Prepping Auto Body Filler for Primer—Flawless Blend (180–400 Grit)

Body filler (Bondo) repairs can make or break your paint job. If sanded unevenly or primed too soon, the repair outline—known as “mapping”—will show through the finish coat. This guide covers how to sand and prep auto body filler to perfection, ensuring a smooth, invisible repair ready for primer and paint.

Why Sanding Matters for Filler Prep

Body filler cures hard but sands quickly when done at the right time. A well-sequenced grit progression removes high spots, shapes contours, and feathers edges into the surrounding metal or existing paint. Too coarse, and you’ll gouge the surface; too fine, and the primer won’t adhere properly.

Recommended Tools

• Dual-action (DA) sander or hand sanding block.
• Wet/dry sandpaper (180–400 grit range).
• Flexible sanding pad for curved panels.
• Guide coat spray for tracking low spots.
• Tack cloth and compressed air for cleaning.

Ideal Grit Sequence

• 180 grit – Shape filler and remove excess material.
• 320 grit – Feather edges and blend repair zones.
• 400 grit – Final smoothing before primer application.

Step-by-Step: Perfect Filler Sanding Before Primer

1. Initial shaping. After filler has cured (usually 15–20 minutes), begin sanding with 180 Grit (25-pack). Use long, cross-hatched strokes to shape the contour and reveal highs and lows. Stop once the repair matches the panel shape.
2. Apply a guide coat. Lightly mist a contrasting color over the repair area. This reveals low spots as you sand the next grit stage.
3. Feather edges. Move to 320 Grit (50-pack) paper to blend the filler edges into surrounding paint. Sand in smooth, overlapping strokes until the transition disappears under touch and sight.
4. Final finish sanding. Use 400 Grit (100-pack) wet/dry paper to remove fine scratches and prep for primer. Keep the surface clean and slightly damp to minimize clogging.
5. Clean and inspect. Wipe with a tack cloth and inspect under low-angle light. Any visible texture or shadowed edge means further feathering is needed.
6. Ready for primer. Apply 2–3 medium coats of high-build primer, allowing flash time between coats. Let cure and sand again with 400 grit before color.

Special Cases

For deep repairs, build filler in thin layers rather than one thick application—it sands smoother and reduces pinholes. On flexible bumpers, use flexible glazing putty for final feathering. Avoid power sanding near sharp body lines; hand-sand these areas to preserve definition.

Pro Tips

• Use a rigid block on flat panels and a flexible pad on curves for even shaping.
• Feather beyond the filler area by at least 2–3 inches to ensure seamless blending.
• Always blow off dust before switching grits; trapped debris scratches the surface.
• Don’t skip the guide coat—it’s the best way to spot imperfections before paint.
• Work patiently; rushing filler sanding leads to visible repair lines after painting.

Aftercare

• Keep sanded filler areas clean and dry until priming.
• Never expose sanded filler to moisture—it will absorb and cause blistering later.
• Store filler and hardener properly; old material can cure unevenly and sand poorly.
• Use a fine mist of primer surfacer to seal before topcoating for best adhesion.

FAQs

• Can I start sanding with 80 grit? Only for heavy shaping—always move to 180 grit quickly to avoid deep scratches.
• Why does my filler feel wavy after sanding? You may be using too soft a pad; switch to a rigid block for consistent flatness.
• Is wet-sanding filler safe? Only after full cure—early moisture can cause bubbles or poor adhesion under paint.

Watch & Learn

By following a consistent sanding sequence from 180 to 400 grit, you can shape and refine body filler to perfection—achieving a smooth, invisible repair that looks seamless under paint.