How to Sand Automotive Body Filler for a Seamless Paint Job (80–600 Grit Guide)

Body filler (Bondo) repairs are only as good as their surface prep. To achieve a flawless car paint finish, sanding must be precise—too rough and you’ll leave visible waves, too fine and the primer won’t adhere. This guide shows you how to sand automotive filler correctly for smooth, invisible bodywork.

Why Sanding Body Filler Is Critical

Fresh filler often shrinks slightly as it cures. Sanding levels the repair area with surrounding panels, removes high spots, and blends edges for an unnoticeable transition. The right grit sequence ensures smooth shaping without clogging paper or cutting too deep into the metal.

Recommended Tools

• Body file or sanding block with curvature matching the panel.
• Wet/dry sandpaper (80–600 grit range).
• Dual-action (DA) sander for large repairs.
• Guide coat spray to reveal low spots.
• Vacuum and microfiber cloth.

Ideal Grit Sequence

• 80 grit – Shape filler while soft for rough leveling.
• 180 grit – Refine and blend edges with surrounding surface.
• 320 grit – Smooth for primer application.
• 600 grit – Final wet-sand for flawless paint base.

Step-by-Step: Sanding Auto Body Filler

1. Shape the filler. Once the filler is firm but not fully hardened, use 80 Grit (25-pack) to rough shape the repair. Sand diagonally across the patch to keep the surface even.
2. Refine the contour. Switch to 180 Grit (50-pack) sandpaper once the filler feels hard. Use a sanding block to maintain flatness and blend seamlessly with the car’s panel lines.
3. Apply a guide coat. Lightly mist a contrasting color over the repair. As you sand with 320 grit, the remaining paint highlights low spots that need more filler or attention.
4. Final dry sanding. Smooth the repair area evenly with 320 Grit (100-pack) before primer. Feel the surface—there should be no visible edges or ridges.
5. Wet-sand before painting. After applying primer and letting it cure, lightly wet-sand with 600 grit for a smooth paint-ready finish. Keep water flowing to prevent clogging and ensure a glossy, uniform base.
6. Clean and inspect. Wipe with a tack cloth to remove residue. Under side light, check for ripples or pinholes before applying color coat.

Special Cases

For fiberglass or plastic bumpers, use finer starting grits (120–150) to avoid cutting too deep. On aluminum panels, keep sanding pressure light to prevent warping. For deep fills, apply multiple thin layers of filler instead of one thick coat—each layer sands more evenly.

Pro Tips

• Sand diagonally to the repair edges to prevent grooves.
• Always use a sanding block—fingers alone can create uneven dips.
• Use guide coats frequently to visualize surface level.
• Switch paper often; body filler clogs quickly.
• Never jump more than two grit levels at once—progress gradually for best smoothness.

Aftercare

• Apply primer immediately after sanding to protect from moisture.
• Wipe panels with wax and grease remover before painting.
• For best adhesion, lightly scuff primer again with 600 grit before color coat.
• Inspect under bright LED light for any texture inconsistencies before final paint.

FAQs

• Can I wet-sand body filler directly? No—only after sealing with primer. Water can weaken raw filler and cause blistering later.
• Why are waves visible after painting? Usually from skipping grits or sanding without a block—always shape filler carefully.
• Can I use power sanders? Yes, but keep the speed low and maintain even movement to avoid heat distortion.

Watch & Learn

By following the sanding progression from 80 to 600 grit and maintaining precise surface control, you’ll create a perfectly smooth foundation that makes your car’s new paint look factory-fresh and seamless.