Plywood is an engineered, layered wood sheet that comes in various thicknesses. The veneers are glued together adjacently, with its grain rotated at 90º on each layer.

It’s manufactured using a cross-graining technique that helps to reduce splitting and creates a strong, consistent panel.

While plywood has been widely adopted for various construction uses, one major flaw is its low resistance to excess moisture exposure.

We've put together this useful guide to help you successfully waterproof your plywood so that it lasts much longer, even in moist environments.

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Table of Contents

1. Is Standard Plywood Waterproof?
2. Can You Make Plywood Waterproof?
3. Can You Buy Waterproof Plywood?
• Class I
• Class II
• Class III
4. Benefits of Waterproofing Plywood
• Prolonged Life
• Prevents Dry Rot
• Prevents Wet Rot
• Prevents Warping
• Prevents Splitting
5. How to Waterproof Plywood
• Surface Preparation
• Epoxy Sealer
• Woodcon
• Hardwax Oil
• Liquid Latex
• Marine Sealer
• Polyurethane Varnish
• Water-Based Stain
• PVA
• Sealing the Edges
6. Conclusion

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Is Standard Plywood Waterproof?

Standard birch plywood sheets are not waterproof!

It’s a great wood for home construction as its laminated structure offers more stability than other sheet timbers.

However, prolonged exposure to moisture will cause the plywood to delaminate, soak up moisture and swell - eventually destroying the wood’s structural integrity.

In fact, the type of adhesive used to bond plywood is the cause of its low moisture resistance.

Most plywood manufacturers use urea-formaldehyde because of its low cost, its availability and resistance to microorganisms that may destroy the wood.

But its high water solubility is the plywood's major disadvantage.

Over time, exposure to water will cause the individual layers of the plywood to split and the glue to disintegrate.

Can You Make Plywood Waterproof?

You can waterproof standard plywood by following the simple steps outlined in this article.

The plywood cross-grain finish and urea-formaldehyde make it very receptive to waterproofing materials like paint or sealant.

Urea-formaldehyde glue has high water solubility, absorbing liquid-like paint applied to the surface.

Also, its cross-grain finish creates a rough surface that prevents paint or sealant from slipping away when applied.

While waterproofing plywood means applying a waterproof substance to its surface and around its edges, many techniques and substances may be employed depending on your available resources.

Can You Buy Waterproof Plywood?

There are many brands of waterproof plywood available in the market.

One thing to note is that these waterproof plywoods are not all of the same quality.

Plywood is available in three types:

1. Commercial Plywood.
2. Boil Water Resistant (BWR) Plywood.
3. Marine Plywood (also called Boil Waterproof).

For waterproof plywood, we recommend purchasing marine plywood for general use.

But the quality of the plywood will depend on the grade you get. There are three grades in the United Kingdom:

Class I

These are plywoods designed for interior use and less humid environments. (Examples are commercial plywood.)

Class II

This plywood is designed for interior use but has a level of moisture resistance and resistance to humid environments like the kitchen and bathroom. (Examples are the BWR plywood.)

Class III

These plywoods are designed for highly humid environments and perform well if exposed to water. It’s great for outdoor use.

Benefits of Waterproofing Plywood

If you've read thus far, you've probably gotten a sense of the advantages of utilising waterproof plywood.

Here are some ways that waterproofing your plywood may benefit you:

Prolonged Life

Exposing plywood to excessive moisture will destroy the wood very quickly.

Applying a waterproofing substance to your plywood can prevent the wood from warping, splitting or deteriorating.

Waterproofing substances add an extra layer of protection to your plywood, which prevents the wood from absorbing moisture, instead causing water to run off its surface.

In addition, this waterproofing material, like sealant or paint, will make the wood stronger than standard plywood.

Fully waterproofed plywood has an expected lifespan of around 10 to 50 years.

Prevents Dry Rot

Despite the name dry rot, it is caused by fungal growth from damp and high-humidity situations.

This virulent fungal infection in the wood can easily spread throughout the plywood and cause major damage.

Also, it is important to note that dry rot will affect plywood with a 20% moisture content.

Waterproofing strategies that help reduce moisture content in the wood or eliminate it must be employed to prevent dry rot.

The fungus will struggle to survive outside damp areas or surfaces.

Prevents Wet Rot

One of the most common causes of plywood deterioration is wet rot, which is caused by exposure to water.

Wet rot may not always cause the wood to crack, but it can cause the plywood to swell and lose its structural integrity.

Plywood rot not only looks ugly, but it also smells horrible, and when this happens, the plywood must be removed since it no longer serves its purpose.

Wet rot may be disastrous for your property's interior or exterior.

Waterproofing your plywood might spare you the trouble of dealing with a slowly deteriorating plywood panel caused by wet rot!

Prevents Warping

Warping destroys plywood!

When plywood warps, it simply means that the moisture content of the different layers of the wood is uneven, causing major bending, deformity and structural disintegration.

For instance, when one part of the plywood dries faster than the other, the dried section shrinks and causes the wood to warp.

Using waterproofing materials such as polyurethane provides a nonporous seal for your plywood.

It’s an excellent technique to prevent warping, as it reduces the amount of moisture the wood absorbs.

Prevents Splitting

Moisture and humidity will cause the plywood to expand, leading to splitting.

The most effective wood splitting solution is to eliminate the moisture that the wood can absorb.

Most waterproofing materials provide an excellent layer of protection and a nonporous seal to the plywood, preventing it from absorbing moisture.

Furthermore, a thoroughly sealed plywood surface keeps the wood compact and stable, preventing fungus from developing or wood expansion.

Waterproofing your plywood before use is the perfect technique for staying one step ahead of this situation.

How to Waterproof Plywood

There are various ways to waterproof plywood.

However, it’s important to consider available resources, the cost, the size of the wood and other factors before you choose a suitable application.

Before treating your plywood, it’s best to wear safety clothing such as glasses and gloves.

In addition, you should work in a well-ventilated area to prevent breathing excessive fumes and follow the manufacturer's directions exactly.

Surface Preparation

Materials

• Sandpaper
• Rag or cloth
• Mineral Spirit
• Primer

Step 1

Prepare the plywood by wiping it clean with a cloth. You want to ensure there are no particles or stones on the surface of the wood.

Step 2

Depending on the grade of plywood you have, gently sand the surface until reasonably smooth.

Grade A plywood does not need sanding, but grade B or other lower grades will need their surfaces sanded.

Step 3

Wipe the surface of the plywood with a rag soaked in mineral spirit. Ensure there is no saw dust left on the surface.

Lastly, apply a primer on the surface using a brush or roller and leave it to dry.

Here's some of the best techniques to waterproof your plywood:

Epoxy Sealer

Epoxy sealer is a glossy protective film or coating applied to the surface of the plywood.

It forms a hard, abrasion resistant coating, and produces a longer-lasting surface than most waterproofing methods.

In addition, its nonporous coat means that the plywood doesn’t trap moisture, instead allowing for water to roll away from its surface.

Epoxy sealers are available in spray and paint versions and are known to make the plywood surface stronger.

Tools

• Roller
• Stock 3" inch foam brush
• Chip brush
• Pair of gloves

Step 1

Pour and measure resin and hardener into separate containers.

There are different resin-to-hardener ratios, depending on the type of Epoxy used.

Ensure you mix the right ratio.

Pour the hardener and resin into separate cups to get the right measurement.

You can get the correct ratio on the package label - for example: 2:1, 1:1 or 3:1.

While mixing the resin and hardener, ensure you’re wearing protective gloves.

Step 2

Mix and stir the resin and hardener in a single container.

After getting the right ratio in a separate cup, you can mix the components together by pouring the resin into the hardener or vice versa.

Use wood or plastic to stir the mixture for at least five minutes.

If you notice bubbles in the mixture, it can easily be removed by heating the surface of the plywood.

Step 3

Pour the mixture on the centre of the plywood and spread using the foam brush.

Pour the mixture carefully on the centre of the plywood, and spread it using a foam brush.

You don't need to worry too much about even spreading as it’s the first layer.

You will need to act quickly as the mixture will harden quickly.

Step 4

Get rid of the bubbles with heat.

After applying and spreading the Epoxy across the surface of the plywood, you may notice small bubbles on the surface.

You can eliminate these bubbles by applying heat from a hair dryer.

Ensure it’s at least 7 inches from the epoxy surface and move it over the wood until the bubbles disappear.

Step 5

Apply another layer of Epoxy.

Let the first layer of Epoxy dry for at least 5 hours before applying another layer.

Pour the mixture on the centre of the wood, and spread evenly using a spatula.

Continue to spread until you achieve the desired flatness and smoothness across the plywood surface.

Leave the plywood to dry for a minimum of 24 hours, although 36 hours is recommended.

Woodcon

The Woodcon technique forms a layer of transparent water repellent coating on the surface of the plywood.

In addition, Woodcon offers fire retardant benefits, which means it's less likely to burn.

It's easy availability makes it a popular choice of waterproofing across the United Kingdom.

Furthermore, it can be applied using a paint brush, roller or sprayed across the surface of the plywood.

Hardwax Oil

Hardwax oil is an excellent nonporous sealant that helps to waterproof plywood.

It’s made of a mix of vegetable oils and wax, with additional chemicals such as volatile organic compounds, colouring, and drying agents.

In addition, the substance provides additional aesthetic appeal when applied to the plywood, forming a plastic-like layer on the wood.

It also helps the wood last longer, making it resistant to wear and water-repellent while keeping the wood’s natural moisture inside.

Here's how it's done:

Step 1

Clean the plywood thoroughly to eliminate any dirt and dust.

If entrenched stains need to be removed, use a clean wipe with a small amount of dish soap or cleaning solution.

Step 2

After that, wipe away any remaining cleaning product.

If some stains are particularly difficult to remove, you can lightly sand the surface of the plywood.

Step 3

After thoroughly cleaning the plywood and removing any cleaning product traces and leftover sawdust, apply an even layer of the hardwax oil, following the product instructions carefully.

Step 4

Allow at least 4 - 6 hours, preferably overnight, to dry.

If you used hardwax oil for the floor, you must wait at least 16 hours before allowing people to walk over it.

Liquid Latex

Liquid latex, also known as liquid rubber, is another inexpensive but effective technique to waterproof plywood.

It comes in spray cans or paint solutions, and is available at local DIY stores.

When applied to the surface of your wood, it provides a layer of waterproof protection.

If you want to waterproof a small sheet of plywood and have a limited budget, then liquid latex technique is best.

Here's how it's done:

Step 1

Remove dust and debris from the plywood's surface using a clean cloth and some detergent.

If you prefer, you can use a 50/50 mixture of water and vinegar to achieve a streak-free finish.

Step 2

Once the plywood is clean and dry, with no traces of cleaning agent, you can apply the liquid latex with a clean paintbrush or roller.

Make sure to apply it in a smooth, even layer whether you spray, brush, or roll it on.

Step 3

After applying the liquid latex, allow the plywood to dry for an hour or so.

It should dry fairly quickly.

Marine Sealer

Marine sealers are excellent for plywood exposed to excessive moisture, like boats and sea vessels.

It produces an air-tight layer over the wood surface.

Step 1

All surfaces of the plywood should be cleaned before applying the sealant.

Step 2

Remove old coats, debris and other materials on the wood.

Step 3

Carry out an absorbency test by sprinkling water on the plywood surface.

If the water doesn’t get absorbed quickly, stripping the wood is necessary.

Avoid applying in hot temperatures of above 950F, or if rain is expected within 15 hours.

Thoroughly mix and stir the marine sealant.

Test on a small section before proceeding to apply over the wood surface.

Apply the first coat using a synthetic paintbrush or spray.

Apply the next coat at least an hour after the first coat.

Allow at least 12 to 24 hours to dry naturally, depending on the temperature.

Polyurethane Varnish

Polyurethane varnish is a tough coat that can be used to add a layer of protection to plywood.

It’s available in water, solvent and oil-based types, with its water production containing less volatile organic compounds (VOC).

Polyurethane varnishes protect materials like plywood from moisture and harsh weather conditions.

You will require the following:

• A small paintbrush
• Tape for masking

Also, don't forget to wear protective gear, as polyurethane contains incredibly harmful chemicals like carbon monoxide and hydrogen cyanide.

Make sure to apply the polyurethane varnishes around the edges of the plywood, as these are most susceptible to absorbing water.

Here's how it's done:

Step 1

Using a brush, apply a coat of varnish to the plywood's edges. Avoid getting varnish on the surface of the plywood.

If varnish gets on the surface of the plywood, it will prevent the wood from absorbing any future treatment/product.

Cover the plywood completely with masking tape, leaving only the edges, to prevent the varnish from getting onto the surface.

Step 2

After painting the varnish onto the edges and following the manufacturer's drying time guidelines, apply another 1 or 2 coats to the edges, allowing for drying time between coats.

Water-Based Stain

Water-based stains are a great way to waterproof plywood.

It offers deeper penetration into the wood than oil-based stains, resulting in a deeper and rich colour with the first coat.

Other fantastic advantages of using a water-based stain for waterproofing include the following:

• Low odour compared to other techniques
• Fast drying
• Less volatile organic compounds (VOC)
• Available in both spray and hand application
• Prevents UVA sun damage

You will need the following supplies:

• Water-based pigment stain
• Cotton rags
• A spray bottle with a pump

Step 1

Apply an even layer of staining product to the entire surface of the plywood using a spray bottle or a disperser provided by the manufacturer.

Step 2

After spraying the entire surface, use a clean rag to remove any excess stain.

Water-based stains only provide a light finish, so don't expect to see dramatic or noticeable colour changes.

Step 3

Allow the product to dry thoroughly according to the manufacturer's instructions on the stain packaging, and don't be tempted to shorten the drying time!

Wait 24 hours after spraying the treated plywood. For best results, allow it to dry completely. When you're sure it's completely dry, apply another coat or two to your liking.

PVA

This is a type of glue that has traditionally been used for bookbinding. PVA has every property that a waterproofing material should have.

It dries clear and thin, adheres nicely to plywood, and is entirely waterproof.

If you wish to seal plywood with PVA, dilute it with water, so it goes on smoother and soaks into the board a little before drying.

Sealing the Edges

To seal the edges of the plywood, do the following:

Apply several coats of varnish around the edges.

Ensure the varnish dries for at least 2 hours.

Since the edges absorb the varnish rapidly, you'll need to apply extra coats to ensure that it cures to produce that protective coating you want.

Conclusion

Waterproofing plywood will help you protect it against warping, rotting and other damage caused by excessive moisture, high humidity and environmental conditions.

The process of waterproofing can be done easily if the steps listed in this article are followed correctly.

Also, an added benefit to the plywood is that it makes it last much longer.