How to Sand Gelcoat on Boats for Repair & Painting (80, 150, 220, 400 Grit Guide)

Boat hulls and fiberglass surfaces with gelcoat often develop oxidation, scratches, or cracks over time. Sanding gelcoat is the essential step before repairs, polishing, or repainting. With the correct grit sequence, you can restore smoothness and prepare for coatings that last. This guide explains how to sand gelcoat step by step using 80, 150, 220, and 400 grit sandpaper.

Why Sanding Gelcoat Matters

Gelcoat protects fiberglass but weathers over time, becoming chalky and brittle. Sanding removes oxidation, smooths imperfections, and creates a surface that accepts primer, paint, or polish. Without sanding, coatings will peel or fail quickly.

Tools You’ll Need

• 80–400 grit wet/dry sandpaper – for step-by-step prep.
• Orbital or palm sander – speeds up sanding large areas.
• Sanding block – maintains even pressure on curved hulls.
• Respirator & goggles – gelcoat dust can be harmful.
• Marine primer & paint – for sealing after sanding.

Best Grit Sequence for Gelcoat

• 80 grit: Strips heavy oxidation or flaking coatings.
• 150 grit: Smooths surface after coarse sanding.
• 220 grit: Refines surface for priming or polishing.
• 400 grit: Final sanding for a smooth, paint-ready finish.

Step-by-Step Gelcoat Sanding

1. Start with 80 grit. Remove chalky oxidation and old coatings using 80 grit (25-pack).
2. Move to 150 grit. Smooth sanding marks with 150 grit (25-pack).
3. Refine with 220 grit. Prepare for primer using 220 grit (100-pack).
4. Finish with 400 grit. Wet sand lightly for a polished, paint-ready surface using 400 grit (25-pack).
5. Clean thoroughly. Wipe with microfiber cloths before painting or sealing.

Special Cases

For deep scratches or cracks, sand aggressively at 80 grit and repair with resin or filler before moving up. For polishing instead of repainting, continue sanding up to 1000–2000 grit, then buff with compound. Always wet sand finer grits for best results.

Pro Tips

• Keep the surface wet to reduce dust and clogging.
• Sand evenly across hull sections to avoid waves.
• Check progress under bright light to spot imperfections.
• Apply marine primer immediately after sanding for long-term protection.

Aftercare

• Wax or seal gelcoat after refinishing for UV protection.
• Rinse boats thoroughly after each use to reduce oxidation.
• Inspect hulls annually and re-sand lightly as needed.

FAQs

• Can I sand gelcoat by hand? Yes, but use a sanding block for control and consistency.
• Do I need to sand all gelcoat before painting? Yes, at least enough to remove gloss and oxidation.
• Is 400 grit smooth enough before primer? Yes—400 grit leaves the perfect surface profile for adhesion.

Watch & Learn

For reliable marine refinishing, explore our medium and fine sandpaper collection.