confirm against the edition referenced by your specification. Ra values are maximums, not averages; every measured reading on the surface must fall at or below the limit.

Two things people routinely get wrong: SF numbers are not a scale from rough to smooth. SF4 is smoother than SF1. And an electropolished finish is not "SF4 or better" by default; it is whichever designation its measured Ra and surface condition actually meet.

How Ra is measured under BPE

Ra is the arithmetic average deviation of the surface profile from its mean line, measured with a contact profilometer per ASME B46.1. BPE specifies the measurement parameters (cutoff length and evaluation length) and requires readings to be taken in a way that represents the whole surface, not a single favorable spot.

Harrison measures and reports Ra on every BPE job. Reports are available per part and per surface. For a plain explanation of Ra versus RMS and how to convert between them, see Ra & RMS: Calculating Surface Roughness.

Surface condition: what else BPE inspects

Ra alone doesn't qualify a BPE surface. The standard also sets acceptance criteria for surface defects on product-contact surfaces, including pits, scratches, inclusions, cracks, porosity, weld discoloration, and residual polishing marks. The general principle is that the surface must be free of defects that could harbor product or contaminants, with specific limits set out in the standard's acceptance criteria.

Electropolishing helps here as well. It removes light scratches and burrs, brightens weld zones, and reveals inclusions that were hidden under a mechanically smeared surface, which is one reason a good electropolish sometimes exposes defects in the base material that a mechanical polish would have covered. That's a feature: it's better to find the inclusion at the finishing shop than in a validated process.

Which designation should you specify?

SF4 (15 µin Ra, electropolished) is the default for most product-contact surfaces in pharmaceutical and biotech equipment: vessel interiors, agitators, process piping, valves, fittings, and sanitary components. If you're unsure, this is the answer.

SF5 and SF6 (20 and 25 µin Ra, electropolished) are used where the geometry or material makes 15 µin impractical, or where the process is less sensitive. Cast components, some fittings, and larger fabrications with limited access are commonly specified at SF5 or SF6.

SF1 through SF3 (mechanically polished) are used where electropolishing isn't required or isn't possible, for example on non-product-contact surfaces, exterior finishes, or components made from alloys that don't electropolish well.

SF0 is for surfaces where the owner has decided finish is not a hygiene concern and will specify it separately, if at all.

For guidance on the drawing callout itself, including how to state the designation, the material, and the standard, see How to Specify Electropolishing on a Drawing.

Materials

BPE surface finish designations apply to the austenitic stainless steels used in bioprocessing (316L is the default), and by extension to the higher alloys used where corrosion resistance demands it: AL6XN, Hastelloy C-22 and C-276, and other nickel alloys. Harrison electropolishes all of these to BPE finishes in-house. Some grades, particularly free-machining and certain ferritic and martensitic stainless steels, do not electropolish to a uniform bright finish; if your design uses one of these on a product-contact surface, call before you finalize the drawing.

Achieving SF4 on real parts

Getting to 15 µin Ra with an electropolished surface starts with the mechanical polish. Electropolishing typically reduces Ra by 30–50%, so a surface that measures 25–30 µin after mechanical polishing will usually come out at or under 15. A surface that starts at 60 µin will not. Harrison provides both the mechanical polish and the electropolish, so we control the whole sequence. If you're supplying parts already polished, tell us the starting Ra and we'll tell you what's achievable.

Weld zones, corners, and internal passages need special attention because current density is uneven in those areas. Fixturing and cathode placement are where experience shows; it's the difference between a part that meets SF4 everywhere and one that meets it on the easy surfaces.

Rouge and de-rouging

Even a properly finished SF4 surface can develop rouge in service, particularly in WFI and clean-steam systems. Class I, II, and III rouge each call for a different removal approach, and re-passivation or re-electropolishing after de-rouging restores the surface. Harrison performs de-rouging and re-passivation for pharmaceutical equipment in-shop and on-site.

Documentation Harrison provides for BPE work

Surface finish report with Ra readings per part and surface; material test reports (MTRs) traceability where required; certificate of conformance to the specified SF designation; passivation verification per ASTM A967 where requested.

Request a BPE electropolishing quote

Send the drawing with the SF designation, material, and quantity. If the finish callout is unclear or you're deciding between designations, send it anyway and we'll help you pick. Harrison has finished bioprocess equipment to BPE requirements from our Houston facility since the standard's early editions, and our team includes a member of the ASME BPE Surface Finish Subcommittee.

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