How to Seal Plywood for Outdoor Use

Materials Needed

• Exterior grade plywood (CDX or marine grade)
• Exterior wood sealer or finish
• Paintbrush or paint roller
• Drop cloth
• Sandpaper (80-100 grit)
• Clean rags
• Mineral spirits or paint thinner

Prepare the Plywood Surface

Proper surface preparation is key to achieving a long-lasting seal on the plywood.

• Inspect the plywood for any flaws, holes, or gaps and fill as needed with exterior grade wood filler.
• Sand the plywood using 80-100 grit sandpaper to roughen and smooth the surface. This helps the sealer adhere properly.
• Wipe away any dust using a clean rag or tack cloth. The surface needs to be free of debris before sealing.
• Use mineral spirits to clean grease or oil spots. Let the plywood dry completely before sealing.

Choose an Exterior Sealer

There are several good options for sealing outdoor plywood:

• Exterior paint/primer – Provides a protective topcoat and seals the wood. Can use latex or oil-based.
• Clear wood sealer – Penetrates into the wood and seals from within. Options like varnish, polyurethane, lacquer.
• Water-sealant – Designed for wood exposed to moisture. Forms a breathable barrier.
• Epoxy resin – Creates a plastic-like coating on the wood with excellent moisture resistance.

Consider durability, appearance, and application method when choosing the right outdoor sealer for your plywood project.

How to Apply the Outdoor Sealer

Carefully follow the application directions on the sealer product. In general:

• Stir or mix the sealer thoroughly before and during application.
• Use a good quality natural bristle brush, paint pad, or sprayer.
• Apply a generous coat of sealer in the direction of the wood grain.
• Allow proper drying time between coats as specified on the sealer product.
• Apply 2-3 coats for maximum protection, sanding lightly between coats.
• Maintain proper ventilation while applying the sealer.
• Allow the final coat to cure fully before use, typically 24-48 hours.

Sealing Plywood Edges

The edges of plywood are especially vulnerable to moisture infiltration. Proper sealing is critical:

• Inspect edges for any gaps, chipping or flaws in the veneer. Fill as needed with wood filler.
• Sand edges smooth for proper adhesion. Break any sharp edges or corners.
• Apply 2-3 heavy coats of sealer on all plywood edges, allowing proper dry time between coats.
• Use a small foam brush to carefully apply sealer into any exposed wood pores along the edges.
• Coat plywood edges before doing the broad surfaces for complete protection.

Sealing Miters, Connections and Fasteners

Where plywood boards meet, butt together, are joined, or where fasteners are used, extra care is needed:

• Sand any surfaces that mate flush to create a good bonding surface when sealed.
• Apply sealer generously to joints, miters, and board connecting points before joining, clamping, or screwing them together.
• Allow to dry before bringing plywood components together for maximum effectiveness.
• After joining, apply additional coats of sealer over fasteners, joints, and miters. Allow to cure.
• Use high quality exterior grade screws, brackets, hinges, etc. Seal fastener holes and hardware for corrosion resistance.

Maintenance and Re-Application

• Inspect sealed plywood regularly, especially during the first year, for any cracks, peeling, or deterioration of the sealer.
• Sand and re-apply sealer promptly to any areas showing wear or damage to maintain protection.
• Depending on usage and exposure, re-apply sealer every 1-3 years for optimal outdoor performance.
• Lightly sand and clean surface before re-coating for maximum adhesion and durability.

Properly sealing your plywood project will protect it from the elements and result in years of service. Always read and follow the specific application directions for your chosen sealer product. With the right preparation and coating system, sealed plywood can withstand outdoor environments for a very long time. Let the properly applied sealer be your plywood’s first line of defense.

Frequently Asked Questions

What is the best exterior sealer for plywood?

For outdoor plywood projects, marine grade spar varnish, exterior grade polyurethane, and epoxy resin sealers provide excellent protection against moisture, UV rays, and weathering.

Should I prime before sealing outdoor plywood?

Applying exterior primer before the sealing topcoat will provide added protection and improves adhesion. Always use an exterior grade primer matched to your topcoat.

How long does sealed plywood last outside?

With proper initial sealing and periodic re-application every 1-3 years, sealed exterior grade plywood can last upwards of 5-7 years or longer before major restoration is needed. Proper maintenance is key.

Does Thompson’s WaterSeal work on plywood?

Yes, Thompson’s WaterSeal is an effective sealer for plywood exposed to moisture. Follow application directions carefully for best results. Its proprietary oils soak into the wood grain to repel water.

What is the most waterproof sealer for plywood?

Epoxy resin provides one of the most waterproof seals for plywood. Multiple coats create a plastic-like moisture barrier that far exceeds the water resistance of varnishes or polyurethanes. Two-part pouring epoxy resins work best.

Conclusion

Applying the right exterior grade sealer is crucial to protect plywood projects in outdoor environments from moisture damage, rotting, and decay over time. With the proper preparation, coating selection, application methods, edge sealing, and periodic re-application, plywood can be made to last for many years even with full weather exposure. Sealing also provides aesthetic benefits and UV protection. Take the time to properly seal your plywood using these techniques for exterior woodworking success.