Bead Blasting Before Anodize: What It Fixes and What It Hides

August 24, 2026

Bead blasting is the quiet workhorse of audio finishing. It is cheaper than brushing, it is not directional so it never has a grain to align, and it hides more machining evidence than any other pre-treatment. It is also badly specified more often than not.

Media changes everything

The word “blasting” covers processes with completely different results.

Media	Effect	Typical use
Glass bead	Rounded, peening action. Soft satin, slightly bright	The default for cosmetic aluminum
Aluminum oxide	Angular, cutting action. Coarser, flatter matte	Stripping, heavy texture, pre-paint
Ceramic bead	Rounded, harder than glass. Fine, dense finish	Premium consumer products
Walnut shell or plastic	Very gentle. Cleans without texturing	Delicate parts, paint removal
Soda	Non-abrasive	Cleaning only

Glass bead is what most people mean. Aluminum oxide on a front panel gives a harsh, grey, flat look that rarely suits hi-fi, and it can embed angular particles in soft aluminum which then show as dark specks after anodizing.

Specify the media. If you only say “bead blast” and the shop has an aluminum oxide cabinet running, that is what you will get.

Grit size

Media size is usually given as a mesh range, for example #80 to #120 for glass bead. Smaller number means larger bead means coarser finish.

Coarse (roughly #40 to #80): obvious texture, industrial, hides a lot
Medium (roughly #80 to #120): the common cosmetic range, soft even satin
Fine (roughly #120 to #220): smooth, almost silky, closest to a polished matte

MIL-STD-1504 and similar standards exist, but in practice a mesh range plus a reference sample is what makes a batch repeatable.

The variables the shop controls

Even with the same media and grit, these change the result:

Pressure. Higher pressure means a deeper, coarser texture and more risk of edge rounding.
Nozzle distance and angle. Closer and more perpendicular means a more aggressive local finish. This is why hand blasting can leave a lighter or darker patch.
Media condition. Glass bead breaks down with use and becomes angular. Tired media gives a different, harsher finish than fresh media. Ask how often it is changed.
Dwell time. Over-blasting rounds sharp edges and softens crisp chamfers. On a panel where the chamfer is the design, this matters.
Automatic versus manual. A cabinet with an oscillating nozzle and a turntable is far more repeatable than a person with a gun, especially across a batch of 100.

For a product you will reorder, ask for automatic blasting and write it on the drawing.

Edges and chamfers

Blasting rounds sharp edges. Usually that is welcome, since it removes the sharpness of a machined edge without a separate deburr operation. On a part where the crispness of a chamfer is the whole point, it is a problem.

If you want the edge to stay crisp:

Say so on the drawing: “do not over-blast, chamfer edges to remain crisp”
Consider masking the chamfer, though masking lines show after anodize
Consider machining the chamfer after blasting, which leaves a bright machined chamfer against a matte face. This is a deliberate and very attractive look on some amplifiers, and it needs to be drawn, not left to chance

Blasting as a matching tool

This is the most useful property of blasting and the reason it appears on so many mixed material products.

A polished or as-machined surface reflects light specularly, so every difference in alloy, mill lot and surface condition is visible. A blasted surface scatters light diffusely, which flattens all of those differences.

If your product mixes a 6061 billet panel with a 6063 extruded sleeve and a 5052 sheet cover, blasting all three to the same media and grit before anodize will do more for the match than anything you can ask the anodizer to do.

Where blasting is wrong

On a face that needs to seal. A blasted surface is rougher, so an O-ring or a gasket face should be masked or machined after.
On a bearing or a sliding fit. Blasting changes the surface and can embed media.
Inside a threaded hole. Media gets in and stays in. Mask threads or blast before tapping.
On acrylic or polished brass. Different processes entirely.
When you want the part to look machined. Some designs want the tool path visible. Blasting erases it.

The drawing note

FINISH: GLASS BEAD BLAST, #80-120 MESH, AUTOMATIC CABINET
UNIFORM MATTE, NO DIRECTIONAL PATTERN, NO PATCHINESS
DO NOT OVER-BLAST: 0.5 x 45 CHAMFERS TO REMAIN CRISP
MASK ALL TAPPED HOLES
THEN ANODIZE PER MIL-A-8625 TYPE II CLASS 2 BLACK, 10-15 MICRONS
APPEARANCE TO MATCH APPROVED SAMPLE

That note, plus a sample in the shop’s hands, gets you the same enclosure next year.

Frequently asked questions

Does bead blasting change my dimensions?

Glass bead peening at normal pressures removes almost nothing, typically under 5 microns, so it does not affect ordinary tolerances. It does work-harden and very slightly stress the surface, which can move a thin flat part. On a panel thinner than about 3 mm, blast both sides or expect a small bow.

Why does my blasted part look darker after anodize?

A matte surface scatters light and holds more dye in the increased surface area. The same black dye on a blasted part reads deeper than on a polished one. That is normal and it is one reason blasting is so useful when parts in different alloys must match.

Can blasting remove tool marks?

It removes the shine that makes them visible, not the geometry. Fine tool paths disappear. A step, a gouge or chatter stays and now has a matte texture on it. Blasting is a leveller of appearance, not of surface.

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Keep reading

This guide is part of Finishing Audio Parts: Anodize, Brush and Blast, one of four sections in the guide library.