Sandblasted Polished and Anodized: How the Three Steps Work Together

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  What sandblasted polished and anodized actually changes on the surface
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  Why the order of blasting, polishing and anodizing is not negotiable
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  Where each step stops helping and starts costing you
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  Alloy behavior under sandblasted polished and anodized processing
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  How to inspect an anodized finish and what the numbers mean
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  Step by step: from machined part to finished surface
• 7
  Choosing a finish route for an aluminum CNC part

Anodizing does not coat aluminum. It converts the top few tens of microns of the metal into aluminum oxide, grown out of the substrate itself. That oxide layer is hard, electrically insulating in the standard sulfuric process, and porous enough to accept dye before it is sealed. Because the layer grows both inward and outward, roughly half its thickness sits below the original surface. A 10 μm anodic film moves the outer dimension by about 5 μm per side.

That single fact drives everything else. The substrate finish you anodize is the finish you keep, only slightly muted. Blasting creates a random dimpled texture that scatters light and hides tool marks. Polishing cuts the surface flat and drops Ra so the metal reflects. Anodize then adds a transparent oxide skin over whatever topography you left behind. Sandblasted polished and anodized parts therefore look different depending on which of the first two steps dominated.

The oxide is also brittle compared with the metal under it. A sharp edge that survived machining can chip its own anodic film during handling, leaving a bright metal speck on a dark part. This is why edge break and corner radius are specified for anodized parts, not just for looks.

On 6061-T6, a clear sulfuric anodize at 5–15 μm is the common call. On 7075, the same bath produces a slightly darker, more yellow-brown tone because of the copper and zinc in the alloy. That color shift is a material property, not a defect, and it is why mixed-alloy assemblies rarely match.

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  Oxide grows from the metalNo plated layer to peel; the film is part of the substrate.
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  Half in, half outA 10 μm film adds roughly 5 μm per side to the part.
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  Topography survivesBlast texture and polish gloss both show through clear anodize.
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  Alloy sets the color6061 and 7075 do not match under the same clear bath.

The sequence is blast or polish, then anodize, then seal. You cannot polish after anodizing without cutting through the oxide and exposing bare metal. You cannot blast after anodizing either, for the same reason. Any mechanical work has to happen before the part enters the anodize line, which means the finish decision is locked at the point you release the part to the finisher.

If a part needs both a matte background and bright highlights, the shop masks during blasting. The raised areas stay protected, the recessed areas take the blast, and the whole part then goes through one anodize cycle. Two separate blast-and-anodize passes on the same part will not match, because the second anodize bath runs at a slightly different temperature and the film thickness will not track exactly.

Polishing before anodizing is common on cosmetic parts, but it has a limit. A mirror polish at Ra 0.05 μm anodized clear comes back slightly hazy. The oxide grows into the surface and the grain structure of the aluminum is exposed as a faint cloudiness. Engineers who expect a chrome-like result from anodized aluminum are usually disappointed. If you want a true mirror, you want bright dip or a plated finish, not anodize.

Media choice in the blast cabinet matters as much as the decision to blast. Aluminum oxide grit at 120–220 mesh gives a fine, uniform matte. Glass bead at 100–170 mesh gives a softer, rounder texture that reads as satin rather than matte. Coarser grit cuts deeper and can leave a texture that survives the anodize layer visibly, which is either the point or a problem depending on the drawing.

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  Mechanical steps come firstBlast and polish must finish before the anodize tank.
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  One anodize cycle per partSplit cycles produce mismatched film thickness and color.
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  Mask for mixed textureProtect highlight areas during blasting to keep gloss.
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  Grit sets the look120–220 mesh oxide for matte, 100–170 glass bead for satin.

Blasting removes material, but not much. On a typical aluminum part, a fine blast pass removes well under 10 μm of surface. That is fine on a cosmetic cover. It is a problem on a thin-wall housing where the wall is already at its minimum, or on a part with a printed or laser-marked feature at 1.5 mm character height that could blur at the edges. Blast before you mark, or mask the marked area.

Polishing is worse for geometry. A polished fillet is a rounded fillet. A polished edge is a softened edge. If the drawing calls a sharp corner at a mating surface, polishing will round it by 20–50 μm and the fit will change. Specify polish only on cosmetic faces and keep functional datums as machined, or accept that the polish step will move your edge condition.

Anodize has its own geometric limits. Deep blind holes and narrow slots anodize unevenly because the electrolyte cannot circulate. A Ø2 mm hole that is 10 mm deep may show a thinner, lighter film at the bottom. Threads anodize too, and a 10 μm film on a 1/4-20 thread tightens the fit noticeably. Most shops mask threads or re-tap after anodize. Ask which one your supplier does, because the two options give different thread class results.

Color matching is the boundary most buyers hit late. Dyed anodize is a bath process, and the dye concentration drifts. Two batches of the same part, same alloy, same bath, can differ by a few units of L*. If your assembly has visible adjacent parts from different batches, specify a tolerance band and expect to sort. Clear anodize has the same issue at a smaller scale because of alloy and film thickness variation.

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  Blast moves the surfaceUnder 10 μm on a fine pass, enough to soften laser marks.
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  Polish rounds edgesExpect 20–50 μm of edge softening on polished faces.
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  Deep features anodize thinBlind holes and narrow slots get a lighter, thinner film.
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  Threads growMask or re-tap after anodize to hold thread class.

6061-T6 is the default for anodized aluminum parts. It takes a uniform clear film, dyes predictably, and the T6 temper holds through the anodize bath because the process runs near room temperature. If your part is 6061 and the finish is clear anodize over a fine blast, the result is repeatable batch to batch.

7075 behaves differently. The zinc and copper content make the anodic film slightly darker and less uniform under clear anodize. Dye covers this better than clear does. Hardcoat on 7075 is common on wear surfaces, and the darker tone is accepted because the application is functional rather than cosmetic.

2024 is the difficult one. Its copper content causes the anodic film to appear blotchy, especially after polishing, and some shops refuse to guarantee color on 2024 clear anodize. If a part must be 2024 for strength and must be anodized, plan for a dyed finish or accept a visible variation.

Cast alloys behave differently again. ADC12 and similar die-casting alloys have silicon and porosity that show up as mottled patches after anodize. Sandblasting hides some of it, polishing makes it worse. On castings, a matte blast plus a dark dye is the most forgiving combination.

On stainless and steel, the same three words mean something else. Blasting and polishing still apply. Anodize does not, because those metals do not grow a useful anodic oxide in a sulfuric bath. Passivation or plating replaces it. If a drawing says sandblasted polished and anodized but the material is 304 stainless, the anodize callout is wrong and someone should catch it before the parts ship.

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  6061-T6Most predictable; clear and dyed both hold well.
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  7075Darker under clear; dye hides the tone shift.
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  2024Blotchy under clear anodize; use dye or accept variation.
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  CastingsSilicon and porosity show; matte blast plus dark dye is safest.

Ra is the first number to check, and it is measured before anodizing, not after. A blasted surface typically lands at Ra 1.6–3.2 μm. A satin polish lands at Ra 0.8–1.6 μm. A fine polish can reach Ra 0.2–0.8 μm. The anodize layer adds a thin oxide over that profile and changes the reading slightly, so the incoming Ra is the one to hold the shop to.

Film thickness is the second number. A standard sulfuric anodize runs 5–15 μm on cosmetic parts, and hardcoat runs 25–50 μm on wear surfaces. Thickness is usually checked with an eddy-current gauge on a flat, accessible face. Reading inside a slot or on a curved surface gives a low number that reflects the gauge, not the process. Agree on the measurement location before the run.

Visual inspection under controlled light is the third check, and it is the one that causes the most arguments. Color variation across a batch is normal within a stated band. Scratches, water spots and dye bleed at masked edges are not. If the part is a visible cosmetic cover, define the inspection light source and the viewing angle in the purchase order. Two inspectors with different lamps will disagree otherwise.

On functional parts, check the dimensions that the anodize layer moved. A 10 μm film adds about 5 μm per side, so a Ø20.000 mm bore becomes roughly Ø19.990 mm. If that bore is a press fit, the change matters. Either machine the bore oversize before anodize or mask it. This is a drawing decision, not a finishing decision.

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  Ra before anodizeBlast 1.6–3.2 μm, satin 0.8–1.6 μm, fine polish 0.2–0.8 μm.
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  Film thickness5–15 μm standard, 25–50 μm hardcoat.
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  Measure on a flat faceEddy-current gauges read low in slots and on curves.
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  Recheck critical boresA 10 μm film shrinks a bore by about 10 μm on diameter.