Uniform Brush on Cast Aluminum: 180–240–360 Grit Guide

Cast aluminum can swing from silky to chalky in the same part: dense chill lines, sandy texture, and machining marks all live side by side. Getting a uniform brush across those changes without gouges or “greasy” streaks is about control, not force. This step-by-step shows how to decontaminate, block-sand with the right grits, and avoid galling—when aluminum smears and clogs your paper—so the surface reads as one consistent satin from every angle.

Why sanding strategy matters (and how to avoid galling)

Aluminum is soft, ductile, and heat-sensitive. Press too hard or use clogged paper and you’ll smear metal (gall), leaving dark, gummy streaks that drag across the surface. Jump too coarse and you carve troughs that print forever; jump too fine and you’ll simply polish highs while lows stay rough. A tight 180 → 240 → 360 progression, kept flat on a hard backer with light lubrication, shears peaks cleanly, refines quickly, and leaves a linear brush that looks intentional.

Tools

• Hard sanding blocks (phenolic/Delrin/steel) in long and narrow sizes; a curved hard backer for fillets and radii
• Silicon carbide wet/dry sheets: 180, 240, 360 (400 optional for extra-fine brush)
• Lubricant: clean water + a drop of dish soap in a spray bottle; for stubborn galling, a little mineral spirits
• Degreaser (wax & grease remover), raking light, pencil for witness marks, and a squeegee
• Guide coat (layout dye or a very light contrasting mist) to reveal lows and machining tracks
• Masking tape for crisp edges, holes, and adjacent materials (plastic, paint)
• Vacuum/air with moisture trap, clean microfiber towels
• PPE: respirator, gloves, eye protection—aluminum fines are real dust

Recommended grit sequence

• Level & unify: 180 grit to break casting skin, knock machining marks, and get the field into one plane.
• Refine: 240 grit to erase 180 lines and remove faint porosity roughness.
• Set final brush: 360 grit to leave a tight, even satin that hides handling and looks premium.

Step-by-step

1. Decontaminate first. Degrease thoroughly—oils make paper skate and smear. If the casting carries shop polish or oxide, clean it now. Mask 1–2 mm shy of edges, logos, and interfaces so you don’t round them while leveling flats.
2. Map the surface with witness marks. Under a raking light, lightly scribble a pencil grid and add a whisper of guide coat. The pencil tells you when an area is truly addressed; the guide coat reveals untouched lows and machining arcs you need to level.
3. Establish flatness at 180—with lubrication. Mist a light soapy spray and sand with 180 on a hard block using long, straight, overlapping strokes. You’re shaving peaks, not grinding. Swap sheets the instant the cut slows to avoid galling. For predictable stock at this opener, reach for 180 Grit Sandpaper (25-pack) and change early—fresh, sharp grains cut cool and clean.
4. Squeegee, inspect, and avoid chasing lows. Pull slurry with a clean squeegee and read the guide coat. If narrow dyed troughs remain, resist poking at them with fingertips—two or three full-width passes keep the plane true and prevent dishes.
5. Refine to 240 to erase the 180 pattern. Change stroke direction slightly (e.g., lengthwise at 180, gentle diagonal at 240). Keep the backer rigid and pressure feather-light. Stock a mid-quantity box like 240 Grit Sandpaper (50-pack) so every station has fresh sheets and you never push a dull, clog-prone piece.
6. Handle curves and fillets with a curved hard backer. Wrap paper around a split dowel or machined radius block so pressure stays distributed. Keep strokes aligned with the part’s length so the final brush “flows.”
7. Set the final brush at 360. Make one or two even, straight passes until 240 lines vanish into a tight satin. Align this last pass with the grain direction you want to keep across the whole part. For consistent results on batch work, finish this step with 360 Grit Sandpaper (100-pack) so the last casting matches the first.
8. Detail edges and stamped marks last. Pull edge tape and with a worn 360 on a tiny hard backer, make two feather-light strokes along edges and around embossing. Crisp beats rounded; don’t freehand with fingers.
9. Clean and preview. Rinse or wipe away slurry, dry, and wipe a bit of mineral spirits to “wet” the grain. If streaks show, you likely polished in arcs; re-enter with two straight, full-length passes at 360.
10. Seal or finish (optional). Bare aluminum fingerprints easily. If the part isn’t going to be painted, consider a clear compatible with aluminum (lacquer, 2K urethane, or wax) applied thinly so you don’t bury the brush.

Special cases

Die-cast vs. sand-cast: Die-cast is denser and smoother—often you can start at 240, then 360. Sand-cast has open pores and “tooth”; stay at 180 a touch longer, but keep pressure light so you don’t open new lows.
Heavy machining marks: If 180 barely moves the grooves, you can briefly test 150 on flats only, then re-enter 180 → 240 → 360. Stop the instant the groove tips are gone—deeper scratches cost time later.
Polish-ready parts: If you’re heading to mirror, continue 400 → 600 → 1000+ after 360 and switch to compounds. Keep everything straight and backed hard or waves will telegraph at high gloss.
Mixed materials (aluminum next to paint/plastic): Mask generously. Keep the block mostly on aluminum; remove tape for a single, light 360 pass to unify sheen right at the boundary.
Contamination from steel tools: Avoid steel wool and cross-used media—iron contamination can create dark streaks and future corrosion. Dedicate abrasives to aluminum.

Pro tips

• Hard backing wins. Foam and fingertips dish lows and create shiny arcs that catch light.
• One direction per grit. Straight at 180, gentle diagonal at 240, straight again at 360—leftover scratches become obvious and removable.
• Change sheets early. A loaded sheet skates and smears (galls). Fresh paper cuts cooler and flatter.
• Work cool. If the surface feels warm, pause. Heat accelerates galling and can “grease” the scratch.
• Use a guide coat. Re-dust between grits and stop the moment witness color disappears evenly—extra strokes only polish and risk dishes.
• Keep lube light. A mist is enough. Puddles hide lows and float the sheet.
• Establish the “grain” direction early. Plan your final 360 strokes so the brush runs the way the eye travels on the part.

Aftercare

• For bare parts, apply a thin clear or microcrystalline wax to slow fingerprints and oxidation; buff lightly to maintain the brush.
• For painted parts, prime within the window after sanding to avoid re-contamination; epoxy primers bury a clean 240–360 scratch well.
• Clean with mild soap/water; avoid harsh alkalis and steel brushes that can gray the surface.
• If light scuffs appear later, a quick two-pass refresh with 360 along the brush direction restores the look.

FAQs

• Wet or dry on aluminum? Wet keeps temps and loading down. Use a light soapy mist; wipe and re-mist often.
• Can I finish at 320 instead of 360? Yes—320 yields a slightly coarser brush. 360 tightens the satin and hides handling a bit better.
• Why is my paper blackening fast? You’re galling—likely dull paper or too much pressure/heat. Swap sheets, add a touch of lube, and lighten up.
• Random-orbital (RO) or block? RO is fine for refining, but do the leveling and final brush with a hard block so the texture is straight and uniform.
• What about anodized castings? Don’t sand unless you intend to remove the anodize. If you must, expect to commit to a full re-finish; spot work will halo.
• Can I use scuff pads? They’re handy but inconsistent on flats and can leave swirls. Use them only for edges after the sheet work is done.

Video

Closing

A clean, even brush on cast aluminum comes from restraint: keep the backing hard, the pressure light, and your grit ladder tight—180 → 240 → 360. Work with a mist of lube, change sheets before they load, and align your final strokes with the grain you want to see. Do that, and the surface reads as one cohesive satin—no galling, no streaks, just confident, professional texture.