Sanding Auto Body Filler: Smooth Prep for Perfect Paint

Auto body filler (commonly called Bondo) is essential for repairing dents and dings before painting a car. But the real secret to invisible repairs is proper sanding. With the right grit sequence, you can blend filler seamlessly into the surrounding metal for a professional finish.

Why Sand Body Filler?

Body filler cures hard but must be shaped to match the contour of the vehicle. Sanding removes excess material, smooths transitions, and ensures primer and paint adhere evenly. Skipping sanding leaves visible edges that ruin the repair.

Recommended Tools

• Coarse and fine wet/dry sandpaper sheets
• Sanding block or longboard for flat panels
• Orbital sander for efficiency
• Tack cloth and shop vacuum
• Respirator and safety glasses

Choosing the Right Grit Sequence

• 80 grit: Shapes filler and removes high spots.
• 180 grit: Smooths filler and feathers edges.
• 320 grit: Prepares surface for primer.

Step-by-Step: Sanding Auto Body Filler

1. Initial shaping with 80 grit. Rough-shape cured filler using 80 grit (25-pack). Sand in a crosshatch pattern to maintain even contours.
2. Refine with 180 grit. Smooth the filler and blend into the surrounding metal with 180 grit (50-pack).
3. Final prep with 320 grit. Create a uniform surface ready for primer using 320 grit (100-pack).
4. Check with guide coat. Lightly spray a contrasting guide coat and sand—any low spots will remain dark, showing areas that need more filler or sanding.
5. Clean thoroughly. Vacuum and wipe down with a tack cloth before applying primer.

Special Cases

Large dents: May require multiple filler layers, sanding each one before reapplying.
Curved panels: Use a flexible sanding block to match contours.
Feathering paint edges: Work carefully with 320 grit to avoid visible ridges under primer.

Pro Tips

• Always let filler cure fully before sanding to avoid clogging sandpaper.
• Sand with even strokes—rushing leads to uneven panels.
• Use a dust mask—filler dust is hazardous when inhaled.

Aftercare & Next Steps

• Apply primer immediately after sanding to prevent moisture absorption.
• Block-sand primer with 400–600 grit before painting.
• Inspect under bright light to catch imperfections before spraying paint.

FAQs

• Can I use a power sander on filler? Yes, but keep speed low to avoid gouging.
• Do I need to sand to bare metal? Only around repair areas—just feather filler smoothly into the paint.
• What if I see pinholes? Fill with glazing putty, then sand again with fine grit.

Watch & Learn

Sanding auto body filler takes patience, but with the correct grits and techniques, you can achieve seamless repairs that disappear under paint. A little extra effort now ensures your finish looks factory-fresh.