How to Sand Fiberglass for Repair and Finishing

Fiberglass is widely used in boats, cars, and household projects because it’s lightweight yet strong. However, sanding fiberglass requires the right technique—too much pressure or the wrong grit can damage the surface. This guide explains how to sand fiberglass safely and effectively for repairs or refinishing.

Why Sand Fiberglass?

Sanding prepares fiberglass for new coatings, smooths out repairs, and removes surface oxidation. Without proper sanding, paint, gel coat, or resin may not adhere, leading to peeling or cracking later on.

Tools You’ll Need

• 9x11 in wet/dry sandpaper sheets
• Orbital sander or sanding block
• Protective gear: respirator, gloves, goggles
• Spray bottle with water (for wet sanding)
• Tack cloth or microfiber rag

Best Grit Sequence for Fiberglass

• 120 grit: Initial sanding for rough areas or oxidation.
• 220 grit: Surface prep before applying coatings.
• 400 grit: Smoothing between primer or gel coat layers.
• 800 grit: Pre-polish sanding for a semi-gloss finish.
• 1500 grit: Ultra-fine sanding for polishing and high gloss.

Step-by-Step: Sanding Fiberglass

1. Inspect the surface. Look for cracks, oxidation, or rough patches.
2. Start with 120 grit. Use 120 grit (25-pack) for leveling problem areas.
3. Move to 220 grit. Smooth the surface evenly with 220 grit (50-pack) to prep for coatings.
4. Refine with 400 grit. Apply 400 grit (100-pack) between layers of primer or gel coat.
5. Polish with 800 grit. Wet sand for a smooth, semi-gloss surface.
6. Finish with 1500 grit. Ultra-fine sanding prepares the surface for buffing and polishing compounds.
7. Clean thoroughly. Wipe down with a tack cloth before painting or sealing.

Special Cases

For boat hulls, sanding should be more aggressive initially to remove oxidation. For auto parts, use finer grits to avoid cutting into thin fiberglass layers.

Pro Tips

• Always wear a respirator—fiberglass dust is hazardous.
• Wet sand finer grits to prevent clogging and improve smoothness.
• Sand in even strokes to avoid gouging the surface.

Aftercare

• Seal with gel coat, paint, or resin immediately after sanding.
• Polish regularly to maintain shine and protection.
• Inspect fiberglass annually for signs of oxidation or damage.

FAQs

• Can I use power sanders on fiberglass? Yes, but use light pressure to avoid cutting too deep.
• Do I need to sand between fiberglass repair layers? Yes—light sanding improves adhesion between resin layers.
• What grit is best before polishing? 1500 grit ensures a smooth base for buffing compounds.

Watch & Learn

eQualle’s wet/dry sandpaper sheets are perfect for fiberglass sanding, available in 25, 50, and 100 packs. Shop sanding sheets here.