Outdoor plywood projects require special treatment and preparation to withstand the elements. Properly treating plywood ensures it resists water damage, rot, cracks and warping when used outside. With the right techniques and products, you can create durable and long-lasting outdoor structures from plywood.
Choosing the Right Outdoor Plywood
When selecting plywood for outdoor use, choose products designed to resist moisture and decay. The two best options are:
Exterior Grade Plywood
Exterior plywood is constructed with water-resistant adhesive and designed to withstand moisture. Look for plywood rated for outdoor exposure, such as Exposure 1 or Exterior grade. The plies are made from water-resistant wood species like Douglas fir or Southern pine. Exterior plywood is a great choice for projects like outdoor furniture, planter boxes, shed walls and decking.
Marine Grade Plywood
Marine plywood is manufactured with top quality veneers and adhesives that resist delaminating from moisture. It’s the best choice for projects that will endure wet conditions like boat hulls, docks and other structures near water. Marine plywood uses durable tropical hardwoods that deter rot. All plies are coated in waterproof glue.
Both types provide protection from moisture, but marine plywood is the ultimate outdoor plywood. It’s more expensive but ideal for applications involving prolonged wetness.
Prep the Plywood Surface
Proper surface preparation is key to achieving maximum coating adhesion and weather resistance:
- Inspect for surface defects like knots, cracks, splinters or voids. Fill small holes with epoxy filler. Patch larger defects with glue and screws.
- Sand the plywood with 80-100 grit sandpaper to rough up the surface. This helps the finish coat penetrate deeper.
- Clean the surface with a rag dampened with mineral spirits to remove grease and dirt. Let dry completely.
- Seal the back, edges and ends with a coat of primer or sealer. This prevents moisture from seeping in from unfinished areas.
- Apply wood conditioner for a more uniform finish absorption. This prevents blotchiness.
- Fill any remaining cracks or checks with thin epoxy filler designed for wood. Allow to fully cure before finishing.
Proper prep removes contaminants, roughens the surface and seals vulnerable areas. This provides the smoothed, clean surface needed for maximum finish adhesion.
Choose an Outdoor Finish
The right outdoor finish protects plywood from water, UV rays and fungus while retaining the wood’s natural appearance. Consider these top options:
Spar Urethane
Spar urethane is ideal for outdoor wood projects. It forms a thick, glossy film that repels water and withstands sunlight, weathering and temperature fluctuations. Spar urethane adheres well to plywood and dries fast. It’s also flexible to resist cracking as wood expands and contracts. Apply 3-4 coats for maximum protection, sanding lightly between coats.
Exterior Oil-Based Paint
Exterior paint stands up to sun, rain, snow and humidity when applied over primer. Look for exterior grade paint with UV blockers and mildew resistance. Oil-based alkyd paints provide better penetration and moisture protection than latex for plywood. Paint allows for color-coding outdoor structures. Plan to apply 2-3 coats for a fully protective film build.
Exterior Stain
Exterior stains are ideal for preserving the look of natural wood while protecting it from moisture. They contain translucent pigments, resins and water repellents that maintain wood’s texture. Look for “above ground” stains rated for decks, fences, siding and plywood projects. Semi-transparent stains offer more protection than clear sealers. Expect to recoat every 2-3 years.
Epoxy Resin
For ultimate waterproofing, consider coating plywood with a clear epoxy resin or varnish. These 2-part coatings cure into a thick, glassy finish that fully seals the wood surface. They’re extremely durable and water-resistant, holding up for many years. Epoxy resins yellow less than varnish but require more coats for UV resistance. Apply over epoxy primer for exceptional bonding and protection.
Marine Varnish
Marine varnish is specially formulated to withstand sun, water, salt spray and humidity in marine conditions. It forms a glossy, flexible and waterproof coating made to resist cracking, peeling and fading. Expect to apply 4-6 coats of marine varnish for a completely sealed finish, recoating annually. It provides excellent protection for outdoor plywood projects.
Consider the level of weather exposure when choosing the optimal outdoor finish. Marine varnish and epoxy resin provide the toughest protection for plywood structures enduring direct sun and moisture.
Apply Finish Outdoors
It’s best practice to finish plywood outdoors whenever possible:
- Work in an outdoor area protected from wind, dust and debris. Avoid direct sunlight which can cause finishes to dry too fast.
- Use drop cloths underneath the plywood to catch drips and spills. For large panels, prop up on sawhorses for easy access.
- Outdoor finishing minimizes exposure to harmful fumes. Wear an OSHA-approved respirator when working with oils, varnish or paints.
- Allow finishes to cure in fresh air for full durability. Oil-based coats cure by oxidation which requires air circulation.
- Outdoor humidity helps water-based finishes fully cure and achieve maximum hardness to resist cracking.
- Dispose of all solvents, rags and clean-up materials properly. Allow oil-soaked rags to fully dry in air before disposal to prevent combustion.
Finish coats achieve better durability when applied and cured in outdoor conditions similar to their end use. Always take safety precautions when using chemical finishes.
Alternative Outdoor Plywood Treatments
Beyond traditional varnish and paint, there are a few alternative ways to treat plywood for exterior applications:
Weatherproof Sealant
Specialty waterproofing products like Thompson’s WaterSeal create an invisible barrier against moisture on wood surfaces. These penetrating sealants contain silicone, acrylics or wax emulsions that resist water absorption. They retain the natural look of wood while protecting against moisture damage. Reapply every year.
UV Protection
Clear finishes like spar varnishes, polyurethanes and wood preservatives can be enhanced with UV absorbers to filter out sun’s damaging rays. Adding liquid UV inhibitors helps prevent fading and graying. Look for products designed to block UV and keep wood from discoloring.
Bleaching/Whitewashing
For a weathered, beachy look, plywood can be whitewashed using a dilute bleach solution. Mix 1 part bleach with 3 parts water and apply liberally with a brush or sprayer. This grays and bleaches the wood for a coastal vibe. A vinegar/steel wool solution produces similar graying effects. Rinse thoroughly and let dry before sealing.
Oxidizing Stains
Greying outdoor wood can be simulated using oxidizing stains containing iron oxide pigments. As the stain weathers, iron reacts with wood tannins to produce a natural, driftwood-like gray patina. Apply over preservative for protection. Regular staining maintains the gray color.
With some creativity, you can develop unique treatments to make outdoor plywood fixtures stand up to the elements while achieving your desired aesthetic. Always test products first on scrap wood.
Maintaining Outdoor Plywood Projects
To maximize the lifespan of exterior plywood projects, implement a regular maintenance plan:
- Inspect plywood annually for cracks, peeling, fungus or moisture damage. Refinish degraded areas immediately to prevent larger failures.
- Lightly sand to remove loose finish and smooth uneven surfaces. Clean with mild detergent before reapplying protective coats.
- Spot treat decaying or discolored wood with a fungicide/wood preservative like copper naphthenate. Replace severely rotten boards.
- Plan to recoat plywood with fresh spar urethane or exterior stain every 2-3 years for optimal water resistance and UV protection.
- Caulk cracks, holes and joints with exterior grade silicone caulk to prevent water intrusion into the plywood.
- Ensure proper drainage so water doesn’t pool on horizontal plywood surfaces. Slope supports away from structures.
- Allow plywood sufficient overhangs and space for air circulation to speed drying after rain or snow.
- Consider installing plywood panels with plastic shims to provide small gaps. This allows ventilation to prevent trapped moisture.
Regular inspection, cleaning, sealing and finish renewal keeps exterior plywood in good shape for lasting performance. Addressing issues promptly is key.
Tips for Successfully Finishing Outdoor Plywood Projects
Follow these best practices when planning your outdoor plywood finishing regimen:
- Before starting, test finishes on scrap wood to check color and confirm compatibility with the plywood. Oil-based coats work best.
- Only apply finishes outdoors or in a well-ventilated area away from sparks or flames. Have fire extinguisher on hand.
- Finishes adhere best when applied at temps between 50-90°F with less than 85% humidity. Avoid extreme heat, direct sun and moisture.
- Apply finish coats when plywood moisture content is below 15% for optimal penetration. Newly purchased plywood may need additional drying time.
- Brush on oil-based finishes using high-quality bristle or foam brushes, maintaining a wet edge between strokes. Work quickly to prevent lap marks.
- Allow at least 24 hours drying time between coats. Lightly sand between layers using 320+ grit sandpaper to improve adhesion.
- Apply at least 3-4 coats of finish for a fully protective film build, including all edges and ends. Heavy use areas may need additional coats.
- Consider thinner coats of penetrating oil finish followed by thicker topcoats of marine varnish for enhanced moisture defense.
- Always follow manufacturer’s instructions for application temperatures, drying times, surface prep, cleanup and storage.
With attention to detail and proper finishing techniques, your outdoor plywood projects will last for many years in tough climates.
FAQs about Treating Plywood for Outdoor Use
What type of plywood is best for outdoor use?
Exterior and marine grades are best for outdoor use. Exterior is the most common and affordable, while marine offers premium water resistance. Both are manufactured with moisture-resistant glues and woods.
Should I use pressure-treated plywood outside?
Pressure-treated (PT) plywood contains preservatives to resist insects and decay but lacks waterproof glues. It requires additional sealing for outdoor use. Non-PT exterior grades offer better performance.
What kind of paint works best on outdoor plywood?
Exterior latex and oil-based alkyd paints withstand weathering better than interior paints when used outdoors over primed plywood. Oil-based enamels provide the toughest, most water-resistant coating.
Can I use polyurethane on outdoor plywood?
Yes, polyurethane is a durable coating for outdoor use. Look for those formulated for exterior applications. Spar urethane provides superior UV resistance and flexibility compared to basic polyurethane.
How do I restore weathered gray plywood?
Lightly sand gray, weathered plywood and apply wood brightener or oxalic acid to extract embedded dirt and revive color. Then seal with exterior stain. Refinishing may be needed for extensive graying.
Should I varnish all plywood edges?
Yes, sealing the edges is critical. Paint or varnish all edges and ends before installing for maximum water protection. Unsealed edges allow moisture penetration.
Can I use linseed oil on exterior plywood?
Linseed oil provides minimal water resistance outdoors unless used as a sealer under epoxy or marine varnish. On its own, linseed oil requires very frequent reapplication on exterior wood.
What’s the best glue for outdoor plywood?
Polyurethane or resorcinol waterproof glues create the strongest bonds for plywood in wet outdoor environments. Epoxy adhesives also perform well. Avoid white/PVA glue.
How often should I refinish outdoor plywood?
Typically every 2-3 years, but it varies based on climate and exposure. Refinish when you notice finish dulling, cracking or peeling. Heavily used horizontal surfaces may need coating yearly.
Conclusion
Outdoor plywood projects require careful preparation and the right finishing system to withstand the ravages of weather, sunlight and moisture. While all plywood lacks native exterior durability, proper sealing makes it a versatile, cost-effective material for outdoor structures. With an exterior grade foundation and heavy-duty finish coat, even plywood can be transformed into fixtures lasting decades outside. Just remember to utilize rugged, water-resistant plywood along with heavy applications of penetrating oil or marine varnish. And don’t forget annual maintenance to maximize service life. With the right treatment, plywood can deliver lasting performance in outdoor settings.