Sanding Fiberglass Surfaces: Prep for Repair and Painting

Fiberglass is widely used in boats, cars, and DIY projects because of its strength and light weight. But whether you’re repairing damage or preparing for paint, sanding is essential. The right grit sequence ensures smooth results without damaging the fiberglass structure.

Why Sanding Fiberglass Matters

Fiberglass surfaces often have a glossy gel coat that prevents paint or resin from sticking. Sanding removes the shine, smooths repairs, and creates a profile that new coatings can bond to. Without sanding, paint may peel, and repairs won’t last.

Essential Tools

• Wet/dry silicon carbide sandpaper
• Orbital sander for flat surfaces
• Sanding block or sponge for curves
• Protective respirator, gloves, and goggles
• Vacuum and tack cloths

Best Sandpaper Grits for Fiberglass

• 80 grit: Remove gel coat gloss or level repairs (80 Grit (25-pack)).
• 150 grit: Smooth surfaces and refine sanding marks (150 Grit (25-pack)).
• 220 grit: Final sanding before primer or paint (220 Grit (50-pack)).
• 400 grit: Optional wet sanding for ultra-smooth finishes (400 Grit (100-pack)).

Step-by-Step: Sanding Fiberglass

1. Clean thoroughly. Wash surface with soap and water, then degrease.
2. Start with 80 grit. Sand glossy gel coat until surface is dull and even.
3. Move to 150 grit. Smooth repairs or transition areas.
4. Switch to 220 grit. Sand lightly for a paint-ready surface.
5. Optional 400 grit. Wet sand for extra-smooth finish before priming.
6. Clean. Vacuum and wipe with tack cloth before painting or applying resin.

Special Cases

On boats, sanding fiberglass hulls should be done evenly to avoid thin spots. For auto body fiberglass panels, avoid over-sanding edges where material is thinner.

Pro Tips

• Always wear a respirator—fiberglass dust is hazardous.
• Use wet sanding when possible to control dust and heat.
• Sand evenly in broad strokes to prevent low spots.

Aftercare

• Prime immediately after sanding to prevent contamination.
• Seal exposed fiberglass with epoxy or resin before painting.
• Inspect regularly for chips or cracks and spot-sand as needed.

FAQs

• Can I paint fiberglass without sanding? No—paint won’t adhere to glossy gel coat.
• What’s the best grit for final prep? 220 grit is standard before primer, 400 grit for ultra-smooth finishes.
• Is dry sanding safe? Yes, but wet sanding is safer and produces less dust.

Watch & Learn

Sanding fiberglass properly ensures strong repairs and long-lasting paint. Explore our fiberglass sanding sheets to prepare your next project.