How to Waterproof Plywood (Do It Right The First Time)

What's In This Article

Plywood can be waterproofed in 5 ways. With epoxy sealer, drying oil, liquid latex, water-based paint or varnish. The general process is to start by lightly sanding the plywood with 100 to 180 grit sandpaper, remove any sawdust, apply the waterproofing finish and then let it dry.

Before we go in-depth, there is more information you should know about waterproofing plywood if you want to make sure you tackle the job correctly and do it right the first time.

To start, wood is one of the most versatile materials in the construction industry, which explains why plywood is usable for all types of structures and projects. Plywood boards are impact-resistant, sturdy, and stable despite their relative thinness.

Plywood is a type of wood that is affordable, making it a staple building material. However, it's vulnerable to water damage and moisture when used for outdoor applications.

Learning how to waterproof plywood can help prolong its service life by preventing decay, warping, and cracking.

Plywood comes in multiple variations, and the most common is water-resistant. The plywood has waterproof adhesives and can repel water molecules. However, the water-repellent degree does not protect you or your home from a constant splash of water.

So, why do many people prefer plywood to waterproof their workpieces and homes instead of other materials?

Well, this is because plywood is engineered wood – this means that it's not wood that comes directly from trees. Instead, engineered wood is a combination of wood and other material layers compressed to make them solid. Plywood has several paper-thin material layers called veneers bound to each other using adhesives.

The adhesives are resistant to water and hold multiple veneers together to create a water-resistant plywood board. These plywood sheets are cheaper than traditional hardwoods and are easy to manipulate for various applications.

While plywood is somewhat water resistant, it is not naturally waterproof. Specifically, it can handle brief spraying or dipping in the water without adverse effects. But you can't leave the plywood sheet exposed for a prolonged time.

Prolonged moisture exposure causes plywood to warp and rot like other wood types. So, plywood is naturally water resistant but not impenetrable.

Fortunately, there are things you can do to strengthen plywood boards and make them more resistant to moisture.

Waterproofing plywood is possible.

The process is not difficult to accomplish, and you can use several methods. Plywood's texture finish is a big reason it bonds with penetrating epoxy sealants and paints.

The sheets may seem smooth, but they have surface imperfections. Microscopic pores in plywood help the sheet bond with other waterproofing materials. Waterproofing plywood is easy, and you can use the material outdoors or even in marine applications!

Waterproofing plywood sheets offers several advantages.

The lifespan for untreated outdoor wood, such as bamboo, is only two to five years. However, untreated plywood may not last long because the material is water-resistant and not waterproof.

Continuous exposure to rain, humidity, and other extreme conditions spell trouble for non-sealed plywood. Some problems include rotting, warping, and infestation of pests like worms and termites.

Waterproofing plywood protects the board, whether you use it outdoors or indoors. A good deal of plywood structures today are waterproof to prevent damage from faucet and drainage leaks or breaks.

A waterproofing material means you are not constantly worrying about damage or multiple replacement jobs. A properly sealed plywood sheet can last anywhere between 20 to 40 years in your home.

An excellent sealing job means you won't spend money and time replacing damaged plywood boards.

Plywood might be an engineered wood, but it's also susceptible to rot. This is because wood is the base material, which eventually days over time.

Poor ventilation and excessive humidity can expedite the decaying process. The moisture causes the different veneer layers in plywood to separate, fall apart, and eventually disintegrate.

But there are ways to slow or stop the natural decaying process of many biological materials, including wood. Sealing can protect plywood from rot. Waterproofing protects the material from outside humidity.

Dry rot (brown rot) is wood decay due to fungi, which can ruin any plywood inside or outside your home. The fungi break down cellulose and hemicellulose components, which give the plywood and other wood products their resilience and strength.

Plywood becomes brittle and weak, with a blocky appearance. Dry rot only affects plywood that is damp and with a moisture content of over 20%. Waterproofing the material helps remove the moisture content.

Dry rot fungus spores that spread through the air. They germinate on landing on plywood exposed to a high moisture level from:

The spores need moisture, a food source (your plywood board), warm temperatures, and oxygen. Without sealing, the affected board starts disintegrating.

Constant moisture exposure and heat warp plywood boards fast. A completely flat plywood sheet can turn into an art sculpture after a rainy summer day.

The accumulated moisture will have the same effect as water seeps into the plywood pores, causing the sheets to swell. Sun rays cause the material to dry out quickly, warping in the process. A continuation of the cycle may lead to terminal damage to the plywood structure.

The curves and bends are artistic but will make your construction look terrible. Waterproofing is an excellent way to ensure your plywood structure looks good for years.

Splitting is a result of exposing plywood to water. The moisture causes the layers within to separate and completely ruin the board.

However, splitting is hard to spot. When you finally notice splitting plywood, the damage may already be affecting other parts of your home or deck. Waterproofing before installing plywood can help prevent splitting.

There are several products you can use for outdoor plywood sealing. Fortunately, most products are affordable and simple to use, and proper application increases their effectiveness.

The product you choose depends on your requirements, budget, and the amount of time available. Here are common products used for waterproofing plywood.

Remember, most plywood waterproofing products are toxic substances. Always cover your nose, mouth, hands, and eyes using Personal Protective Equipment (PPE) when applying them. Also, lay down a tarp and seal off everything you wouldn't want the solutions to get onto or into.

An epoxy sealer (epoxy resin) is an oil-based thick coat that penetrates wood pores and makes plywood waterproof from the inside. Epoxy sealing offers protection against water damage by blocking out moisture and air infiltration, which may lead to mold growth.

The sealer comes in two parts; the epoxy and hardener, which you mix to create a solution. Apply the mixture to the surface and edges of the plywood, but do so quickly because it dries fast. Follow manufacturer instructions for the best results.

Epoxy resin is clear unless you decide to mix it with certain dyes for added color. A clear epoxy lets you maintain the beauty of the wood grain on plywood.

Epoxy sealers are notably thick and hard after careful curing, so they also offer protection against general damage. The product is a common feature in boat construction. If the product is reliable for waterproofing boat parts, it should protect your plywood from moisture.

You can run sandpaper over the dried epoxy surface and paint over it!

Seals, preserves, & protects hard and soft woods from mold, mildew, and salt damage. Its slower cure rate gives the epoxy time to saturate deeply. All without the odor & fumes of other epoxies.

Drying oil is an excellent waterproofing product for outdoor use. The oil lets plywood maintain some of its natural flexibility and moisture.

Drying oil soaks into plywood sheets to provide an effective protection coat, especially in areas where you need flexibility. The product is a better choice if you want to apply plywood in high-moisture areas.

A drying oil works differently from epoxy sealers because it protects plywood against water deterioration instead of blocking water. Drying oils extend the life of plywood without making it fully waterproof, so it's advisable to reapply over time.

The most common variant is Tung oil. Just wipe a thin coat of the oil across your plywood and let it dry. You have the option to apply multiple coats, but make sure each coat dries completely before the next.

Waterproof Sealant - Wood Stain and Sealer in One - 1 Gallon & Clear

The easiest way to seal plywood is liquid latex. You can purchase the product as an easy-to-apply spray can for minor projects or in large quantities with a paint sprayer for larger jobs.

Liquid latex creates a protective layer on plywood. The substance seals the porous layer and prevents water from going inside the surface. Latex sprays require multiple coatings depending on the sealant strength and plywood water exposure, but two to three layers are enough.

Liquid latex products decorate your plywood well for outdoor use. The method is ideal for layers experiencing direct exposure to the elements. The product's versatility, cheapness, and durability make it the perfect waterproofing method for your deck or beachside balcony plywood.

Liquid Rubber Waterproof Sealant works by creating a watertight seal to keep moisture where you want it. With multiple uses, the possibilities are endless.

Water-based paints work well when waterproofing your outdoor plywood. However, the method does not create a long-lasting effect like drying oil or epoxy. You need to touch up or repaint the surfaces every few years.

Problems with water-based paint surfaces on plywood are easy to fix and protect the board from moisture damage. Paints give you a major advantage other products on this list don't offer; color!

Provides a 15 year waterproofing guarantee, resists 20 psi water pressure, nationally voc compliant.

Varnish is a thin resin type that creates a protective, hard coating after curing. Think of varnish as something that makes any wood type look better. The product gives your plywood surfaces a rich, dark brown color that transforms the poor aesthetics of low-quality wood to look like dark, rich oak.

Varnish is ideal for above-the-water line uses. The product will resist moisture, mildew, and salt as long as your plywood is not submerged.

Formulated to work in both fresh and salt water settings, these paints, primers and additives work hard to keep maintaining your wood easier than ever.

Seal and waterproof your plywood before using it outdoors by applying several layers of epoxy sealer, drying oil, liquid latex, paint or varnish. Use fine to medium grit sandpaper and clean thoroughly before sealing each side at a time.

Give each coat the recommended drying time, and pay special attention to the edges.

Some items you require to seal your plywood include:

There are three key steps to waterproofing plywood. Let's examine each of these steps in a little more detail.

Prepare the plywood before applying any sealer. Preparation is the same no matter the sealant product you choose.

First, clean the surface using a dry cloth or soft brush and remove glue, grime, and dirty spots. Inspect your plywood for imperfections such as knots, dents, and holes. These imperfections can create an uneven surface that makes bonding for sealing agents harder.

Use wood filler on most imperfections to create a stable, more solid surface. Give the filler at least 30 minutes to dry, then use some sandpaper to smooth it out.

Wipe the plywood with a damp cloth after the wood filler dries. A little moisture will help raise the grains a bit and remove pollen and fine dust. Make sure that water is not running down the plywood from your cloth.

Finally, lightly sand the plywood using 100 to 180 grit sandpaper. The step ensures the sheet is smooth and removes rough spots likely to interfere with the sealer and waterproofing. Don't apply too much pressure to the sandpaper.

You can apply most waterproofing products with a roller or brush, but some require a sprayer. The sealer application doesn't require special equipment.

When using a brushed-on sealing product, paint it on the plywood with a roller or brush as you’d normally do. Get enough coverage and allow the first coat to dry before applying the second. Adding a third coat is not necessary, but some products and waterproofing requirements may require it.

When spraying, read the manufacturer's instructions carefully. An improper spray-sealer application can lead to:

Waterproofing the edges will prevent water from seeping into your plywood and ruining it. Apply a thicker amount of sealant to the edges.

Alternatively, use edge banding tape. The tape makes sure your plywood is durable and prevents water damage. Make sure you’re using PVC edge banding tape as it's more durable and can withstand moisture.

Give your plywood a few hours to dry. The length will depend on the waterproofing products you’re using. It's advisable to wait at least 24 hours before installing your waterproofed plywood.

Add some sealant to the screws or nails you’ll use for installation. The extra step ensures everything stays watertight.

Inspect the plywood sheets every couple of months to ensure moisture is not damaging them—a simple eyeball test is enough!

Now that we’ve gone through the different plywood waterproofing methods and major steps, it's time to answer another critical question hobbyists ask—what are the different types available?

Plywood manufacturers use letters to grade the panel and glue type used on a board. The American Plywood Association (APA) uses grades A, B, C, and D to show the plywood panel quality. Two letters, such as AB, AC, or AD, indicate the board has two panels glued together, with each side representing a different quality. The X on the label stands for the exterior grade resin glue, which is not waterproof but of high quality.

From Grade A, the consecutive letters are indicators of more faults in the panels. The faults or defects include knots, splits, and cracks. A Grade A plywood panel has few defects, and those it has are fixable before selling.

Here are six of the most common plywood boards.

Also known as marine grade, ABX plywood is the highest grade. The boards are also available as AA, AB, and BB. Construction of the board features Douglas Fir or Western Larch wood while the glue is of the highest quality permanent WBP glue.

Marine plywood standard BS 1088 dictates the thickness of the face and back veneers for multi-ply construction be between 1.3 mm and 3.8 mm. Three-construction plywood should be about 0.5 mm thick.

Each sheet in ABX plywood should have no defect in the face or the back. The inner layers should have no holes or voids and be stronger and denser than the exterior sheets. ABX plywood edges are cut and sanded cleaner than those of standard boards.

ACX plywood is a high-grade board with a smooth, sanded front face and the back side is not finished or sanded. The plywood can resist heavy blows because of its uniform strength in any direction. ACX plywood is made from different species of wood, including pine and cedar.

The plywood is applicable in cases where a good appearance or smooth finish is critical. One critical use for ACX plywood is for subflooring under a vinyl floor. It's also an excellent material for furniture and shelving projects that require painting.

ADX plywood is much the same as ACX, but the back panel is a knottier, rougher variety. The plywood is a cheap option because only the front face is a pricier grade. ADX plywood is applicable for outdoor construction and subfloor projects.

Boiling Water Proof (BWP)-grade plywood is a moisture-resistant board that withstands atmospheric changes and is boil-proof. The plywood can resist exposure to moisture and water when used in cabinets or furniture without experiencing quality damage.

BWP-grade plywood is commonly used in bathroom or kitchen cabinets, under-sink cabinets, and lofts where there's continuous water exposure. Phenol-formaldehyde glue in the plywood features advanced technology to ensure zero gaps in the veneer and resistance to termite attack.

Moisture-Resistant (MR)-grade plywood can resist moisture with ease but is not waterproof. MR is a preferred choice in hot and humid areas.

MR-grade plywood is ideal for indoor use with excellent internal strength. Common applications include office and home furniture. The plywood has a high internal density that makes it fold-resistant and robust.

The plywood has resistance to termites because of the additives and adhesives used. However, MR plywood is not on par in terms of durability or strength in comparison to BWP or BWR plywood.

Boiling Water Resistance (BWR) plywood is highly resistant to humidity and water. The exterior-grade board is suitable for semi-outdoor and outdoor uses, including making garden tables, lawn chairs, and outdoor swings.

Waterproofing characteristics of BWR plywood come from grade A phenol-formaldehyde adhesive or phenolic resins used during manufacturing. The plywood is warp-free and the core layer contains multiple toxicant chemicals that make it termite-proof.

It's a mistake to assume all plywood is the same and vulnerable to water damage. However, plywood is available in a variety of configurations, which makes each type ideal for specific applications. Some boards come with waterproofing pre-treatment, so you don't have to seal them!

The best plywood variants for outdoor use are ABX or marine-grade plywood and CDX plywood. ABX plywood has a smooth, high-quality finish and offers the best waterproof characteristics. The plywood is suitable for multiple outdoor applications, including decks and waterfront projects.

CDX plywood is a budget-friendly option capable of withstanding extreme weather conditions. The plywood has a clean finish that is aesthetic, and resistant to wear and tear.

Plywood is an excellent building material for multiple applications. The material is versatile, strong, durable, and easy to work with. However, plywood is usually not waterproof.

Learning how to waterproof plywood can help protect your outdoor construction jobs from rotting and warping. The process is simple and easy to complete within a day!

Chief Building Officer at Green Building Elements, joined us after a successful career and business owner at Alexander Architecture, which served the New York City Metropolitan area. Prior to running his own firm, John was an architect working for Gensler projects in Boston which is the largest architecture firm in the United States. John holds both a bachelors and masters degree in architecture from the Rhode Island School of Design. John is the current contact for InformeDesign and The 1 Percent program of Public Architecture.