Brush Cast Aluminum Without Galling (220–400 Grit)

Cast aluminum can look chalky and blotchy right out of the mold, and if you attack it the wrong way you’ll raise burrs, smear metal (galling), and end up with a patchwork of shiny streaks. The goal here isn’t mirror polish—it’s a uniform brushed satin that hides casting texture and stays consistent across parts. The trick is to keep pressure low, the backing hard, and the grit jumps tight so you cut clean chips instead of rubbing metal smooth.

Why sanding discipline matters on cast aluminum

Aluminum is soft, gummy, and heat-conductive. Coarse paper on a soft pad digs valleys that trap light and dirt; ultra-fine paper on a DA burnishes highs and leaves lows untouched. Worst of all, too much heat or pressure makes swarf weld to the surface—galling—which tears the metal and forces you to drop way down in grit. A disciplined, hard-backed ladder—typically 220 → 320 → 400—cuts fresh metal with shallow, straight scratches that read as an even brush. A light lubricant at the finer steps keeps swarf from loading and helps the scratch stay tight and consistent.

Tools & supplies

• Hard sanding blocks: long flat for faces, narrow block for flanges, tiny detail block for fillets
• Silicon carbide sheets: 220, 320, 400 (9×11 in)
• Optional file/scraper for proud gates and flash before sanding
• Raking light, soft pencil for witness marks; optional layout dye
• Lubricant at 320–400: water + drop of soap, mineral spirits, or dedicated lube (check finish compatibility)
• Masking tape for adjacent coatings, threads, and critical dimensions
• Vacuum with brush tip, microfiber towels, compatible panel wipe
• PPE: respirator/dust mask, eye protection, gloves

Recommended grit sequence

• Open & unify (220): Shear casting texture and tool marks into a single plane without gouging.
• Refine (320): Erase 220 tracks and tighten the pattern; begin to develop the final brush direction.
• Set the brush (400): Create a shallow, uniform satin that accepts clear or primer cleanly.

Step-by-step

1. De-burr and map the surface. Knock down gates, parting-line flash, and proud bosses with a sharp file or scraper. Degrease thoroughly so you’re cutting metal, not oil. Under raking light, pencil a loose grid or wipe on layout dye—witness color vanishes first on highs, telling you where to stop.
2. Open the field at 220 on a hard block. Wrap a fresh 220 sheet and make long, straight, overlapping strokes that bridge highs and lows. Keep pressure feather-light and the block fully supported—no fingertip circles. Replace sheets the instant the cut slows; dull paper rubs aluminum and heats it. Staying stocked helps you swap early instead of pressing harder; start the cut with 220 Grit Sandpaper (25-pack).
3. Read, don’t chase. Vacuum, then pull a rubber squeegee across the face. If faint dye islands remain in micro-lows, take two full-width passes with the block rather than spot-digging; distributed pressure narrows lows without carving troughs that glare after finish.
4. Refine to 320; set the brush direction. Change your stroke angle slightly (gentle diagonal) so any leftover 220 tracks pop, then return to the final brush direction for your last passes (usually lengthwise on visible faces). At this step, a thin film of lube keeps swarf from loading and reduces galling. Keep the backer hard. For predictable, cool cutting across multiple parts, move to 320 Grit Sandpaper (50-pack).
5. Detail features deliberately. For ribs, slots, and fillets, back the paper with a tiny rigid block or a split-dowel (hard) so you bridge the geometry. Never wrap paper around a finger—that dishes corners and leaves shiny arcs. Maintain the same brush direction you chose on the main face.
6. Set the final satin at 400 with light lube. One or two even passes are enough to tighten the 320 pattern into a smooth, uniform brush. Mist or wipe a thin lubricant film—damp, not soaked—to keep swarf moving. Finish consistently with 400 Grit Sandpaper (100-pack) so the last bracket sands like the first.
7. Edge protocol. Keep tape on sharp arrises through 320. For the final 400 pass, remove the top layer of tape and make a single feather-light stroke parallel to the edge with a worn sheet to unify sheen without rounding.
8. De-dust and test wetting. Vacuum with a brush tip, then panel-wipe. To preview the look, damp-wipe with your chosen solvent; the “wet” appearance should read as a single tone with no zebra bands or shiny trenches. If bands appear, re-enter at 320 (two full-width passes), then finish at 400 again.
9. Seal, clear, or prime within the window. Freshly cut aluminum oxidizes quickly. If you’re clear-coating, apply light, even coats with proper flash; for paint, follow your system’s etch/adhesion promoter or primer recommendations. Avoid heavy first coats that can undercut the brush pattern.

Special cases

Porous castings / pits: Don’t chase deep voids with coarse grits; you’ll dish the surrounding field. Spot fill after 320, block flat, then re-enter 320 → 400.
Anodized or clear-coated castings: Strip the film chemically per spec before sanding; scrubbing at it loads paper and smears. Once bare, proceed 220 → 320 → 400.
Machined + cast hybrids: Machine faces cut differently than raw cast. Keep the backer hard and stroke direction identical across both zones so the brush reads continuous.

Pro tips

• Hard backing wins. Blocks bridge highs and keep planes true; foam pads and fingertips dish lows and burnish highs.
• One direction per grit. 220 straight, 320 gentle diagonal (then straight in final direction), 400 straight.
• Change sheets early. Loaded paper rubs instead of cuts, raising heat and galling risk.
• Use lube thoughtfully. A thin film at 320–400 controls loading; flooding floats swarf and creates random scratches.
• Mind the light. Inspect under raking LEDs from two directions; bands you miss now will shout under clear or anodize-look paints.

Aftercare

• Handle fresh satin with clean gloves; fingerprints can darken uncoated aluminum.
• If you’re leaving parts bare, expect a soft oxide bloom; a light 400 pass restores the brush quickly.
• For coated parts, clean with pH-neutral soap; avoid abrasive pads that polish the brush into blotchy gloss.
• Log your recipe—grits, stroke directions, lube, and finish brand—so future parts match.

FAQs

• Why not start at 180? Use 180 only for heavy texture, then re-tighten to 220 → 320 → 400. Coarser grits raise galling risk.
• Can I do this with a DA sander? A DA with a firm pad helps on big flats, but finish each grit by hand on a hard backer to keep the brush straight and the plane true.
• Wet or dry? Dry at 220; light lube at 320–400. Keep liquids out of cavities unless you can dry immediately.
• Do pads like Scotch-Brite replace paper? Handy for tiny transitions, but on flats they leave a mixed scratch. Use hard-backed sheets for faces.
• How do I match an existing brush? Photograph the original direction/tempo, mimic it at 320, then lock it with your 400 pass.

Video

Closing

For cast aluminum that looks intentionally finished—not patchy or polished in streaks—keep the backing hard, pressure light, and the ladder tight: 220 → 320 → 400. Swap sheets before they skate, add a thin lube at the finer steps, and let raking light and witness color call the stops. Follow this plan and you’ll get a clean, repeatable brush that holds up under clear, paint, or daily handling—without the heartbreak of galling.