N4 surface finish explained for stainless steel and machining drawings

What N4 surface finish means

N4 surface finish is useful shorthand, but it is not a universal finish callout. In stainless steel supply, N4 often means No. 4 or #4 finish: a directional brushed or satin-polished surface used on sheet, panels, enclosures, trims and equipment covers. In machining and engineering drawings, N4 can also mean a legacy roughness grade equal to about Ra 0.2 µm, or about 8 microinches.

These two meanings are not interchangeable. A No. 4 stainless finish describes appearance and the general production route. An N4 roughness grade describes a measured surface texture value. If a drawing, purchase order or project specification only says N4, the safest approach is to confirm which system is intended before ordering material, releasing production or accepting finished parts.

This article compares both meanings, explains the standards language behind them, and gives practical wording for clearer specifications. For related articles, see our surface finishes section.

The two common meanings of N4

The confusion comes from the way different industries use short finish codes. Stainless steel stockists, fabricators and polishers may use N4 as shorthand for No. 4 stainless steel finish. Precision machining drawings, especially older drawings or drawings influenced by ISO roughness grade notation, may use N4 as a surface texture class.

Term on drawing or order	Common meaning	Main thing it controls	What it does not fully define
N4, #4 or No. 4 stainless finish	Directional brushed or general-purpose polished stainless steel finish	Visual texture, grain direction and general polished appearance	A precise Ra value unless separately specified
N4 roughness grade	Legacy surface roughness grade, commonly treated as Ra 0.2 µm or 8 µin	Measured roughness magnitude	Material, polish pattern, reflectivity, color or lay appearance

For this reason, N4 should not be used by itself when the result matters. A brushed stainless panel can be visually correct as No. 4 without meeting Ra 0.2 µm. A machined or lapped component can meet Ra 0.2 µm without looking like a No. 4 brushed sheet.

N4 as No. 4 brushed stainless steel finish

In stainless steel, No. 4 finish is one of the most common decorative and functional finishes. It normally has a consistent linear grain, moderate reflectivity and a satin appearance. It is less mirror-like than No. 7 or No. 8 finishes and more refined than rougher ground finishes.

ASTM A480/A480M descriptions for flat-rolled stainless products identify No. 4 as a general-purpose polished finish. Public summaries of the standard describe it as a linearly textured surface that may be produced by mechanical polishing or rolling, with average roughness generally up to 25 microinches. That is approximately 0.64 µm Ra. ASTM descriptions also note that roughness measurements for adjacent polished finishes can overlap, which is one reason appearance samples are often used in procurement.

In European stainless terminology, the closest language is usually found around EN 10088-2 finish codes. A 2J finish is commonly described as cold rolled, brushed or dull polished. A 2K finish is a more controlled satin-polished finish often associated with lower roughness requirements for architectural and external uses. The exact match between ASTM No. 4 and EN finish codes depends on the supplier, starting material, abrasive sequence and acceptance criteria.

Commercial No. 4 stainless is often associated with abrasives in the 120 to 150 grit range, but grit alone is not a complete specification. Different belts, pressure, speed, base finish, lubricant, polishing direction and post-cleaning practice can produce visibly different results. For exposed panels, elevators, kitchen equipment, appliances and enclosures, visible lay and uniformity may matter as much as numerical roughness.

N4 as a surface roughness grade

In engineering surface texture notation, N4 usually means a fine surface roughness grade with a nominal Ra value of 0.2 µm, commonly rounded to 8 microinches. This is a numerical texture requirement, not a brushed stainless finish.

Older drawing systems often used N grades from N1 to N12 to represent a ladder of Ra values. N4 sits toward the fine end of that ladder. Surfaces in this range are commonly produced by careful grinding, honing, lapping or polishing rather than ordinary rough machining. In modern specifications, it is usually clearer to write the actual parameter and value, such as Ra 0.2 µm maximum, instead of relying only on the N-grade symbol.

There is also a standards update to understand. ISO 1302:2002, a familiar reference for indicating surface texture on technical product documentation, has been withdrawn and replaced by ISO 21920-1:2021 for profile surface texture indication. In the United States, ASME B46.1 covers surface texture concepts including roughness, waviness and lay. For new drawings, use the current standard required by the project and state the measured parameter clearly.

Ra is not the same as Rz, Rt or Rq. Ra is an arithmetic average of profile deviations, so it can hide differences in peak shape, valley depth and scratch pattern. Two surfaces can have the same Ra but behave differently in sealing, cleanability or visual appearance. Where function is critical, specify additional parameters, measurement direction, cutoff, sampling length and inspection method.

Why No. 4 stainless and N4 roughness are not interchangeable

The common error is assuming that N4 brushed stainless and N4 roughness grade describe the same surface. They do not. The numerical comparison shows the gap. The roughness-grade meaning of N4 is about Ra 0.2 µm. The commonly cited ASTM No. 4 roughness guidance is generally up to 25 microinches, which converts to about 0.64 µm Ra. Some No. 4 material may be smoother than that, but the name No. 4 by itself does not guarantee Ra 0.2 µm.

Question	No. 4 stainless finish	N4 roughness grade
Is it mainly visual or measured?	Mainly visual and process-based, with possible roughness guidance	Mainly measured by roughness value
Does it require linear grain?	Usually yes	No, unless lay is specified
Typical material context	Stainless steel sheet, strip, plate, panels and fabricated parts	Machined, ground, honed, lapped or polished components in many materials
Does it define corrosion resistance?	No; alloy, environment, cleaning and fabrication quality still matter	No; it only states a roughness level unless combined with material and process requirements

For corrosion and hygiene, surface finish is only one variable. Smoother surfaces can reduce dirt retention and make cleaning easier, but they cannot compensate for the wrong stainless grade, chloride exposure, poor drainage, embedded iron or heat tint left after welding. When parts are used in coastal, food, pharmaceutical or chemical environments, the specification should combine material grade, finish, cleaning, passivation or pickling requirements, and inspection criteria.

How to specify N4 clearly

A clear specification removes the guesswork. Start by deciding whether the requirement is visual, functional or both. If the part is an exposed stainless panel, appearance and grain consistency may be the priority. If the part is a sealing face, bearing surface or precision sliding component, measured roughness and lay may be more important.

For brushed stainless sheet or fabricated panels

Use stainless finish terminology and add acceptance details. A typical wording might be: 304 stainless steel sheet to the applicable ASTM or EN material standard, No. 4 brushed finish on exposed side, grain direction lengthwise, protective film during handling, appearance to approved sample, and Ra limit if required by the application. See also: Buying Guides.

If the project uses European notation, the specification may refer to EN 10088-2 finish 2J or 2K, with grade, thickness, grain direction, surface protection and roughness limit stated separately. Do not assume that 2J, 2K and ASTM No. 4 are automatically identical across all suppliers.

For machined or precision surfaces

Use surface texture notation rather than a decorative finish name. A clearer callout would be: Ra 0.2 µm maximum, measured according to the selected ISO or ASME surface texture standard, with lay direction and measurement conditions defined where necessary. If sealing, fatigue or wear performance is important, consider specifying Rz or other parameters in addition to Ra.

For purchasing and inspection

• State the material grade and product form, such as sheet, plate, bar, casting or machined component.
• Identify the finish system: ASTM No. 4, EN 2J or 2K, or measured Ra 0.2 µm.
• Define whether the finish applies to one side, both sides, all exposed surfaces or selected drawing zones.
• Specify grain direction, especially for architectural panels and formed parts.
• Use approved samples for visible stainless work because Ra alone cannot describe appearance.
• Define measurement method, cutoff and inspection location for functional roughness requirements.
• State cleaning, passivation or contamination-control requirements if corrosion performance matters.

Inspection issues that cause disputes

Surface finish disputes usually come from incomplete specifications rather than bad material. A supplier may deliver a correct No. 4 sheet by commercial practice, while the buyer expected a tighter Ra value. A machinist may meet Ra 0.2 µm but leave a lay direction that is unsuitable for a seal.

Profilometer readings can vary with instrument type, stylus condition, cutoff selection, measurement direction and sampling location. Measuring parallel to a brushed grain can give a different result from measuring across it. Edges, weld zones, bends and heat-affected areas may not match flat sheet areas. If the requirement is critical, the drawing should say where and how the surface will be checked.

Visual inspection needs its own controls. Lighting angle, panel orientation, protective film residue and adjacent batch variation can all change perceived appearance. For decorative stainless assemblies, approved reference samples, consistent grain direction and batch control are often more useful than a roughness number alone.

Practical selection guidance

Choose No. 4 stainless finish when the goal is a clean, satin, directional appearance with reasonable resistance to showing minor handling marks. It is widely used for appliance panels, elevator interiors, door plates, work surfaces, kitchen equipment, splashbacks and general architectural metalwork.

Choose an N4 roughness-grade requirement when the functional surface needs a fine measured texture. Examples may include sealing lands, sliding interfaces, precision ground surfaces, mold details or parts where friction, wear, cleanliness or contact behavior is controlled by surface texture. In these applications, the material and process may vary, so the finish should be specified by parameters, not by a stainless appearance name.

If both appearance and function matter, specify both. For example, an exposed stainless component may need No. 4 directional brushing for appearance and a maximum Ra value for cleanability. That combined requirement is more demanding than either one alone, so it should be agreed with the supplier before production.

Frequently asked questions

Is N4 the same as No. 4 stainless steel finish?

Sometimes, but not always. In stainless steel trade language, N4 may be used as shorthand for No. 4 brushed finish. In machining, N4 can mean a roughness grade of about Ra 0.2 µm. Check the context before placing an order or approving a drawing.

What Ra value is N4 surface finish?

As a roughness grade, N4 is commonly treated as Ra 0.2 µm, or about 8 microinches. As a No. 4 stainless finish, the Ra value is not fixed by the name alone and should be separately specified if it matters.

Is No. 4 stainless finish suitable for outdoor use?

It can be used outdoors, but the finish alone is not enough. The stainless grade, chloride exposure, surface roughness, drainage, cleaning frequency and fabrication quality all affect performance. For severe exterior or coastal conditions, a more controlled satin finish and a suitable stainless grade may be needed.

Can a No. 4 finish be polished to N4 roughness?

It may be possible, but it should not be assumed. Achieving Ra 0.2 µm while maintaining a consistent directional No. 4 appearance requires controlled polishing and inspection. Confirm feasibility with the processor before specifying it.

What is the best way to avoid N4 confusion?

Do not write N4 by itself. State whether you mean No. 4 brushed stainless finish or Ra 0.2 µm surface roughness. Then add material grade, standard, grain direction, measurement method and acceptance sample where appropriate.