Marine Sanding Guide: Preparing Fiberglass and Gelcoat (80–2000 Grit Sequence)

Boats and marine equipment face constant exposure to water, UV rays, and abrasion. Over time, fiberglass and gelcoat surfaces fade, scratch, or blister. Sanding is an essential step for repairs, refinishing, or prepping for paint and polish. This guide covers the best grit sequence and techniques for long-lasting marine results.

Why Sand Marine Surfaces?

Sanding removes oxidation, smooths out imperfections, and prepares surfaces for gelcoat repair, paint, or polish. Without sanding, coatings won’t adhere properly, leading to premature failure in harsh marine conditions.

Best Tools for Marine Sanding

• 9x11 in silicon carbide wet/dry sandpaper sheets
• Sanding block or orbital sander
• Bucket of water with mild soap
• Microfiber towels
• Marine-grade polishing compound

Recommended Grit Sequence

• 80–120 grit: Remove heavy oxidation or damaged gelcoat.
• 220 grit: Smooth surface after coarse sanding.
• 400–600 grit: Level repaired areas before polishing or painting.
• 1000–2000 grit: Wet sanding for high-gloss finishes.

Step-by-Step: Sanding Fiberglass & Gelcoat

1. Wash the surface. Remove salt, dirt, and grease with soap and water.
2. Start with coarse sanding. Remove oxidation or blisters with 80 Grit (25-pack).
3. Smooth with 220 grit. Blend surfaces and eliminate scratches from coarse sanding. Use 220 Grit (50-pack).
4. Refine with 400–600 grit. Prepare for polishing or repainting. Stock up on 600 Grit (100-pack).
5. Finish with 1000–2000 grit. Wet sand for a mirror-like polish before buffing.
6. Polish. Use marine-grade polishing compound to restore shine.
7. Seal. Apply wax or protective coating to protect against UV and water.

Special Cases

For hull blister repairs, sand damaged areas aggressively with 80 grit before applying new gelcoat. Always finish with wet sanding before polishing.

Pro Tips

• Always wet sand marine surfaces to reduce dust and heat.
• Work in sections to maintain even sanding.
• Use crosshatch sanding motions for better leveling.
• Check progress frequently by rinsing and drying.

Aftercare

• Rinse thoroughly after each sanding stage to prevent residue buildup.
• Apply UV protectant regularly to maintain gloss.
• Store sandpaper flat in a dry, sealed container on board.

FAQs

• Can I dry sand fiberglass? It’s possible but not recommended—wet sanding reduces clogging and heat.
• How often should I sand gelcoat? Only when refinishing or repairing—regular maintenance requires only polishing and waxing.
• Do I need to reapply gelcoat after sanding? Only if you sand through the outer protective layer.

Watch & Learn

For marine refinishing, explore our reliable Extra Coarse Sandpaper Collection and Super Fine Sandpaper Collection for complete coverage from hull prep to final polish.