How to Waterproof Plywood- 5 Proven Ways to Waterproof Plywood - WoodCritique (2024)

Plywood is a famous construction material because of its affordability, versatility, and durability. However, it is susceptible to water damage when used on decks and other outdoor projects. So, let’s learn How to Waterproof Plywood.

You can waterproof plywood with the application of polyurethane, epoxy finish, or paint.

Lightly sand your plywood using 180-grit sandpaper, and clean the resulting sawdust.

Then, apply the chosen finish with a brush or pour it if using epoxy.

Knowing how to waterproof wood helps prolong its life span by preventing warps, cracks, and decay. In addition, we have multiple waterproofing strategies.

Thus, read the article to the end for the specifics of each method.

How to Seal Plywood for Outdoor Use

Apply a water seal to preserve plywood from water damage. Further, consider using a garden pump-up sprayer to apply thin, multiple coats to the surface.

However, scuff sand and prepare the surface before application for better adhesion. This way, you will guarantee a professional outcome.

You can also seal the plywood with water-based paint. But start by applying the corresponding plywood primer.

In addition, apply the finish with a high-quality roller or chip brush for a smooth, consistent surface.

Finally, lightly sand between coats and allow each layer to dry to the touch before adding the next.

Then, apply several sealant coats to deliver superior resilience against moisture penetration.

How to Waterproof Plywood for Outdoor Use

We have a few standard methods for waterproofing different types of plywood. The good news is that most of them are relatively simple and affordable.

Thus, you do not need to be a professional to achieve a lasting result.

Even better, these strategies are more or less equally effective with proper application. So, getting the absolute best way to waterproof plywood may be tricky.

Nonetheless, take a look at the top six recommended plywood waterproofing techniques.

• Applying Epoxy Sealer

An epoxy sealer is perhaps the most popular way to seal the plywood. It strengthens the lumber by forming a scratch, dent, and water-resistant layer.

The epoxy soaks into the plywood and goes clear. Hence, the wood will retain its natural color.

After running sandpaper over the dried epoxy, you can still paint the surface any color you like.

The sealers come in paint and spray forms, tinted paint, and transparent hues. In addition, unlike polyurethane varnish, it has a thicker consistency.

This thickness makes spraying less viable, requiring you to use a paint roller.

Apply the formula along and against the grain for the best outcome. Moreover, remember to tip the wood with a foam brush immediately after application.

This way, you’ll even out the texture left behind by the paint roller.

Lastly, let each coat dry for about one hour and sand it gently before applying subsequent layers.

• Using Drying Oil

Unlike most waterproofing products, drying oil does not form a hard coat. Instead, it penetrates the plywood’s pores, seals them, and makes the sheet impervious to moisture.

In addition, the formula does not entirely block moisture from the lumber but strengthens it against water deterioration.

This aspect is beneficial as it preserves the wood’s natural moisture and keeps it flexible.

However, getting a more robust sealer, such as epoxy and wood varnish, is advisable for the lumber’s edges.

The oil only strengthens the wood against moisture damage, not making it watertight.

As a result, you’ll significantly increase the wood’s life cycle without making it 100 percent waterproof in the conventional sense.

This technique is the best when planning to use plywood in high-moisture locations like boats and docks. It also comes in handy when you live in a wetter environment.

• Spraying Waterproof Paint

Waterproof paint is the best if you are not a clear coat fan. Besides, the product comes in multiple colors.

Hence, getting something that matches or complements your outdoor decor and color theme is easy.

Ensure you use oil-based paint to facilitate a lasting bond with the plywood.

The formula seeps into the lumber’s pores and blocks them, making the surface resistant to water and damaging UV rays.

Please note that you still need to seal the plywood edges with varnish or epoxy while coating both plywood sides with paint.

Moreover, consider applying an oil-based primer before spraying the paint.

The finish may also take some hours to dry between coats. So, a little patience is necessary to complete the project.

• Polyurethane Varnish Method

Polyurethane varnish is among the best plywood waterproofing sealers. It cures into a tough coating, resulting in water and scratch-resistant surfaces.

The product is suitable for coating wooden floors, stairs, and surfaces in high-traffic areas. Besides, it is a perfect way to seal hardboard, plywood, chipboard, and parquet.

A wood sealant is also an excellent alternative, whether the plywood is outdoors or indoors.

Further, a quality sealant delivers a rigid, long-lasting coat on the lumber’s surface.

So, you won’t have to stress about water, moisture, and wear and tear damage.

Your local hardware store should have a wide range of sealants. Consult with the sales expert and follow the manufacturer’s directives on the package.

Apply varnish with a brush, paint roller, pad, or sprayer. Also, work with thin, even coats and give the finish enough time before adding another layer.

The easiest way to apply the formula is with a paintbrush. However, try to get a size appropriate for the plywood. It will make your work much faster.

Ensure that you cover the entire surface and remove any streaks or bubbles. In addition, give the sealant enough time to dry.

The drying time for oil-based sealants is 24 hours, whereas water-based products only need four to six hours.

Finally, consider applying more than one polyurethane coat for enhanced durability. Two to three coats will do!

The resin mixture forms a robust, transparent, water-resistant coat. Thus, your plywood boards will be resilient to harsh outdoor conditions.

Further, polyurethane varnish is not as noticeable as paint or thick as epoxy. But you will enjoy various benefits.

The finish leaves an appealing glossy appearance after drying. It is also ideal for above-water line marine applications.

Hence, you can use it for above-water parts of a dock or upper areas of wooden boats.

Even better, the product resists mildew, salt, and moisture for years as long as you do not submerge it in water.

• Liquid Latex

Liquid latex comes in liquid form, as its name suggests. Besides, it is also an easy way to waterproof your plywood for exterior use.

In addition, you will deliver adequate coverage as the liquid bonds well to plywood and lasts a couple of decades.

However, water will enter the plywood through gaps left by poorly sprayed latex. Therefore, ensure you completely cover the surface.

The formula is available in larger containers for more demanding applications.

Also, you can consider using a paint sprayer when working with a dozen plywood sheets.

Spray-on liquid latex does a perfect sealing job. But you need to waterproof the wood’s edges with polyurethane varnish or epoxy.

Remember to give the finish sufficient time to dry. Fortunately, it only needs an hour or so to cure to the touch.

Finally, when necessary, you can add another liquid latex coat after the first one dries.

• PVA Glue

PVA glue dries thin and transparent, bonds perfectly to plywood, and is impressively waterproof.

However, consider diluting it with water to improve its lumber penetration qualities.

Place the plywood on a flat worktable before applying a glue layer. Then, wait for it to absorb well into the surface.

Add another layer and carry on with these steps until you see glue spots on the sheet’s wrong side.

Next, turn over the veneer and repeat the waterproofing procedure.

Lay the plywood on a flat area and allow it to dry naturally. But please note that this method has a long waiting time.

Also, you have to protect the project from moisture for three to four days.

Choose the most suitable method and correct supplies for your project. This way, you will be sure of a successful project.

The materials include safety goggles, a drop cloth, medium and fine-grit sandpaper, a protective face mask, clean rags or cloths, and hand gloves.

Then, follow the steps below.

• Prepare the Workstation

The first thing is to select a well-ventilated location for your task. It could even be a sheltered space like a garage or carport.

Wear a safety mask as some waterproofing substances produce toxic fumes. Moreover, you’ll need to sand the wood, and you do not want to inhale the dust.

Put on hand gloves to protect your skin from touching the sealer. You can also consider wearing safety goggles to safeguard your eyes.

Finally, lay down a tarp or painter’s tape to cover the floor and anything the sanding dust and sealer could affect.

• Fill Grooves and Rough Spots with Wood Filler

Check the plyboard to confirm uniformity on the surface and the sides. Then, fill out the grooves or noticeable rough areas with wood filler.

Let the filler bulge slightly above the wood to give room for sanding.

Also, consult the manufacturer’s manual on the best hardening and drying duration to give the surface.

Please note that this step ensures no gaps impede waterproofing. Thus, it is essential for a successful result.

Wipe the plywood sheet with a wet rag once it dries. However, you will observe that the small moisture amount will raise the lumber’s grain.

But do not worry about this as it helps the sealer bond properly with the wood.

Next, sand the entire plywood with medium-grit sandpaper and wipe the sanding dust.

Move to fine-grain paper such as 180-grit and scuff sand the surface till it smoothens.

Remove the sanding dust and debris with a dry cloth or natural-bristled paintbrush.

Finally, gently sand the plywood with 60-100 grit. It helps smooth the surface and remove any spots. Otherwise, they will ‘gunk up the sealing/painting.

• Apply the Sealer

Paint the formula with a roller or brush as you usually would. Also, ensure adequate coverage and allow the first layer to dry before adding another.

However, some supplies and waterproofing requirements may need a third coat. Thus, do not shy away from adding an extra coat when necessary.

Any chosen product will have a recommended application technique and drying duration between coats. Please follow the instructions thoroughly.

Usually, these guidelines are available on the product label.

The sealer application methods include using a foam or chip brush, spray bottle, or paint roller.

Consider using a roller for epoxy or varnish. Then, tip the finish immediately with a high-quality chip or foam brush to level the texture.

It is crucial to tip immediately before the wood gets tacky. And follow the grain for a consistent outcome.

Alternatively, you can spray the formula. But you’d need to cover the surface with more thin coats than a few thick ones.

Further, improper use of a spray-on product could result in a spotty concentration. You’ll have lots of formula in some areas and little in others.

In addition, the technique needs more sealer to deliver complete coverage.

These risks occur because spray-on sealers are harder to control and see than brushed-on ones. Therefore, please be intentional with where you point the can.

Also, plywood edges are particularly susceptible to water because the material has several layers. So, ensure you waterproof them.

Luckily, sealing these edges is relatively simple. Apply several varnish coats and allow each to dry for one hour.

The plywood edges will soak up the formula quickly. Thus, you need more layers to ensure the formula dries into a protective surface.

• Let the Finish Dry, and Keep an Eye

Give the plywood a few hours to dry, up to 24 hours. Then, feel free to install it once it completely dries.

Consider adding sealer over the nails and screws to ensure the surface remains watertight.

Still, keeping an eye out for gaps in the sealed lumber is advisable. The exercise will go a long way in preventing critical damage to the structure.

Lastly, although the plywood sheets are now waterproof, assess them after every few months.

This way, you can confirm that water is not wreaking havoc without you knowing.

Fortunately, water damage is easy to spot. Thus, the process will not feel like a comprehensive inspection.

On top of that, check out a few tips for waterproofing plywood.

• Mount cupboards, drawers, and furniture onto spacers or feet to keep the wood from contacting water.
• Stick tar paper underneath the structure to keep water from seeping through the edges.
• Use an exterior plywood grade for outdoor applications. And caulk all your joints inside out for a better outcome.
• Design your project with no vertical joints or seams that will collect water.
• Use the recommended plywood type for your work. For instance, stick to BWP graded lumber for harsh weather conditions and heavy usage.
• Utilize a high-grade epoxy coating on the plywood edges or other exposed areas touching the ground.
• Pitch the plywood’s surface, like a roof if it will be staying outside. It will prevent water from pooling on the lumber.
• Alternatively, use an aluminum sheet to cover the wood for structures sitting outside.

Video On Waterproofing Plywood:

Why Is Plywood Susceptible to Water?

Plywood is vulnerable to water because of its layered structure. Besides, the material comprises thin wood sheets layered and glued together into a board.

Creating plywood involves gluing multiple wood layers under heat and pressure. Further, the process sucks out water from the lumber, giving a robust sheet of bonded veneers.

Thus, the edges retain a significant porosity level where moisture can seep in, causing individual layers to expand. Then, they pull away from the glue and split.

Moreover, the separation process can take a few months to years, depending on the water levels.

This damage can be irreparable, and the structure is no longer helpful.

Rot occurs when water thoroughly corrupts the wood, and it cannot hold weight anymore.

Spots with nails and screws are more prone to rot. The plywood sheets become completely loose as they cannot hold tightly to the fastener.

Warping also results from water damage. It leaves bent and carved wood that looks horrible and is useless in building applications.

Remember that plywood is water-resistant, not waterproof. In addition, small water amounts will not affect it. Only prolonged exposure is disastrous.

Why Should You Waterproof Plywood?

You may wonder why go through the above hassle waterproofing plywood, yet it is water-resistant?

Well, we have multiple benefits of doing a waterproof treatment on plywood. They are as follows.

• Prolonged Life

The atmosphere always possesses a specific humidity level that falls or rises depending on the weather. Hence, untreated lumber will eventually absorb some moisture.

Besides, untreated plywood boards like 2×4s will last approximately two years outside. Even worse, plywood will fail even quicker when left exposed to elements.

Similarly, an indoor water leak is as deadly to untreated lumber as snow and rain. But you can expect adequately treated plywood to go for 20 to 40 years.

In addition, sealing introduces water resistance attributes that keep the wood structurally sound.

• Prevents Rotting

Prolonged moisture exposure leads to rot even if the plywood does not split fully. Further, wood rot spells disaster for your home, deck, or boat.

Dampness can also provide a foothold for bacteria and fungi, leading to rot.

Worse still, wood rot does not just look ugly; it smells bad. Hence, you will notice a foul odor from the lumber.

Neither of the above conditions is desirable. And thankfully, you can seal plywood to prevent them from occurring.

Besides, waterproofing the structure will save you the headache of handling a slowly crumbling plywood panel.

• Curbs Dry Rot

Wet and dry rot have similar effects, but their causes are different. The latter occurs when microscopic fungi grow on the wood.

And it is almost impossible to see the organisms until it is too late.

Also, the process does not need much moisture. The damage occurs after the lumber dries and the fungi are already growing.

• Protects from Moisture and UV Rays

Repeated heat and moisture exposure warps plywood quickly. Moreover, a flat plywood sheet can turn into a modern art sculpture after a rainy summer.

Over time, accumulated moisture seeps into the lumber pores, causing the plywood sheets to swell.

The resulting bends and curves may be interesting. But they deliver an ugly floor, deck, or siding.

On the other hand, plywood dries out quickly when hit by sun rays. It warps in the process and compromises the structure’s integrity.

Thankfully, there is hope!

Waterproofing the lumber is an excellent way to enhance its longevity. The process protects it from water and UV rays damage.

• Reduces Splitting

Splitting is among the most damaging effects of moisture exposure on plywood. Further, it causes the veneers to separate, completely ruining the material.

The occurrence is sometimes hard to see. And the water will likely damage other deck or boat parts when you finally notice it.

Therefore, waterproof plywood before installing it.

How Many Coats of Epoxy Does It Take to Waterproof Plywood?

Generally, you need only one coat of epoxy resin to waterproof plywood.

However, the hardener used by the manufacturer determines the application procedure and drying duration.

Can You Waterproof Plywood With Paint?

Painting plywood is an easy and quick way to waterproof it. Further, exterior latex paint is better if the structure is outdoors.

On the other hand, interior latex paint is perfect for indoor applications.

But both options will waterproof the lumber properly, so it should not be a primary consideration.

When applying the paint, we have two extra steps: priming the surface and adding a topcoat.

A strong bonding primer bonds the paint to the plywood, preventing later peeling. Also, you do not have to sand the surface before painting.

Sanding scratches the plywood to facilitate adhesion. Thus, the bonding primer makes it unnecessary.

Exterior paint alone does not waterproof the plywood. Consider applying a top coat to add extra protection.

Finally, the best finish to use for latex paint is water-based polyurethane. It is clear and does not fade or discolor over time.

Should I Seal Plywood Before Painting?

It is advisable to seal plywood before painting. Apply two sealer coats and let them dry in-between each layer.

Sealing delivers an easier surface to apply primer and paint coats. In addition, it removes any chances of Support Induced Discoloration.

Apply a minimum of three heavy layers to seal the lumber entirely. This way, the surface will withstand harsh outdoor conditions.

Alternatively, apply two paint coats over two primer coats.

But either way, the idea is to seal the plywood and enhance its longevity.

How Do You Waterproof Plywood for a Boat Floor?

Generally, manufacturers use marine plywood to construct ships and boats because it is moisture resistant.

Besides, the material is affordable, durable, and does not show wear and tear signs. So, it is an excellent choice for several outdoor and indoor projects.

However, marine plywood still deteriorates when submerged in water. Also, harmful UV rays eventually damage the material.

Therefore, a few adjustments will enhance the wood’s water resistance.

We have several techniques used to waterproof plywood for boat floors. But the best is applying a waterproof sealant or paint.

Below is a simple procedure to follow.

• Apply a Wood Filler

Sometimes, the plywood is not smooth. So, you need to apply a wood filler.

The product fills any holes in the lumber, allowing effortless and successful sealant application.

Some wood filler types require direct application to the plywood. Or you can mix the paste with a hardener for better quality.

Also, use a putty knife to apply and smooth out the paste.

Fortunately, this step is not necessary when using high-quality plywood. Thus, you’ll save time and energy by getting the correct material.

• Sand Down the Plywood

Sanding the lumber should be the first step before sealing it. Moreover, the process removes the outer layer and facilitates even application.

Sand both plywood sides and do not ignore the edges. This way, you guarantee that the surface is ready for sealing.

Consider using 80-grit sandpaper to remove any roughness, and don’t stop until you remove the dust and debris.

• Clean the Plywood’s Surface

Remove dirt from the wood to guarantee a long-lasting finish. Use an acetone-saturated towel to deliver the best outcome.

Allow the acetone to dry for a few hours. Otherwise, the damp surface may affect its attributes. Then, apply a sealant layer.

Alternatively, you can use water to clean the wood. Use a clean cloth and remove the remaining debris.

• Apply a Wood Sealant

A quality wood sealant seals the plywood’s edges and prepares it for other procedures. In addition, it guarantees that moisture will not affect the material.

The formula protects the wood from moisture decay and mold, guaranteeing a longer lifespan.

It also makes the surface more UV rays resistant. Therefore, you can use it for boats and other marine projects.

Leave the waterproof wood sealant to dry for about 24 hours before adding another finish.

Further, do not panic after noticing a dark hue on the surface. The process tints the lumber’s color while increasing its durability.

• Use Clear Epoxy to Coat the Plywood

Clear epoxy is another suitable material to use with plywood. It penetrates deep into the lumber, delivering increased water resistance.

In addition, the formula allows other waterproof paint layers to slide effortlessly. Thus, it is easy to deliver a professional paint finish.

You can use a roller or foam brush to apply clear epoxy. Only ensure that the formula saturates the applicant for sufficient coverage.

Spread the product on all plywood sides and pay attention to the edges.

Also, epoxy dries fast. Thus, please avoid applying another layer until the first one completely dries.

Transparent epoxy is longer lasting and requires less maintenance than other sealant types. Better still, you only need to apply a new coat if the plywood shows damage signs.

Other sealant types need reapplication every few years to guarantee a desirable protection level. Therefore, you’ll have to conduct frequent maintenance.

As a rule of thumb, apply as many epoxy coats as the plywood layers. But sand lightly between layers to enhance adhesion and uniform coverage.

Standard plywood possesses four to five layers, while the marine type has ten or more layers for added durability.

Although sanding in-between coats is a time-consuming task, it is an essential one.

It changes the plywood attributes to make it waterproof and able to withstand submersion.

• Apply a Spray-On or Paint

You can get away with having one paint coat when painting indoors, but not with an outdoor application.

Moreover, it is advisable to paint both plywood sides and use multiple coats for added protection.

Also, work on the edges and wait for the first coat to dry before adding subsequent ones.

Do the same with a spray-on formula. Apply a liberal spray amount and spread it with a brush.

Let it dry, and then add the second layer.

The spray-on formula allows water to slide off the wood’s surface. It does not penetrate through the surface, preventing rot or deterioration.

Finally, consider working with high-quality exterior paint to withstand the elements.

Can Painted Plywood be Used Outside?

You can use painted plywood outside. However, ensure you use exterior latex paint for guaranteed protection.

Alternatively, you can use exterior plywood for outdoor applications. The most common types are ACX or CDX.

The manufacturer uses an X to denote its use and the construction grade. Further, the first letter refers to the plywood’s front, while the second points to the back.

‘A’ is a premium grade suitable for high-end uses or cabinetry where appearance is crucial. Besides, CDX is more affordable and sturdy.

In addition, it is a construction-grade option and will not crumble after a rainstorm. And the layers can come from high-density wood, veneers, and light hardwood.

Other exterior plywood types include overlaid, pressure-treated, T1-11, and oriented strand board (OSB).

Pressure-treated plywood has chemicals adding to its resistance against mildew and mold growth.

Overlaid plywood is similar to CDX or ACX but delivers a cleaner finish. Therefore, one can expect it to look more attractive while holding up better against wear and tear.

Lastly, OSB is an engineered substitute suitable for areas semi-exposed to elements, whereas T1-11 is ideal for siding.

Frequently Asked Questions

The most frequently asked questions around the subject include:

• Should I Seal Flex Seal on Plywood?

It is not advisable to use Flex Seal on plywood. Although the product is moisture-resistant, it is not watertight.

Thus, harsh weather elements like precipitation and humidity may seep into the wood’s core, damaging it.

Besides, Flex seal is better for piping, plastic, and porcelain, not outdoor use.

• Is It Okay to Put Epoxy Over Plywood?

Absolutely! You can apply epoxy over plywood. The exercise waterproofs the material and enhances its longevity.

The paste penetrates the wood and becomes transparent, retaining or highlighting the surface’s natural color.

We have two primary approaches to this process. The first one involves installing self-leveling concrete over the wood. Then, add an epoxy layer.

The second strategy is installing a flexible epoxy layer over the lumber. Then, add a standard self-leveling epoxy mortar.

• Does Staining Plywood Make It Waterproof?

Stains are among the primary products used in waterproofing lumber. Besides, woodworkers prefer the product as it enhances the lumber’s color and makes it weather resistant.

However, the finish only protects wood and concrete from moisture. It does not stop water movement through the concrete from the outside.

• Which Is the Best Water-Resistant Plywood

Marine plywood is the best moisture-resistant plywood type. Moreover, it is the highest grade available.

The lumber typically comes from Western Latch or Douglas Fir. Then, the manufacturer uses WBP, a high-quality phenolic glue, to attach the layers.

Also, depending on your project requirements, you can get the plywood in A-A, A-B, or B-B.

• Is Thompson Water Seal Good for plywood?

Thompson’s WaterSeal is among the best weatherproofing products for plywood. In addition, it is safe for the veneers and delivers a robust, productive coat.

You can also use the formula on pressure-treated wood and expect excellent moisture resistance.

• What Is a Good Plywood End Grain Sealer?

Edge banding is an adhesive veneer for various materials. Further, it is a decent option when sealing plywood’s end grain.

Position a thin plywood strip with a slight edge overhang. Then, iron it with a household iron and smooth it out using a woodblock.

Lastly, trim off the excess at the ends and gently sand the surface for a professional outcome.

Conclusion

Plywood is a versatile, commonly used building material in woodworking.

It is also durable and affordable and comes in veneer sheets.

But there are concerns about whether the material will hold up with water exposure, leading to the discussion:

How to Waterproof Plywood

We have more than a few ways to waterproof plywood and to summarize:

As you plan to waterproof your plywood, start by sanding the surface. Remember to do a light sanding with the help of a 180 grit sandpaper.

After sanding the surface, clean it to remove all the sawdust then apply the finish of your choosing. You can use a polyurethane, paint or epoxy to waterproof your plywood.

Besides, you can keep the lumber water-resistant for longer, whether you use drying oil, epoxy sealer, polyurethane varnish, or liquid latex.

Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.