How to Sand Fiberglass for Repair and Painting

Fiberglass is durable and versatile, used in boats, cars, and home projects. But before painting or repairing, sanding is essential to smooth imperfections and create proper adhesion. Here’s a step-by-step guide to sanding fiberglass correctly.

Why Sand Fiberglass?

Sanding removes oxidation, levels repairs, and scuffs glossy surfaces so that paint, gelcoat, or resin bonds securely. Without sanding, coatings may peel, bubble, or fail prematurely.

Tools You’ll Need

• 9x11 in wet/dry sandpaper sheets
• Orbital sander or sanding block
• Spray bottle with water
• Protective mask and gloves
• Microfiber cloths

Best Grit Sequence for Fiberglass

• 80 grit: Removes rough patches and old coatings.
• 150 grit: Smooths repairs and levels the surface.
• 220 grit: Final sanding before applying paint or gelcoat.
• 400 grit: Optional fine sanding for polishing or clear coats.

Step-by-Step: Sanding Fiberglass

1. Clean the surface. Wash away dirt, oils, and loose debris.
2. Start with 80 grit. Use 80 grit (25-pack) to strip coatings and smooth rough areas.
3. Smooth with 150 grit. Apply 150 grit (50-pack) for leveling repairs.
4. Finish with 220 grit. Use 220 grit (100-pack) before priming or painting.
5. Optional 400 grit. Wet sand for a polished base before clear coat or buffing.
6. Clean thoroughly. Wipe with microfiber cloth to remove residue.

Special Cases

For boat hulls, always wet sand to control dust and prevent clogging. For automotive fiberglass parts, feather edges with finer grits (220–400) for seamless blending.

Pro Tips

• Always wear a mask—fiberglass dust can irritate lungs and skin.
• Sand evenly to avoid waves or thin spots.
• Wet sanding reduces dust and helps achieve smoother finishes.

Aftercare

• Apply primer, gelcoat, or paint immediately after sanding.
• Store leftover sandpaper flat in a dry place.
• Re-polish fiberglass regularly to maintain gloss and protection.

FAQs

• Do I need to sand between fiberglass repair layers? Yes—light sanding ensures each layer bonds properly.
• What grit before painting? 220 grit gives the best adhesion balance.
• Can I sand fiberglass by hand? Yes—hand sanding works for small repairs or detail areas.

Watch & Learn

For marine and automotive projects, use eQualle’s wet/dry sandpaper sheets, available in 25, 50, and 100 packs in grits 80–400. Shop fiberglass sanding sheets here.