Ask two machinists how to finish the same 6061 aluminum enclosure and you will get two different answers before either one checks the drawing. One reaches for Type II anodizing, betting on corrosion resistance and a clean satin look. The other wants to bead blast it, betting on a uniform matte texture that hides every tool mark. Both are defensible, and both can be wrong. Anodizing grows a hard aluminum-oxide layer into and out of the surface; it changes dimensions, resists corrosion, and takes dye. Bead blasting does none of that. It peens the surface with fine glass or ceramic media, dulling reflections and erasing machining marks while leaving the metal bare underneath. The right choice depends on what the part must survive, how it has to look, and how much dimensional change the tolerance can absorb. This article lays the two processes side by side so you can decide without a phone call.
What anodizing actually does to the surface
Anodizing is not a coating in the paint sense; nothing is deposited on the part. It is an electrochemical process that converts the aluminum surface itself into a controlled layer of aluminum oxide. The part is hung on a rack, made the anode in an acid bath — most often sulfuric acid — and current is run through it. Oxygen released at the surface reacts with the metal to build a porous oxide layer that is harder than the base aluminum and grows both inward and outward from the original surface.
That two-direction growth is the detail that trips up tolerances. Roughly half the oxide grows outward and half penetrates inward, so a finished coating that measures 10 μm thick sits about 5 μm proud of the original surface on each face. On a 25 mm cube that means the part comes back about 10 μm larger per dimension. It sounds small, and for loose work it is. For a bearing bore or a press fit it is the difference between a part that assembles and one that does not.
Because the layer is porous, it drinks up dye before it is sealed. That is why Type II anodizing can be delivered in near-black, red, blue, gold, and dozens of other colors, and why the color holds rather than flaking off the way paint can. Sealing — usually in hot deionized water or a nickel-acetate bath — swells the pores shut and locks in both the dye and the corrosion resistance.
Alloy choice matters more than most buyers expect. Wrought alloys such as 6061 and 6063 anodize cleanly and evenly. High-silicon castings are a different story: the silicon does not oxidize the same way, and the coating comes out gray, blotchy, and less protective. If a cast part needs to look like a machined 6061 part, it usually will not, and that conversation is better had before the first rack goes in the tank.
Type II versus Type III, and why the thickness matters
Anodizing splits into two broad grades that are quoted and priced very differently. Type II, the decorative grade, builds a layer typically 5 to 15 μm thick. It is what most people mean when they say anodized. Type III, often called hardcoat, builds 25 to 100 μm and is specified when the part will see sliding wear or abrasion.
The difference shows up fastest in the dye. Type II holds vivid colors across the full palette because its pores are large and regular enough to accept dye evenly. Type III's layer is denser and its pores smaller, so hardcoat rarely dyes past dark gray, black, or olive tones. If the drawing calls for a bright red face and hard wear resistance in the same breath, the two requirements are in tension and one of them usually has to give.
Hardcoat also moves the dimensions more, and it is far less forgiving about what it does to a surface. A 50 μm hardcoat raises each face by roughly 25 μm, which is why hardcoated bores and threads are routinely masked or machined oversize before coating. For most consumer and general-industrial parts, Type II is the default; Type III gets pulled out when a sliding surface, a wear edge, or a high-cycle contact is on the print.
Bead blasting: media, pressure, and what it changes
Bead blasting is mechanical, not chemical. A stream of media is accelerated at the part, and the impact deforms the surface into a fine, even texture. The most common media for aluminum is glass bead, run at 40 to 80 psi; ceramic and fine aluminum-oxide grits are used where a rougher key or a faster cut is wanted. The result is a matte, satin-gray finish that hides tool marks, minor scratches, and the visual chatter that anodizing tends to reveal rather than hide.
Three things decide the result: media size, pressure, and standoff. A finer bead at lower pressure gives a uniform haze that barely changes a machined surface; a coarser grit at higher pressure cuts a more aggressive texture and can round sharp edges and knock the peaks off fine detail. Because it is impact rather than growth, bead blasting does not add thickness the way anodizing does. If anything it removes a few microns of material, which is why we blast before final measurement on anything with a tight callout.
The catch is what bead blasting does not do. It provides no corrosion protection on its own. It leaves a bare aluminum skin that will oxidize naturally, unevenly, and often within days, picking up fingerprints and blotches. That is why a bead-blasted part is so often anodized afterward: the blast establishes the texture, and the anodize seals it.
Corrosion, appearance, and the finish that actually survives
The fastest way to choose between the two is to ask what the part will be exposed to. Anodized aluminum resists corrosion well; a properly sealed Type II coating will survive hundreds of hours in neutral salt spray, which is why it shows up on marine hardware, outdoor enclosures, and anything that lives near salt air. A raw bead-blasted part has no such defense. Bare aluminum forms a thin natural oxide that protects it from mild indoor environments and nothing else.
Appearance is the other axis, and it is where most buyers actually make the call. Anodizing delivers a clean, directional satin sheen with the option of color. Bead blasting delivers a uniform matte that reads as more industrial and is excellent at concealing the cosmetic fingerprints of machining. If the part is a consumer housing where consistency across a thousand units matters, anodizing's predictability usually wins. If the part is an internal bracket that only an engineer will ever see, bead blast is often enough.
When a design wants both — the matte texture and the corrosion resistance — the two processes are not competing; they are sequenced. Bead blast first to establish the texture, then anodize to seal and color it. The blast roughens the surface just enough that the anodize still forms a complete layer, and the end result is a matte, colored, corrosion-resistant part that neither process alone can produce.
Dimensions and tolerances: where the two finishes collide
If the drawing carries tight tolerances, finishing is not a cosmetic afterthought; it is part of the tolerance budget. Anodizing grows the part, and the growth is not perfectly uniform. A hole that measured Ø10.000 mm before coating will shrink toward Ø9.985 mm after a 15 μm Type II layer, because coating builds on both sides of the bore. Threads are worse: a Class 3 fit can become a no-go after anodize if the pitch diameter was not machined with coating allowance or the threads were not masked.
Bead blasting works the other direction and by a smaller amount, but it has its own hazard on fine geometry. Impact rounds sharp corners, dulls knife edges, and can erode tiny features such as fine lettering or a 0.2 mm wall if the operator is not careful with pressure. On a robust bracket the effect is negligible; on a delicate fin or a sealing surface it is not.
"We have lost more parts to unmasked threads than to any other finishing call. Anodize shrinks a tapped hole, and nobody notices until the customer's bolt will not start. Mask the threads and bill the five minutes."
The practical rule is to budget for the finish in the print, not after the part is cut. Mask threaded holes, bores, and sealing faces before anodizing, and specify the coating thickness so the machinist can leave the right allowance. For bead blasting, call out any edge or feature that must stay sharp so it can be protected.
Cost, lead time, and what each finish is actually worth
On cost, bead blasting is the cheaper process and it is not close. It is a few minutes per rack with inexpensive media and no chemistry to maintain, which is why it is the default when the spec only says "break the edges and give it a matte finish." Anodizing costs more because it is a batch chemical process: the part is racked with titanium contacts, run through tanks, dyed if color is required, and sealed. Type III hardcoat is the most expensive of the three, driven by longer cycle time, tighter current control, and more masking.
| Parameter | Bead Blast | Type II Anodize | Type III Hardcoat |
|---|---|---|---|
| Process type | Mechanical impact | Electrochemical | Electrochemical |
| Layer result | Removes 2–5 μm | Adds 5–15 μm | Adds 25–100 μm |
| Dimensional change | −2 to −5 μm per surface | + half coating per surface | + half coating per surface |
| Surface look | Matte gray | Satin, dyable | Dark, limited colors |
| Corrosion resistance | Poor (bare metal) | Excellent (sealed) | Excellent |
| Wear resistance | None | Good | Excellent |
| Typical Ra after | 1.6–3.2 μm (rougher) | Smooths slightly | Smooths slightly |
| Relative cost | $ | $$ | $$$ |
| Lead time | 1–2 days | 3–5 days | 5–7 days |
The numbers are order-of-magnitude for a typical 100-piece aluminum job, and they move with geometry, rack size, and color. What matters less than the exact figure is the shape of the comparison: bead blast is fast and cheap but protects nothing; Type II anodize costs a modest premium and buys corrosion resistance and color; hardcoat is the serious option for wear, and it charges accordingly.
Choosing between them without guessing
Walk the decision in this order and most of the ambiguity disappears:
- Corrosion or outdoor exposure. Anodize — Type II for general use, hardcoat if wear is also on the table.
- Tight bores, threads, or press fits. Mask them before anodize, or choose bead blast if corrosion is not required.
- Need a specific color. Type II anodize; hardcoat will not hit bright or pastel tones.
- Need a matte, uniform texture. Bead blast, alone or before anodize.
- Sliding or abrasive contact. Type III hardcoat.
- Cosmetic internal part on a tight budget. Bead blast.
- Sharp edges or fine features that must survive. Bead blast with masking, or skip blasting entirely.
The practical takeaway: finishing is a design decision, not a checkbox at the end of the quote. Decide the finish before the part is machined so tolerances, masking, and coating allowance are built in — not patched over afterward.
Whatever finish the drawing calls out, the options live on our surface finishing page — anodizing, plating, passivation and bead blasting, all inspected after coating.