Plywood is one of the most versatile building materials for construction and home improvement projects, but its susceptibility to moisture can lead to warping and rotting. If you want to make your plywood last longer and keep it from deteriorating, the best way to do it is by waterproofing it.
If you’re new to waterproofing plywood, don’t worry—we’re here to help. This guide will walk you through the process of waterproofing plywood from start to finish so you can save money on repairs and keep your projects looking as good as new.
Why Waterproof Plywood?
Since plywood is made of bonded layers of wood, it’s absorbent. This means that untreated plywood may swell, warp, or rot when it comes into contact with moisture, which can cause significant damage over time.
Additionally, the multi-layer quality of plywood is a breeding ground for all kinds of fungi like mold and mildew, potentially leading to health hazards. However, plywood’s absorbent qualities make it perfect for holding onto waterproofing sealants, so you can prevent all these problems by treating the plywood ahead of time.
Preparing Plywood for Waterproofing
Before applying any waterproofing treatment to the plywood, there are a few steps you need to take. Don’t skip the preparation process—these steps make for better adhesion and more effective waterproofing.
Clean the Surface
Remove any dirt, dust, or debris on the plywood using a brush or vacuum cleaner. Then, use a damp cloth to wipe the surface of the plywood clean. Allow it to dry completely before proceeding.
Sand the Plywood
Sanding the plywood creates a smooth surface for better adhesion of waterproofing products. Use medium-grit sandpaper (between 60 and 120-grit) to lightly sand the entire surface, paying extra attention to rough spots or splinters. After sanding, clean the surface with a brush or vacuum cleaner to remove any dust.
Fill Gaps and Holes
Inspect the plywood for any gaps, holes, or dents. Fill these imperfections with wood filler using a putty knife. Once the filler dries, sand the filled areas to create a smooth, even surface. After sanding, clean the surface with a brush or vacuum cleaner again to remove any dust.
Make sure to fill any gaps, holes, or dents in the plywood with wood filler. Any cracks in the wood can result in gaps in the waterproofing, which allows moisture to settle.
Methods for Waterproofing Plywood
There isn’t just one right way to waterproof plywood, but each method has its own advantages. Feel free to choose the best option based on your project requirements and skill level.
Epoxy Sealant
Epoxy sealant is a popular choice for waterproofing plywood due to its durability and strength. It creates a clear, hard coating that protects the wood from moisture while improving its natural appearance.
To apply epoxy sealant:
- Mix the epoxy according to the manufacturer’s instructions.
- Apply a thin, even coat using a paintbrush or roller.
- Allow the first coat to dry completely before applying additional coats.
- Sand lightly between coats for better adhesion.
Epoxy sealant is ideal for projects that require a strong, waterproof finish, such as outdoor furniture or boat decking.
Latex Paint
Latex paint offers a flexible and long-lasting waterproofing solution for plywood. It’s easy to apply and comes in a variety of colors, making it a versatile option for both interior and exterior projects.
To waterproof plywood with latex paint:
- Apply a clear latex primer designed for use on wood surfaces.
- Once the primer has dried, apply two or three coats of exterior-grade latex paint.
- Allow each coat to dry completely before applying the next.
Latex paint is an excellent choice for projects that may require some flexibility, such as outdoor structures that experience temperature fluctuations.
Penetrating Oil
Penetrating oil is a natural option for waterproofing plywood. It soaks into the wood fibers, providing protection from within rather than creating a surface barrier.
To apply penetrating oil:
- Use a brush or cloth to apply the oil generously to the plywood surface.
- Allow the oil to soak in for 15 to 20 minutes.
- Wipe off any excess oil with a clean cloth.
- Apply additional coats as needed, allowing each to dry completely.
Penetrating oil is ideal for projects where maintaining the natural look of the wood is important, such as indoor furniture or decorative items.
Polyurethane Coating
Polyurethane is a durable, clear coating that provides excellent water resistance. It’s available in both water-based and oil-based formulations.
To apply polyurethane:
- Choose between water-based (faster drying, less odor) or oil-based (more durable) polyurethane.
- Apply a thin coat using a brush or roller, following the wood grain.
- Allow the first coat to dry completely, then lightly sand the surface.
- Apply two or three additional coats, sanding between each.
Polyurethane is great for projects that require a clear, hard finish, such as tabletops or shelving units.
Waterproofing Plywood for Different Applications
Depending on what type of project you’re working on, you may need a specific type of waterproof sealant. Here are some recommendations for a few common uses.
Outdoor Furniture and Structures
For any projects outdoors in the elements, use epoxy sealants or high-quality exterior latex paint. If you want to go above and beyond, apply a combination of penetrating oil before sealing with epoxy for multiple layers of protection.
Marine Applications
Boat building and other marine applications require the highest level of water resistance. We recommend epoxy resin systems since they create a tough, impermeable barrier against water. To be safe, finish the plywood with a marine-grade varnish for UV protection and enhanced waterproofing.
Roofing and Siding
When using plywood for roofing or siding, we recommend using a combination of penetrating oil and polyurethane coating. This mixture offers double protection to withstand extended exposure to moisture.Â
Tips for Effective Waterproofing
To get the best results when waterproofing plywood, consider these helpful tips:
- Always work in a well-ventilated area, especially when using chemical-based products.
- Wear protective gloves when applying waterproofing treatments.
- Follow the manufacturer’s instructions carefully for each product.
- Pay extra attention to the edges and end grain of the plywood, as these areas are particularly susceptible to splintering and water absorption.
- Allow sufficient drying time between coats and before using the waterproofed plywood in your project.
For a better idea of the safety procedures for wood staining, refer to the Occupational Safety and Health Administration’s guide on woodworking finishing chemicals.
If you’re unsure what method of waterproofing to use, do a small test application on a scrap piece of plywood to help you gauge the effectiveness before committing to the entire project. If you don’t like the look of it, you can always choose a different waterproofing agent and try again.
Maintaining Waterproofed Plywood
Proper maintenance can extend the life of waterproofed plywood and keep it protected against moisture. Here are some maintenance tips:
- Regularly inspect the waterproofed surface for signs of wear or damage.
- Clean the surface periodically to remove dirt and debris.
- Reapply waterproofing treatments as needed, typically every one to three years, depending on exposure and use.
- Address any damage promptly to prevent moisture from penetrating the plywood.
If you notice areas where the finish is wearing thin or peeling, sand and reapply the waterproofing products so the wood doesn’t get damaged.
Final Thoughts About Waterproofing Plywood
It’s always a good idea to treat your wood and give it a protective coating, but it’s especially important for plywood because of its layered design. Whether you’re building outdoor furniture, constructing a shed, or working on interior projects, taking the time to waterproof your plywood will pay off in the long run. Waterproofing may be more effort initially, but you’ll have long-lasting structures that are free of mold and rot as your reward.