Uniform Brushed Cast Aluminum Without Galling

Cast aluminum can look beautifully industrial when it wears a clean, linear brushed finish. Getting there, however, isn’t as simple as “sand until it looks right.” Aluminum loads abrasives, smears when it gets hot, and can gall—welding tiny bits of metal into the paper—leaving streaks and random scratches. This guide gives you a dependable, low-risk method to turn rough or blotchy castings into a consistent brush without tearing up the surface or chasing swirl marks for hours.

Why sanding aluminum (the right way) matters

Unlike steel, aluminum is soft and ductile. Push too hard with a coarse grit and the metal plows instead of cutting cleanly; stay too long in one area and heat rises, encouraging galling and slurry that drags across the surface. The cure is simple: firm backing, controlled pressure, frequent sheet changes, and light lubrication—all paired with a grit sequence that removes texture just fast enough, then refines to the final linear grain.

Tools & supplies

• Hard, flat sanding block (phenolic or steel) sized to your workpiece
• Silicon carbide wet/dry sandpaper sheets: 180, 240, 320 (optionally 400)
• Light lubricant: water with a drop of dish soap, or a few drops of mineral spirits
• Masking tape to protect crisp edges and adjacent features
• Pencil/marker for witness coats and arrows (grain direction)
• Bright raking light and a straightedge
• Shop towels, soft brush, and a vacuum
• Gloves, eye protection, and a dust mask or respirator

Recommended grit sequence (cast aluminum brushing)

• Leveling & texture knockdown: 180 grit on a firm block to remove casting sand texture and machining rings without digging lows.
• Refining: 240 grit to erase 180 lines and blend the field.
• Final brush: 320 grit (optionally 400 for a tighter, finer grain) in a single, consistent stroke direction.

Step-by-step

1. Map the surface. Clean the casting, then use a pencil or layout dye to haze the area. Draw arrows in the direction you want the final grain to read. These witness marks make it obvious when you’re flat and help you keep the final lines straight.
2. Protect edges and features. Mask crisp breaks, logos, and holes you don’t want to roll over. On thin fins or ribs, add a couple of wraps of tape so accidental pressure doesn’t blunt them.
3. Level with 180 grit on a hard backer. Lightly mist with your chosen lubricant and sand in long, overlapping strokes that ride the natural plane. Keep pressure even and the block flat—no tipping onto corners. Swap sheets early; if the cut slows or you feel drag, change immediately. For reliable stock that stays consistent on larger castings, use 180 Grit Sandpaper (25-pack) and keep several fresh sheets within reach.
4. Check flatness, not just shine. Vacuum, re-mark the surface, and sight under a raking light. If witness color remains in isolated spots, you have lows; resist poking at them with finger-backed paper. Instead, widen your passes so pressure spans highs and lows equally—this keeps the plane true.
5. Refine to 240 to erase the 180 pattern. Change direction slightly (e.g., diagonal) so remaining 180 lines stand out. Use a light lubricant, keep the block flat, and stop the moment the 180 pattern is gone. For continuous work without mid-project shortages, reach for 240 Grit Sandpaper (50-pack)—a practical fit when you’re processing multiple parts.
6. Set the final brush with 320 in straight lines. Align your strokes with the arrows you established earlier and make long, even passes end to end. Overlap slightly so the grain reads continuous. If the piece is large, complete it in zones, feathering overlaps so they disappear. Finish consistency is easiest when every sheet cuts the same; the 320 Grit Sandpaper (100-pack) keeps your finish uniform from the first panel to the last.
7. De-slurry and inspect. Rinse or wipe away any gray slurry and dry with a clean towel. Under raking light, you should see a uniform, linear 320 scratch with no random swirls or shiny islands.
8. Optional: Tighten the grain. If you want a finer brush, make a brief follow-up with 400 in the same straight direction. Avoid circular motions—random arcs are what make brushed aluminum look amateur.
9. Clean for coating. If you plan to paint or clear-coat, degrease after brushing and coat promptly. For bare aluminum, finish with a non-chloride cleaner and handle with gloves to prevent prints.
10. Record what worked. Note lubrication choice, stroke count, and grit stops. Castings vary; a short log makes repeat jobs faster and more predictable.

Special cases

Porous or pitted castings: Don’t chase pits with aggressive local sanding—it makes the surface wavy. Flatten the overall plane through 180→240, then decide whether a light filler or a slightly coarser final brush (240) looks more consistent.
Curved housings and radiused edges: Use a slightly compliant hard backer (thin cork under a phenolic block) to follow the arc while keeping pressure distributed. Finish strokes should run in the final grain direction around the curve, not across it.
Machined rings or milling marks: If rings remain after a normal 180 pass, re-ink and take longer, full-width strokes with the block bridged across multiple rings. Avoid narrow fingertip sanding—it digs channels between rings.
Heat buildup: If the surface feels warm or the slurry darkens, stop and cool the part. Heat encourages galling and can smear softened aluminum across your scratch pattern.

Pro tips

• Keep pressure light and even. Let the grit cut; heavy hands smear aluminum and bend thin features.
• Change sheets early. A loaded sheet drags and seeds random scratches.
• Lubricate sparingly. You want a slick glide, not a puddle. Too much liquid floats swarf and leaves streaks.
• Finish with straight, end-to-end strokes in the final grit only. Cross-hatching at earlier grits is fine; the last passes should all run the same way.
• Work in zones, then blend overlaps with two or three feather-light passes so the grain looks continuous.
• On parts that will be coated, stop at 320 for most primers; scuff 320–400 lightly again after primer surfacer for show finishes.

Aftercare

• If leaving bare, clean periodically with an aluminum-safe cleaner; avoid chlorides that can stain.
• Handle with gloves to prevent fingerprints that oxidize into uneven patches.
• For outdoor parts, consider a clear protective coat after brushing to slow oxidation.
• If scuffs appear later, re-align with a few straight 320 passes rather than random hand buffing.

FAQs

• Why not start at 80 or 120 to go faster? Coarser grits cut deep, making it harder to erase tracks on a soft metal like aluminum. 180 balances speed and control on most castings.
• Do I have to wet sand? A little lubrication reduces loading and keeps the scratch crisp. Many pros use a damp, soapy mist or a few drops of mineral spirits for glide.
• What direction should the final grain run? Whichever you choose, keep it consistent and straight—usually along the part’s longest dimension.
• Can I use a power sander? Yes—with a firm interface pad and very light pressure. Still finish the last few passes by hand in straight lines to align the grain.
• My finish looks streaky. That’s usually old slurry dragged into new passes or mixed stroke directions. Clean between grits, lube sparingly, and finish with straight, uniform 320 strokes.

Video

Closing

Brushed cast aluminum rewards discipline: firm backing, light pressure, fresh sheets, and strokes that run straight and true. Move through 180→240→320, keep things cool and clean, and you’ll land a uniform grain that looks intentional and professional—without the galling, streaks, or low spots that plague rushed jobs.