Marine-Grade Plywood Preparation

Use sealing epoxy and gluing epoxy to ensure all the fibers are sealed.

Marine-grade plywood is a vital element in the practice of boat building and other marine-related projects. As the term suggests, marine-grade plywood is a type of wood that is constructed specifically to withstand extensive subjection to water. Thus, its preparation differs from that of regular plywood in the sense that it is made from special plywood veneers whose grain pattern and density make it suitable for outdoor applications. However, once purchased, marine-grade plywood still requires further preparation before it can be used in boat building or similar construction projects.

Things You'll Need

• 120-grit sanding paper
• 220-grit sanding paper
• 340-grit sanding paper
• 2-inch nylon-bristled brush
• ½-inch nylon-bristled brush
• 2-inch badger brush
• Sealing epoxy
• Gluing epoxy
• Spar varnish

Instructions

1. Sand the panels on all sides with 120-grit sanding paper. Sand the panels again on all sides using 220-grit sanding paper. Finish the surface with 340-grit sanding paper until an ultra-smooth, glass-like surface is obtained.

2. Apply one coat of sealing epoxy to the top side of the panel, using a 2-inch nylon-bristled brush. Allow this coat to dry. Consult the tin for exact drying times; these can be between three to 28 hours. Sand the coat gently with fine 340-grade sandpaper. Apply a second coat of the sealing epoxy with a brush and allow to dry completely.

3. Turn the panel over and repeat the previous step for the underside of the panel. Apply two coats and sand the first coat before applying the second. Allow the sealant to dry completely.

4. Apply one coat of sealing epoxy to all the edges of the panel, using a ½-inch nylon-bristled brush and wait until it dries. Sand the coat with 340-grit sanding paper. Apply a second coat and sand this coat when it is fully dry. Apply a third coat of the sealant and sand when fully dry.

5. Repeat Steps 2 through 4 to apply the gluing epoxy. Use a 2-inch nylon-bristled brush for the sides and a ½-inch nylon-bristled brush for the edges. Allow the gluing epoxy to dry fully. This takes between half an hour to two hours.

6. Apply one coat of spar varnish to the top side of the panel, using a 2-inch badger brush. Lift the brush at an angle when you move it backwards into the wet edge. Use as few brush strokes as possible in the process. Allow the coat to dry fully for between 24 to 48 hours. Sand with 220-grit sandpaper and apply a second coat. Repeat this step for the underside of the panel.

Tips & Warnings

• Apply sealing epoxy to any new edges should you cut the panels into smaller sections. Make sure to maintain all the panels at least once per year. The gluing epoxy does not penetrate the wood, therefore applying both the sealing epoxy and gluing epoxy ensures all the fibers of the wood are sealed, not just the outside.

• Wear safety goggles, chemical-resistant rubber gloves and chemical-resistant clothing when applying the epoxy.

References

• Dave Carnell's Boat Building Page; Epoxy Knowhow
• "Boatbuilding With Plywood"; Glenn L. Witt; 1989