How to Sand Fiberglass for Repairs and Smooth Finishing (80–600 Grit)

Fiberglass is strong and versatile, but repairing or refinishing it requires proper sanding. Whether you’re fixing a boat hull, car panel, or custom project, the right grit sequence ensures smooth results without damaging the fibers.

Why Sand Fiberglass?

Fiberglass surfaces develop scratches, oxidation, or resin bumps over time. Sanding removes imperfections, feathers repair areas, and prepares the surface for paint, gelcoat, or clear coat.

Tools You’ll Need

• Respirator mask and safety goggles (fiberglass dust is hazardous)
• Gloves and long sleeves for skin protection
• Orbital sander or sanding block
• Wet/dry sandpaper sheets (80–600 grit)
• Vacuum or tack cloth

Recommended Grit Sequence

• 80 grit: For heavy material removal and shaping fiberglass patches.
• 150 grit: Smoothing after coarse sanding.
• 220–320 grit: Prepping fiberglass for primer or paint.
• 400–600 grit: Final wet sanding before topcoats or polishing.

Step-by-Step Process

1. Clean the surface. Wash with soap and water, then dry thoroughly.
2. Start with 80 grit. Use 80 grit (25-pack) sheets to shape fiberglass repairs and remove rough resin.
3. Refine with 150 grit. Smooth the surface using 150 grit (50-pack) wet/dry sheets.
4. Prep for primer. Sand with 220 grit (100-pack) to create adhesion for coatings.
5. Finish sanding. Use 400–600 grit wet sanding for a silky surface before painting or polishing.
6. Clean thoroughly. Vacuum and wipe with a tack cloth to remove fiberglass dust.
7. Apply finish. Prime, paint, or polish as desired for your project.

Special Cases

For boats, always wet sand to minimize dust. For automotive fiberglass panels, use guide coats to spot imperfections before painting.

Pro Tips

• Always sand in stages—skipping grits leaves scratches that show through paint.
• Wet sanding extends sandpaper life and keeps fiberglass dust contained.
• Use light pressure to avoid sanding through fiberglass layers.
• Replace clogged sheets quickly for consistent results.

Aftercare

• Rinse the work area thoroughly to remove fiberglass dust.
• Apply protective coatings (paint, gelcoat, or wax) to extend surface life.
• Inspect repairs periodically for cracks or wear.

FAQs

• Can I use power sanders on fiberglass? Yes, but always use light pressure and keep the surface cool.
• Is dry sanding fiberglass safe? No—always wear a respirator and prefer wet sanding when possible.
• Do I need to seal fiberglass after sanding? Yes—paint, gelcoat, or clear coat protects the fibers from UV and moisture.

Watch & Learn

Sanding fiberglass requires patience, but the payoff is a smooth, durable finish. Stock up on coarse grits, medium sheets, and fine sandpapers to tackle any fiberglass repair or refinishing project.