How to Achieve a #4 Brushed or #8 Mirror Finish on Stainless Steel

Quick Answer

To get a #4 brushed finish, work stainless through a grit ladder to 120-180 grit, landing about 0.2-0.8 um Ra (ASTM A480; ASSDA). For a #8 mirror, continue past bonded grit through successive buffing compounds to reach roughly 0.02-0.1 um Ra. Never skip more than one grit step, and use iron-free inox tooling throughout.

What a #4 and a #8 finish actually mean

On stainless steel, "#4" and "#8" are named decorative finishes defined under ASTM A480 / A480M and EN 10088, not loose descriptions. They are specs, and the way you hit them is a disciplined abrasive sequence ending on a defined grit or polish stage.

• No.4 (brushed/satin): the architectural and food-contact "workhorse" finish, a uniform directional grain produced with roughly 120-180 grit, landing around 0.2-0.8 um Ra (Voyage Metal; ASSDA, 2026).
• No.8 (mirror): a near-reflective decorative finish, typically below 0.1 um Ra and reaching down toward ~0.02 um, produced by mirror polishing and buffing well beyond bonded grit (Voyage Metal; ASSDA, 2026).

Both finishes are Ra targets reached by a grit ladder, so the controlling skill is sequencing, not buying one magic disc. The table below sets out where the common mill and polish finishes land.

Sources: Voyage Metal; ASSDA; ASTM A480/A480M (2026).

Why the grit ladder is non-negotiable

Each abrasive grit cuts thousands of microscopic scratches. A coarse grit leaves deep, wide valleys; a fine grit leaves shallow, numerous ones. The job of every step is to fully remove the previous grit's scratch pattern before you step up (Empire Abrasives; Uneeda, 2026).

The governing rule is never skip more than one grit step, sometimes called the Golden Rule of Sanding. A related quantitative form is the ~50% / "less than double" rule: the next grit number should be no more than roughly 50% higher than the current one, and never more than double it. So 80 to 120 (skipping 100) is acceptable; 80 to 150 or 80 to 220 is not (Empire Abrasives; Uneeda, 2026).

The physics is simple. A fine abrasive lacks the cutting depth to reach the bottom of a coarse grit's valleys, so it only polishes the peaks, leaving the deep scratches buried until light or a finishing pass reveals them (Empire Abrasives; eQualle, 2026). The final Ra of a finish is governed by the coarsest scratch that survives, not the last grit you used. Jumping more than one step (for example 80 to 220) leaves base scratches that finer grits cannot fully erase, so the final Ra is worse than a proper 80 to 120 to 180 to 220 ladder would give (Surface Finish Ra note; Astro Pak, 2026).

Verification tip: draw light pencil lines across the work before each step. When the lines are fully gone, the prior scratches are gone and you may step up (Empire Abrasives, 2026).

How to get a #4 brushed finish on stainless (step by step)

The #4 band is 120-180 grit / 0.2-0.8 um Ra, which is exactly where flap discs, surface-conditioning discs and fine belts work best (Surface Finishing note; Voyage Metal; ASSDA, 2026). Start no coarser than the deepest defect requires, then climb the shortest legal ladder that reaches the band.

1. Knock down welds and heavy defects (36-80 grit). On a welded part, level the proud bead with a coarse flap disc (36-40 grit, worked at a steeper 15-25 degrees), then blend the leveled weld into the parent metal with 60-80 grit at a shallower 0-15 degrees (Grit Progression note; Empire Abrasives, 2026). Use zirconia or ceramic alumina grain here: plain aluminium-oxide flap discs burn up and glaze over under weld-blending pressure, which is the same silent ladder failure as skipping a grit (Empire Abrasives, 2026).
2. Refine to 120 grit. Remove the coarse 60-80 scratch pattern completely. This is the lower edge of the #4 band; on lighter material with no welds you can start here.
3. Finish on 120-180 grit for the directional grain. Land the uniform brushed grain that defines No.4. A fine belt or a non-woven surface-conditioning disc produces the consistent long-grain look; 120-180 grit puts you in the 0.2-0.8 um Ra target (Surface Finishing note; ASSDA, 2026).
4. Even out with a non-woven pass if needed. A medium (maroon, ~100-150 grit equivalent) or fine (blue/green, ~180-220 grit equivalent) non-woven grade unifies the grain and de-tints heat marks without cutting real material (Non-Woven Abrasive note; EMI Supply; Combat Abrasives, 2026).

Throughout, keep the abrasive iron-free. INOX-marked discs (under 0.1% iron, sulfur and chlorine) and dedicated tools prevent the iron embedding and cross-contamination that cause rust spotting later. The wrong disc anywhere in the ladder ruins the corrosion resistance no matter how fine you finish (Grit Progression note; Empire Abrasives, 2026).

How to get a #8 mirror finish on stainless

A #8 mirror sits below 0.1 um Ra and reaches toward ~0.02 um, which is beyond what bonded or coated grit alone delivers (Surface Finish Ra note; Voyage Metal, 2026). You first build a flawless fine-ground base, then move to compounds.

1. Reach a clean fine-ground base first. Carry the surface up through the ladder to a fine grit (320-400 and finer) so no coarse scratch survives. The mirror stage only reveals scratches you left behind, so the base has to be visually scratch-free under raking light before you start buffing.
2. Polish through successive buffing compounds. A No.8 finish is produced by mirror polishing and buffing through progressively finer compounds, not by a single disc (Surface Finishing note; Surface Finish Ra note, 2026). Each compound stage erases the prior stage's pattern, the same coarse-to-fine logic as the grit ladder.
3. Use non-woven grades at the satin/pre-polish end. Ultra/super-fine non-woven grades (grey, ~600-1200 grit equivalent) handle satin and decorative polishing prep before final buffing (Non-Woven Abrasive note; EMI Supply, 2026).
4. Keep pressure and heat controlled. Apply light-to-medium pressure on non-woven media (about 3-6 lb); excess pressure glazes the web and melts the resin into a smooth, non-cutting face, stalling the finish (Non-Woven Abrasive note; Flexovit, 2025).

Mirror work is slow by design. The eye resolves micro-scratch under gloss, so the ladder runs much longer than a #4 and every shortcut shows.

Grit-to-Ra is a guideline, not a certificate

A grit number does not map to one fixed Ra. The same 240 belt can leave anywhere from ~10 to ~25 microinches Ra depending on grit size distribution, tool load, abrasive condition, feed rate, traverse rate, the metal being worked, and lubricant (Surface Finishing note; finishing.com, 2026). The approximate landings below are starting points for steel, not guarantees.

Sources: Surface Finishing note (Astro Pak; Cratex; finishing.com, 2026).

For QC, validate with a profilometer when a drawing calls a number; never certify Ra from grit alone (Surface Finishing note; finishing.com, 2026). Note also that ISO 21920-2:2021 has replaced ISO 4287 for profile roughness, and the same surface can return a slightly different Ra under the new standard, so confirm which standard a customer's drawing references before reporting numbers (Surface Finish Ra note; Digital Surf; Quality Magazine, 2026). US prints most often cite ASME B46.1 (Surface Finish Ra note, 2026).

Process control is part of the answer. Peer-reviewed work on robotic polishing shows that holding a constant contact force with an active high-bandwidth end-effector reached a 0.4 um Ra finish, evidence that surface quality is driven by controlled process parameters, not the consumable alone (Li et al., 2020, IEEE Access). Belt-grinding research likewise treats the coated abrasive as an engineered finishing tool whose abrasiveness is a measurable, qualifiable property (Mezghani and El Mansori, 2008, Surface and Coatings Technology).

Why the right consumable protects the whole ladder

The sequence assumes each grit cuts to a predictable, consistent depth. If a "120" disc actually carries scattered 80-sized grains, it leaves rogue deep scratches that the next step cannot remove, silently breaking the ladder (Grit Progression note, 2026). Tight grading to a published standard is therefore the precondition for any progression to work, which is why honestly graded, stamped grit matters more on a finish job than on a rough cut.

Two stainless-specific cautions sit alongside grading. First, grain choice is part of the ladder — use zirconia or ceramic alumina for weld blending so the disc keeps cutting instead of glazing. Second, keep the abrasive contaminant-free so you do not embed iron that rusts later. Non-woven media help at the finishing end because their cool, low-pressure cut avoids the bluing and discolouration that heat-sensitive stainless is prone to (Non-Woven Abrasive note; Flexovit, 2025).

The Whitby Abrasives recommendation

Whitby Abrasives stocks the full stainless ladder at value-tier pricing: coarse-to-fine flap discs for weld knock-down and the #4 band, non-woven discs for blending and de-tinting, and polishing wheels for the mirror end. The wedge is not the lowest price alone but honestly graded, stamped grit plus a published grit-to-Ra map, so the disc you buy lands the finish band it claims. The common objection that a value disc must be a low-quality disc gets the ladder backwards: on a finish job, consistent grading protects every downstream step, while premium grain wasted on a light scuff pass just costs more.

• For the #4 brushed band and weld blending, run zirconia or ceramic flap discs.
• To unify the grain and de-tint without cutting metal, step to non-woven surface-conditioning discs and choose the right maroon/blue/grey grade.
• For the #8 mirror end, finish on polishing wheels and buffing compounds.

Everything is stocked in our Whitby, Ontario warehouse for fast domestic fulfillment.

Frequently asked questions

What grit do you need for a #4 finish on stainless steel?

A No.4 brushed finish is produced with roughly 120-180 grit, landing about 0.2-0.8 um Ra (Voyage Metal; ASSDA, 2026). Reach it by climbing a grit ladder rather than starting at 180, so no coarser scratch survives underneath.

What Ra is a #8 mirror finish?

A No.8 mirror finish typically sits below 0.1 um Ra, reaching toward about 0.02 um, and is produced by mirror polishing and buffing well beyond bonded grit (Voyage Metal; ASSDA; Surface Finish Ra note, 2026).

Can I skip grits to save time on stainless?

No. Skipping more than one grit step leaves deep base scratches that finer grits cannot fully erase, so the final Ra is worse and the job actually takes longer. Skip at most one step, for example 80 to 120 (Grit Progression note; Empire Abrasives, 2026).

Why do my flap discs stop cutting on stainless welds?

Plain aluminium-oxide flap discs burn up and glaze over under weld-blending pressure, polishing the peaks instead of cutting. Use zirconia or ceramic alumina grain for weld work and stainless (Grit Progression note; Empire Abrasives, 2026).

Does grit number guarantee a specific Ra?

No. The same belt can leave a range of Ra values depending on grit size distribution, tool load, abrasive condition, feed rate, the metal worked, and lubricant. Validate with a profilometer when a drawing calls a number (Surface Finishing note; finishing.com, 2026).

Why does stainless need dedicated, iron-free abrasives?

Free iron from carbon-steel tooling embeds in stainless and rusts later. INOX-marked discs (under 0.1% iron, sulfur and chlorine) and dedicated tools prevent the contamination that ruins corrosion resistance regardless of how fine you finish (Grit Progression note; Empire Abrasives, 2026).

Sources

• Grit Progression and Sequencing (WA Abrasives Knowledge Base) — never-skip-more-than-one rule, ~50%/double rule, flap-disc weld ladder (36-40 to 60-80 to 120 + non-woven), zirconia/ceramic over aluminium oxide, INOX/contaminant-free for stainless, pencil-line verification. Underlying: Empire Abrasives (2026), Uneeda (2026), eQualle (2026).
• Surface Finishing (WA Abrasives Knowledge Base) — stainless decorative finish bands (No.3, No.4, HL, No.7, No.8), No.4 = 120-180 grit / 0.2-0.8 um Ra, grit-to-Ra table, grit-is-not-a-fixed-Ra warning. Underlying: Voyage Metal, ASSDA, ASTM A480/A480M, Astro Pak, Cratex, finishing.com (2026).
• Surface Finish Ra (WA Abrasives Knowledge Base) — Ra definition (ISO/ASME B46.1), #4 ≈ 0.8 um / 32 uin at 180-240 grit, #8/mirror ≤0.05 um / ~2 uin, coarsest-surviving-scratch governs final Ra, ISO 21920-2:2021 replaces ISO 4287. Underlying: Astro Pak, Engineers Edge, Digital Surf, Quality Magazine (2026).
• Non-Woven Abrasive (WA Abrasives Knowledge Base) — grade bands (C/M/F/VF/UF-SF) and approximate grit equivalents, colour-not-a-standard caution, operating speed 2,000-9,000 SFPM, light-to-medium pressure ~3-6 lb, glazing failure mode. Underlying: EMI Supply, Combat Abrasives, Flexovit (2025).
• Standards bodies referenced: ASTM A480/A480M (stainless finish definitions); EN 10088; ASME B46.1 (surface texture); ISO 21920-2:2021 (profile roughness, supersedes ISO 4287).
• Jian Li, Yisheng Guan, Hao Chen, Bing Wang, Tao Zhang, Xineng Liu, Jie Hong, Danwei Wang (2020). A High-Bandwidth End-Effector With Active Force Control for Robotic Polishing. IEEE Access. https://doi.org/10.1109/access.2020.3022930 (open access; reached 0.4 um Ra under active force control).
• S. Mezghani, M. El Mansori (2008). Abrasiveness properties assessment of coated abrasives for precision belt grinding. Surface and Coatings Technology. https://doi.org/10.1016/j.surfcoat.2008.08.058