How To Waterproof Plywood -5 Most Effective Methods

Plywood is a fantastic wood for construction. It's really durable which makes it suitable for most projects.

Not only is this versatile wood sturdy and strong but it is also relatively inexpensive.

This makes it a great choice for any project as you don’t have to compromise on quality or price. 

However every cloud has its silver lining, and plywood’s is that water and moisture are its ultimate weakness.

If you leave the wood as it is, it’s going to massively limit which projects you can use it for as outdoor projects won’t be suitable since the wetter weather would ruin the wood. 

Luckily, you can waterproof plywood to make it withstand the elements. It can prolong the wood's life from warping, cracking, or decaying. 

In this article, we’ll go over all the ways that you can do this so that you can start any outdoor project your heart desires. 

Sealing Plywood For Outdoor Use

If you want to be able to use plywood for outdoor projects, the very first thing that you are going to need to do is apply a couple of layers of epoxy sealant, waterproof paint, drying oils, spray-on latex, or polyurethane varnish. 

 When you do this, make sure that you lightly sand the plywood and clean it before you seal it.

You want to seal each side one at a time. It’s also incredibly important that you seal the edges of the wood.

For the sides, apply multiple coats to seal them tightly. You will want to wait around an hour between each application to ensure that each layer is dry before starting the next. 

Everything That You’ll Need

• Applicator (Brush, Roller, or Spray bottle)
• Clean cloths or rags
• Waterproof sealant of choice
• Pair of gloves
• Protective face mask
• Drop cloth
• Safety goggles
• Sandpaper (medium and fine grit)

Step-By-Step Guide

Now that you’ve gathered everything you’ll need we can begin the steps that you will need to follow to get the job done. 

Step One: Preparation & Safety

When deciding where to start your project, you’re going to want to pick somewhere that is well-ventilated. A garage is a perfect example. 

The very first thing you should do is wear your safety mask.

The substances that you’ll be handling can produce toxic fumes that you’re certainly not going to want to be inhaling.

You will also be sanding in later steps and it's not advisable to be breathing in sanding dust. Safety should always be your first priority when beginning any project. 

Next, pop on your gloves. Just as you don’t want to be breathing in anything nasty, you don’t want your skin coming into contact with any harmful chemicals.

Wearing gloves will prevent the solvent from touching your skin. Safety goggles will also shield your eyes from fumes and dust. 

Lastly, you’ll want to cover the area with a tarp so that sanding dust and sealer don’t end up making a mess of your work area. 

Step Two: Fill The Grooves 

Your workstation should now be prepared, and you should be in full safety gear, therefore you are ready to begin. 

What you’ll want to do first, is check your plywood for any obvious rough spots or grooves.

If your board does have these grooves and spots, you will want to fill them out with wood filler.

Use enough wood filler so that there is a slight excess. This will be sanded down later. 

It is really important that you make sure there are absolutely no holes that you miss or this will massively affect the waterproofing. 

Leave the filler to harden and make sure it has had enough time to dry.

If you’re unsure about how long this will take, always refer to the manufacturer's instructions or recommendations. 

Step Three: Prep the Plywood

At this point, it is time to start sanding the plywood. You want to sand it enough so that the excess wood filler is now completely flush against the rest of the board.

Try to get it as even and level as possible. 

Before you begin sanding though, you’ll want to use your damp rag or cloth to wipe down the wood around the areas that you’ve just filled to elevate the grain.

We do this so that the sealer can bond with the plywood better. 

Once you have done this you can sand the plywood with medium-grit sandpaper. Be sure to remove the excess dust.

Then, with the fine sandpaper, scuff sand until the whole sheet is smooth. 

Again, remove any excess debris. For best results use a cloth or a natural-bristle brush. 

Step 4: Apply The Sealer

Once you’ve picked your sealer of choice, you’ll want to give the manufacturer’s recommendations a read.

These will let you know how to apply the sealant and how long to wait in between coats. 

Because all sealants will have completely different instructions depending on which you choose, I can’t advise you further here.

You will need to check the product label and follow the information that you are given.

In most cases, you can choose between a paint roller, brush, or spray bottle for your application method. 

If you're applying varnish or epoxy, I would recommend using a roller and then tipping it instantly with a good quality brush to level any of the texture that the paint roller leaves.

You need to tip it straight away because if you leave it, the surface will become tacky.

In most other cases, spraying is the preferred method. So if you have the tools handy, or the money to go buy them, I would suggest doing it this way. 

Regardless of which method you choose, it is always best practice to opt for multiple thin layers than one or two thick coats.

Make sure that each layer is fully dry before you being your next coat. 

Step 5: Dry & Seal Screws

A lot of sealers can be dry in just a couple of hours, although I would always recommend leaving the plywood for a minimum of 24 hours before you start using it in a project.

This is to give it adequate drying time. 

Once you have started installing your plywood, it is always a good idea to seal any nails and screws too just to be certain that it is 100% waterproof.

While doing this, keep a watchful eye for any other weak spots that may need filling. 

This may seem like a lot of effort now but it will give your wood the longevity to withstand even the worst weather conditions. 

What Methods Are Available

Now that you’ve learned how to waterproof plywood, let's talk about all the different methods there are on the market.

There are quite a few different types of sealants so sometimes it can be confusing knowing which one is the most suitable for you.

We will go into more detail about these now. 

1. Polyurethane Varnish

Polyurethane is probably one of the better options for waterproofing plywood.

When it dries, it has this really tough outer coating that turns the wood resistant to moisture and water. 

These varnishes are usually for coating wooden stairs and floors.

Essentially their purpose is to protect areas that usually sustain damage from high-traffic areas which means that they’re going to keep your wood durable and strong too.  

Polyurethane can be applied with a paint roller, a brush, or a spray bottle. So you can choose whichever feels the most comfortable or easiest for you. 

You just need to be sure that you only use thin coats and give ample drying time before adding the next layer.

Once it’s dry it’ll work wonders for your plywood, but you’ll come into issues if you don’t allow it the time it needs to dry.

2. Epoxy Sealant 

Epoxy sealants are fairly similar to the aforementioned varnish.

It too dries leaving a tough and strong layer behind which will keep your wood free from scratches, dents, and water or moisture. 

Epoxy is usually in two forms, clear or tinted paint. While polyurethane is quite thin, this sealant tends to be a lot thicker.

Because it’s a lot thicker, it reduces the application methods you can use to administer it.

I would opt for either a brush or a roller for this sealant as you will struggle massively with a spray bottle. 

For the best results, you will want to apply the within and against the grain.

You’ll also need to tip the surface immediately after so that you can even out any texture that the paint roller causes.

Much like the method before, you’ll need to make sure that you let each coat dry the recommended time and then sand it gently before starting to apply the second coat. 

3. Latex

Spray-on or liquid latex is exactly what it sounds like. It is a liquid form of latex that usually comes in a spray can.

This is probably the easiest of all the methods. You can usually get it in small and larger bottles so that whether it's a small or large project it’ll be up for the task. 

If you’ve got a large quantity of plywood to waterproof, you can spray the sealant using a paint sprayer to coat them all in no time at all. 

Latex also does a fantastic job of sealing plywood, so not only is it easy but it's also effective.

If you do choose this option though, you will still need a small amount of polyurethane or epoxy for the edges of the wood. 

4. Waterproof Spray Paint

If you find clear coats difficult to work with, or perhaps are just not keen on the way clear coats look or just want to opt for a bit more color, then waterproof paint will be the most favorable option.

There’s a really wide variety of colors on the market, so you’ll be sure to find the right color to match your aesthetic. 

One thing that you will need to be careful of, is ensuring that you choose an oil-based paint so that it will take to the plywood.

Oil-based paint will soak into the pores of the wood and block water and moisture from seeming in. It also makes the wood resistant to sun damage. 

Like with latex, the edges will need to be sealed with epoxy or varnish. You’ll also want to use an oil-based primer before any paint comes into contact with the plywood.

Be patient when it comes to applying the coats for paint, you will need to wait for at least one to two hours before starting your second layer.  

5. Drying Oil

Drying oil works a little differently than a lot of the other methods we have discussed.

This one doesn’t form a thick or hard coat like the others, alternatively, it actually seeps into the plywood's pores and seals them shut, making them waterproof. 

Of course, this does mean you’ll need a more sturdy sealant on the edge of the wood. Again epoxy and varnish are your best bets.

What’s great about working with drying oil is that it enables the plywood to preserve some of its natural moisture and flexibility.

The oil you add then makes the plywood stronger and more resistant to water damage rather than watertight. 

What Makes Plywood Defenceless Against Water

Plywood has a really layered structure and because of this, it can be pretty susceptible to water.

Plywood is essentially made from loads of fine sheets of wood that are all then layered and glued together to make one final piece of wood. 

This process involves a lot of pressure and heat which then sucks the moisture and water completely from the wood.

This creates tightly bonded veneers. Because of the structure, the edges of the wood tend to be quite porous. 

If water gets into these layers, it will be detrimental to the wood as they will expand, become unglued and begin to separate.

If this happens there is no way to fix the wood. 

Why Waterproofing Plywood Is Important 

So by now, it’s been well established that the whole point of waterproofing the wood is to keep the moisture away from the core of the wood.

But why exactly is this so vital? Well, there are plenty of reasons. We’ll go through these together now. 

Protection Against The Elements 

If you are using the plywood for a project that is permanently outdoors you’re going to need it to be able to withstand all types of weather.

And while we’ve already established that waterproofing will protect the plywood from moisture such as rain and snow, you also need to consider that the sun can also cause just as much damage. 

When it’s a scorching summer day, the rays from the sun can dry out and warp the wood. If this happens multiple times it is going to lead to irreversible damage.

Waterproofing not only ensures that your plywood is protected against the rain but also against harmful UV rays.

Longer Lasting Plywood

Humidity will fluctuate based on the weather and season and this will also affect the wood.

If you haven’t treated your plywood, throughout these fluctuations the board will eventually start to absorb the moisture in the air.

Unless treated, this is not preventable. 

Untreated plywood is likely to have a lifespan of a matter of months. This can increase to twenty to forty years if you use a sealant.

This is a massive jump and so is definitely worth the time it takes to administer. 

Stops It Rotting

If you get the layers of plywood damn, they are eventually going to begin to rot.

This isn’t something anyone wants as it will then produce a very pungent and unpleasant smell.

It will also become a massive breeding ground for fungi and other forms of bacteria. This will only lead to further rotting known as dry rot. 

Obviously, no one wants this to happen to anything they are constructing, which is why it’s lucky that sealing plywood eradicates this problem.

Frequently Asked Questions

Can You Make Plywood Waterproof? 

Yes! Not only is it possible, but it’s also pretty much essential!

As mentioned throughout this article, it’s fairly easy to make sure your plywood is waterproof with the use of a decent sealant such as varnish, waterproof paint, or epoxy. 

What’s The Best Way To Waterproof Plywood?

There are loads of different ways that you can waterproof plywood and really it can come down to personal preference as to which method you believe is the most effective. 

In my opinion, epoxy is the most effective sealer.

While a lot of the other methods are still perfectly fine options, they usually still require you to use epoxy on the edges.

Using this all over just means less hassle changing between methods. It also comes in paint and sprays so you can pick your favorite application method.   

How Do You Seal Plywood Against Water?

You’ll need to coat the plywood in a thin layer with a seal of your choice. If you have a garden pump-up sprayer that will work wonders to apply an even fine coat. 

Between each coat, sand the wood ever so slightly and make sure you’ve given each layer enough time to dry.

You’ll want to make sure you have quite a few layers on the wood before you have finished making sure that it is completely resilient to any water or moisture that may try to penetrate the surface. 

What Is The Best Choice Of Plywood For Outdoor Projects? 

When choosing your plywood for your next outside project, you’ll be best off picking either marine grade plywood or ACX.

This is because this plywood is a little stronger and designed to withstand some of the harsh conditions the weather causes. 

Though they are designed for outdoor use, I would still always recommend sealing the wood.

This will give you super durability and you can be certain that it will withstand whatever the weather may throw at it. 

Which Plywood Is The Most Water-Resistant?

The highest grade of plywood you can purchase is Marine plywood, this is the most resilient against water.

You can buy varying ranges in grade for marine plywood. The higher the grade, the better the quality and thus the best water-resistance. 

However, I would still advise waterproofing even this high grade of plywood with a seal to be certain you have the optimal protection. 

Can You Use Flex Seal On Plywood? 

Flex seal isn’t really appropriate for plywood. Flex seal is water-resistant but is by no means watertight and so you still run the risk of the wood absorbing moisture. 

Flex seal cannot withstand the harsher elements such as precipitation and humidity and so water is likely to make its way to the core of the wood.

This can result in the layers in the plywood coming undone and also increases the likelihood of the wood rotting. 

How Do You Seal The Edges?

For the edges, you’ll need to apply a layer of wood filler and then let that dry. Once it’s dried you can then sand the edge until it's smooth. 

You can also use a technique named edge banding where you apply a small amount of adhesive veneer.

Simply place the strip on the edge and use an appliance such as an iron to apply heat. Once you've done this you can then smooth it out with a woodblock.

Can you Seal Plywood With PVA Glue? 

PVA isn’t just useful for arts and crafts. You can actually use PVA glue to seal plywood too.

However, if you’re going to do this you’ll need to thin it out with some water first.

This method is a perfectly decent option for smaller projects, or if it’s all you’ve got to use, but I wouldn’t advise using this for large projects. 

What Is A Decent Plywood End Grain Sealer?

If you want to conceal the end grain of plywood, edge banding may be the best option.

All you’ll need to do is out a fine strip of veneer with a slight overhang on the edge you’re sealing.

Then iron it and smooth it out with a woodblock. Last but not least, you’ll then trim the edges at each end and finish off by sanding them slightly.

This will give your wood a beautiful professional finish. 

Final Thoughts

Plywood is a strong and durable construction material that can seem like the perfect choice when you’re starting a project.

And If you’ve decided it's the material for you, you’d be absolutely right. Not only is it strong and sturdy it’s also nice and cheap. 

The only thing you need to remember is that while it is fairly heavy-duty, it is still incredibly porous.

This is something you will need to rectify before you begin installing anything as it leaves you susceptible to rotting, and it’s likely that the moisture will also make the wood fall apart - so it won’t stay durable for long. 

Luckily, as I’ve hopefully demonstrated, waterproofing this wood to become a foundation that can stand the test of time is a relatively simple task.

A few layers of sealant will protect your project from whatever harshness the elements will throw at it. 

I wish you the best of luck in whatever project you begin to embark on.