Anodizing, brushing, or polishing: how do you finish aluminum after you buy it?

How do you finish aluminum after you buy it?

Match the finish to the part's job. Hidden structure keeps its mill finish and nobody cares. A visible panel gets brushed with sandpaper and a Scotch-Brite pad in an afternoon. A show piece gets sanded and buffed to a mirror. Parts that need colour or a harder skin get anodized or powder coated, both usually send-out jobs.

First, the honest baseline. Every bar and extrusion on our rack arrives as mill finish: the surface the mill made, nothing added. On extrusions that means faint die lines running the length, plus the light scuffs and handling marks any real warehouse leaves. That's normal metal, not damage. A bracket, a jig, or a frame rail never needs more than that. The moment a part gets looked at instead of bolted in, pick its finish before you buy the metal, because the alloy decides how some finishes turn out. Our aluminum buying guide covers choosing the alloy; this guide covers what happens after the saw.

How do you get a brushed aluminum finish?

Sand in one direction, in even grit steps, and never wander off the line. Degrease first with dish soap or isopropyl alcohol, because sanding over oil grinds it into the grain. Then work along the part's length: 180 or 220 grit to erase the mill surface, 320 to refine it, and a maroon Scotch-Brite pad for the final passes.

Technique is most of the result. Back the sandpaper with a block so the scratch pattern stays flat. Run full-length strokes, clear past the ends, with even pressure; short strokes leave start-stop marks that catch the light. If your hand drifts, clamp a straightedge beside the part as a fence. Wipe the part clean between grits, so a stray coarse grain doesn't ride into the fine pad and plow its own trench.

The final grit sets the look. Stopping around 220 leaves a bold, toolbox-lid grain. The maroon pad (sold as very fine, roughly a 320-400 grit scratch) gives the classic appliance brush. A grey ultra fine pad softens that to satin. Whatever you choose, every stroke goes the same direction, because a single crosswise pass shows up the second light hits it. Know the trade too: brushed aluminum is bare metal, so it'll dull slowly toward grey and it shows fingerprints. Trim that lives outdoors usually gets brushed first, then clear anodized to lock the look in.

How do you polish aluminum to a mirror finish?

Climb the sanding grits, then switch to buffing compounds. Aluminum is soft, so it shines fast once the deep scratches are gone; the whole job is letting each step fully erase the one before. Sand to at least 600 grit before a buffing wheel touches the part, 800-1000 wet if you want the mirror to come easy.

Buffing compounds come in colour-coded classes. Black emery is the coarse cut, for rough or damaged surfaces. Brown tripoli is the workhorse cut for aluminum and brass. White rouge is the final colour pass. Keep one wheel per compound and label it; a wheel that's seen tripoli will haze a rouge finish forever after. Two safety habits: work at or below the wheel's centreline, so a snag throws the part down and away instead of at you, and wear sealed eye protection and a dust mask. A buffer that grabs an edge launches it hard.

Now the part nobody puts on the can: the mirror is temporary. Bare aluminum starts regrowing its oxide skin the moment you stop buffing, and a fresh mirror hazes in weeks. Paste wax or a polymer sealant, renewed every few months, holds most of the shine on indoor parts. A spray clear coat protects far longer at the cost of a little sharpness, and once the clear gets scratched, the fix is stripping the whole panel. Polished aluminum outdoors is a maintenance schedule, not a one-time event. If you want permanent bright, that's the job clear anodizing over a polish was invented for.

What does anodizing actually do, and why does 6063 take it best?

Anodizing grows the metal's own oxide skin until it's thousands of times thicker. That layer is transparent, so the alloy underneath decides the look: 6063 anodizes bright and even, 6061 runs a touch greyer. The process is electrochemical. The part hangs in dilute sulfuric acid as the positive electrode (the anode, hence the name). Current then builds hard, clear oxide straight out of the surface. Fresh from the tank the layer is porous, so it drinks dye; a hot water seal then locks the pores shut. The colour can't chip or peel the way paint does, because the layer is converted metal, not a film sitting on top.

Two grades cover almost everything. Type II is the everyday decorative anodize that takes dye; Type III hardcoat is the same chemistry run colder until the layer is several times thicker, built for wear instead of looks. Hardcoat moves dimensions: the oxide grows roughly half inward and half outward, so a 0.002″ hardcoat leaves each surface standing about 0.001″ proud. Flag threads, bores, and sliding fits to the shop. Regular Type II is thin enough that most shop fits never notice it.

The alloy story is simple chemistry. 6063 carries less copper and iron in its recipe, so the transparent oxide comes out clean and even, which is exactly why storefront and railing extrusions are 6063. 6061 anodizes fine for brackets and tool parts, but it can pull a faint grey or yellow cast and can streak across a wide visible face. The full strength-versus-surface tradeoff lives in our 6061 vs 6063 guide. Welds bring one more wrinkle: weld beads anodize darker than the metal around them. Common 4043 filler turns dark grey in the tank, while 5356 comes close to matching, and our 6061 welding guide covers picking between them. Weld first, anodize second, and tell the anodizer the part is welded.

Can you anodize aluminum at home?

Yes. Hobbyists anodize knife scales, RC parts, and bike bits every week, and the results can be genuinely good. Plain talk first, though: this is the most serious chemistry on this blog. The bath is dilute sulfuric acid, around 10-15%. A regulated DC supply pushes 12-18 volts through it for 30-60 minutes, and the tank gasses hydrogen and lifts a fine acid mist the whole time. Current control is the skill: hobby guides size the amperage to the part's surface area, and guessing buys burnt edges or no colour. Decide whether you want all that in your garage before you price the parts.

If you do it, do it like a chemist. Always add acid to water, never water to acid. Chemical gloves and sealed goggles or a face shield whenever the tank is open. Work outdoors or with a fan pulling air away from you. No kids, no pets, no exceptions. Keep baking soda open beside the tank for drips. Spent acid never goes down a drain: neutralize it and take it to your municipality's household hazardous waste depot. The lye etch most guides add is caustic enough to deserve the same respect. And budget for blotchy first parts; anodizing punishes lazy cleaning, and the first batch is where everybody learns that.

The alternative is easier than most people expect: anodizing shops take small batches. The catch is the math. Shops price by the rack and the tank run, not the part, so most carry a minimum lot charge, commonly $75-150, and one small part costs nearly as much as ten. Save parts up into one batch, and ask to ride along on a colour the shop already runs; clear and black almost always are. For one project's worth of parts, sending it out beats building the chemistry set almost every time.

When does powder coat beat anodizing on aluminum?

When you want solid, opaque colour, or the surface has history to hide. On steel, powder coat earns its keep fighting rust. Aluminum never red-rusts, so on aluminum powder is pure cosmetics plus UV protection, and that changes the comparison. Anodizing keeps the metallic look and wears harder. Powder covers in any colour the coater stocks and hides the surface underneath.

That hiding power is real. Powder lays on a solid film a few thousandths thick, so die lines and minor scratches disappear under it, where a transparent anodize would telegraph them. Chips behave differently too: chipped powder on aluminum shows silver metal, not a spreading rust stain, so touch-ups are cosmetic rather than urgent. Ask for an outdoor polyester powder on anything that sees sun, since epoxy powders chalk outside. The process is the same send-out, oven-cured story we covered for steel in the rust guide. One tip carries over: ask the coater to ease sharp edges first, because powder pulls thin exactly there.

Why does prep decide the finish you get?

Because every process on this page magnifies the surface you hand it. Anodize is transparent and thinner than a sheet of paper, so scratches, sanding swirls, and fingerprint oils telegraph straight through. A mirror shows everything by definition. The anodizer's etch (that uniform matte look) blurs fine scratches and not much more; it won't swallow a gouge or an 80-grit scar. Etch hides minor, not major.

So the routine is boring and absolute. Degrease before any abrasive touches the part. Sand the whole face to one even final grit, not just the scratch you're chasing. Once a part is prepped for the tank, handle it with clean gloves, because finger oils show up as ghost marks after anodizing.

Prep also starts at the buy. Trim and railing profiles headed for the anodizer want 6063, which we rack as angle, channel, and the tube family. Brushed and polished shop parts are usually 6061, the deepest pile on our aluminum rack, and bent or formed sheet work is 5052 or 3003. Then buy yourself a practice piece. Burning one 12″ stick learning the grit ladder is the cheapest finishing lesson there is, about $5 as of June 2026.

Type your length in inches on any product page and the price shows up, with every cut free to ±1/8″ and volume discounts of 5-15% applied automatically. Order by 1pm Atlantic on a business day and it usually ships the next one, up to 96″ anywhere in Canada. Need a profile we don't rack, or a batch cut and ready for the anodizer's next run? Send a quote request; sourcing runs 2-21 days, and you'll see the price before you commit.