How to Sand Fiberglass for Painting & Repairs (120, 220, 320, 400 Grit Guide)

Fiberglass surfaces are strong and lightweight but require proper sanding before painting or repairing. Whether working on a boat, car body kit, or household item, the right sanding method ensures smooth finishes and strong adhesion. This guide explains how to sand fiberglass safely and effectively.

Why Sanding Fiberglass Matters

Fiberglass has a glossy gel coat that resists paint and adhesives. Sanding removes the shine, levels imperfections, and provides the right surface texture for coatings or repair compounds.

Tools & Materials

• Respirator and gloves (fiberglass dust is irritating)
• Wet/dry sandpaper sheets
• Sanding block or orbital sander
• Vacuum and tack cloth
• Primer or gel coat

Best Grit Sequence for Fiberglass

• 120 grit: Removing gel coat gloss and rough shaping.
• 220 grit: Leveling repairs and smoothing surfaces.
• 320 grit: Prepping for primer or paint.
• 400 grit: Final wet sanding before finishing.

Step-by-Step: Sanding Fiberglass

1. Wear protection. Fiberglass dust can irritate skin and lungs—always use a respirator and gloves.
2. Start with 120 grit. Use 120 grit (25-pack) to dull the glossy gel coat and rough-shape repairs.
3. Smooth with 220 grit. Switch to 220 grit (100-pack) to refine the surface and feather edges.
4. Prep for primer. Sand with 320 grit (25-pack) to create an even, paint-ready surface.
5. Final sanding. Finish with 400 grit (50-pack) wet to achieve a smooth base for coatings.
6. Clean thoroughly. Vacuum dust and wipe with a tack cloth before priming or painting.

Special Cases

• Boat hulls: Sand larger areas with orbital sander; always wet sand finer grits to reduce dust.
• Car parts: Pay special attention to curves and edges to avoid uneven sanding.
• Fiberglass patches: Blend edges carefully at 220–320 grit for seamless repairs.

Pro Tips

• Always wet sand at 320 grit and finer to avoid clogging.
• Work in small sections to ensure even sanding.
• Sand in long, overlapping strokes for consistency.
• Clean between grit changes to prevent scratches.

Aftercare

• Apply primer immediately to prevent moisture intrusion.
• Seal painted fiberglass with clear coat for UV protection.
• Regularly wax or seal fiberglass to extend finish life.

FAQs

• Can I sand fiberglass by hand? Yes, but larger surfaces are easier with a sander.
• What grit before primer? 320 grit creates the best adhesion surface.
• Do I need to remove all gel coat? No, just scuff and level it for coatings to stick.

Watch & Learn

Fiberglass requires the right sanding sequence for durable, attractive results. Stock up on 120 to 400 grit sandpaper sheets in bulk packs for marine and automotive projects. Explore the complete fiberglass sanding collection today.