How to Sand Cast Aluminum Repairs Smooth Before Powder Coating (180–800 Grit Sequence)

Cast aluminum parts—like wheels, brackets, or housings—often need filler repairs for pinholes or porosity before powder coating. If those repairs aren’t perfectly leveled, the powder will highlight every ripple and crater. Here’s how to sand aluminum repairs flush and smooth so the finish cures mirror-flat.

Why Sanding Matters Before Powder Coat

Porosity traps air and oil, which can bubble under powder when baked. Sanding ensures the filled zones blend seamlessly with the parent metal and gives the coating a uniform surface energy for adhesion.

Recommended Tools

• Die grinder or rotary tool with carbide burr (for initial leveling).
• Rigid sanding block or flat file.
• Dual-action (DA) sander for large castings.
• Aluminum-compatible filler (epoxy or TIG weld).
• Tack cloth and compressed air.

Best Grit Sequence for Leveling Aluminum Repairs

• 180 grit: flatten filler high-spots and rough-shape the repair.
• 320 grit: blend filler to parent metal without gouging.
• 800 grit: final smoothing before chemical cleaning and powder.

Step-by-Step: Flush-Sanding Porosity Repairs

1. Inspect the casting. Circle any pits, voids, or weld spots with a marker. Clean oil and oxide off using acetone.
2. Fill defects. Apply aluminum-filled epoxy or weld material. Let it cure or cool completely.
3. Level with 180 grit. Sand across the repair using a rigid block so pressure stays even. Stop when you just kiss surrounding metal.
4. Blend using 320 grit. Feather the transition zone until the surface feels continuous to the touch.
5. Refine with 800 grit. Use wet/dry sheets and light water mist to remove micro-scratches that can telegraph through powder.
6. Clean thoroughly. Blow out all dust and wipe with solvent—no residue or fingerprints should remain.
7. Apply pre-treatment. Perform a light etch or conversion-coat rinse if required by your powder system.
8. Powder and bake. Follow manufacturer’s cure schedule; a uniform matte gray undercoat means sanding prep was ideal.

Special Cases

For curved or finned castings, wrap sandpaper around a foam pad so it conforms without rounding edges. For deep pockets, use narrow sanding sticks to maintain control.

Pro Tips

• Avoid coarse grits below 150—they dig into soft aluminum fast.
• Always sand perpendicular to grinder marks before progressing finer.
• Use compressed air—not rags—to clear abrasive dust from pores.

Aftercare

• Handle bare aluminum with gloves—skin oils cause fisheyes in powder.
• Store parts in a dry area before coating.
• Inspect under raking light; any shadow means more leveling needed.

FAQs

• Can I skip 800 grit if I’m priming first? Not recommended—primer won’t hide sanding scars under powder heat.
• What filler sands best? Metal-filled epoxies stay crisp and resist clogging better than polyester putties on aluminum.
• Will sanding expose porosity again? Sometimes—repeat fill/sand cycles until surface stays closed after 320 grit.

Watch & Learn

Outfit your bench: keep coarse, medium, and fine grits on hand—each helps bring aluminum repairs to a powder-ready finish.