Remove Mill Finish From Aluminum Extrusions Without a Patchwork Grain

Define the finish before choosing a grit

“Mill finish” is not one defect. An extrusion may carry lengthwise die lines, handling scratches, oxide, lubricant residue, or simply an uneven sheen. Decide whether the target is paint preparation, a uniform satin grain, or a dimensionally critical bare surface. Those outcomes do not share one automatic grit sequence.

First clean a small area with a method compatible with the alloy and the next coating. Sanding through oil can drive residue across the work and load the sheet. Mark deep handling damage, edges, stamped areas, and mating faces before abrasion. If a scratch is too deep to remove without changing fit or wall thickness, accept it, blend a wider area only if the drawing permits, or reject the part instead of digging a hollow.

For a directional cosmetic finish, make a sample from the same extrusion lot. The straight-grain jig guide explains how a fence and supported stroke help keep brush lines parallel. If the final goal is a coating rather than visible grain, use the coating maker’s required profile and the broader mild-steel preparation guide only as a process comparison; aluminum needs its own cleaner, primer, and timing.

Use two ladders: removal and refinement

A grit ladder should be driven by the deepest mark that must disappear and the scratch pattern the next operation can accept.

A useful shop ladder might begin with 120 only when a coupon proves that the cut is necessary, then progress through intermediate grades before 320. Do not jump directly from 120 to 320 and expect the fine sheet to erase every coarse line efficiently. A shallow cosmetic job may begin at 220 or 320 instead. Grit labels describe abrasive size, not an entitlement to remove a fixed amount of metal.

For a coarse test where 120 grit is suitable, the 120-grit ten-sheet option offers a cut-to-fit pack. For final refinement where 320 is specified, use 320-grit sheets and compare available quantities in the 320-grit collection. The 120-grit collection is useful only after the coupon shows that grade belongs in the process.

Control feed rate by overlap, not a magic speed

Hand feed rate is the distance traveled per stroke divided by time, but a stopwatch alone does not control the result. Pressure, block length, abrasive condition, stroke overlap, lubrication, and contact area all change the cut. Establish a repeatable stroke on the sample instead:

1. Support the abrasive on a flat block long enough to bridge minor local variation without rocking over the profile.
2. Move parallel to the extrusion, using light pressure and full-length strokes where the part allows.
3. Overlap each track consistently. Do not scrub a stubborn line in place.
4. Stop after a fixed small number of passes, clean the sample, and inspect it under low-angle light.
5. Adjust one variable—grit, pressure, overlap, or travel rate—before the next test.

If fast travel leaves disconnected shiny islands, increase overlap or reduce travel slightly. If slow travel creates heat, dark smearing, or a hollow, reduce pressure and dwell, refresh the abrasive, and clear swarf. A fresh sheet that cuts cleanly where a loaded sheet smears has identified an abrasive-condition problem, not a need for more force.

Keep a simple sample card with alloy or part number, starting condition, each grade, dry or lubricated use, passes, approximate travel time, and inspection result. That record makes “same finish” a reproducible instruction rather than a visual memory.

Protect edges and geometry

Extrusions concentrate pressure at corners, ribs, grooves, and thin lips. Mask mating faces that must not change. Break burrs with an appropriate deburring method before finish sanding; do not use the cosmetic sequence to chase a sharp projecting defect. On narrow faces, use a support that registers on that face instead of wrapping around both corners.

Measure any dimension that matters before and after the sample process. A satin appearance can hide a rounded edge or tapered flat. If the part has a protective conversion coating, anodizing, plating, or an unknown surface treatment, stop and identify it—removing that layer is a different job from refining bare mill finish.

Inspect the grain before scaling up

Rinse or wipe away swarf with a compatible method, let the surface dry, then view it in both diffuse and raking light. Rotate the part. The accepted sample should show parallel lines, no surviving cross-scratches, no dark packed-metal streaks, and no abrupt change near clamps or ends. Compare production pieces directly with that sample at the same orientation and lighting.

Scale up only after the coupon proves the complete ladder and stroke. Change the abrasive when cutting slows or metal packs onto the face. The goal is not to spend equal time everywhere; it is to leave every required face matching the approved grain while preserving the extrusion’s geometry.