How to Waterproof Plywood and Keep it from Rotting

Seal plywood for outdoor use
How to waterproof plywood

Plywood is an excellent construction material because it is durable, versatile, and relatively inexpensive. However, it can be vulnerable to moisture and water damage when used on decking or other outdoor applications. 

Knowing how to waterproof plywood for outdoor use can help prolong its service life by preventing it from cracking, warping, and decaying.

How to Seal Plywood for Outdoor Use

To use plywood outdoors, seal it by applying multiple thin layers of polyurethane varnish, epoxy sealant, spray-on latex, waterproof paint, or drying oils like Thompson’s WaterSeal.

Read: Thompson’s Water Seal Problems

Ensure you sand lightly and clean the plywood before sealing it one side at a time, allowing the recommended drying time between each coat. 

Even more importantly, seal the edges tightly by applying several coats of varnish all around, allowing about one hour of drying time for each coat.

You can also use painter’s tape to cover the edges after applying the first layer of sealant and then adding one or more coats on top. 

Whichever method you choose, here is a breakdown of the procedure to follow.  

What you will need for the exercise 

  • A spray bottle, brush, or roller, depending on the application method you choose 
  • Two or more clean rags or cloths 
  • A waterproof sealant 
  • A pair of hand gloves 
  • Protective face mask 
  • Drop cloth 
  • Safety goggles 
  • Medium and fine-grit sandpapers 

The steps to follow 

This section provides the step-by-step procedure to follow in completing this task.

Step 1: Prepare your workstation. 

The first thing you want to do is choose a well-ventilated area for your project. This could be any sheltered space such as a carport or garage. 

Start by wearing a safety mask. Some of the substances you will use to waterproof plywood produce fumes that you don’t want to inhale.

Also, you will need to sand the board to prepare it for sealing, and you don’t want the sanding dust in your lungs.

Put on a pair of hand gloves to protect your skin from coming in contact with the sealer you will be using. You may equally wear safety goggles to protect your eyes. 

Next, lay down a tarp to cover the floor and anything the sealer and sanding dust could affect within your workstation. 

Step 2: Fill any grooves or rough spots with wood filler 

Once you have assembled all the supplies you need for the sealing project, check the board to ensure it is even on the surface and the sides. 

If there are any grooves or noticeable rough spots, especially on the sides, fill these out with wood filler to even out the sheet. Let the filler bulge slightly above the surface to allow room for sanding. 

This part is essential in ensuring there are no gaps that could impede waterproofing. Allow the filling product to harden and dry for the amount of time the manufacturer has recommended before moving on to the next step. 

Step 3: Prep the plywood for sealing 

This is the part where you sand the bulging wood filler to let it sit flush with the rest of the surface.

Before sanding, use a damp rag to wipe down the sheet around the filled areas to raise the grain slightly. This activity helps the sealer bond better to the plywood sheet.

Next, sand the entire sheet with medium-grit sandpaper, and wipe the sanding dust. Switch to fine-grain sandpaper such as 180-grit piece and scuff sand the plywood sheet till it is smooth to the touch

Wipe the sanding dust and any debris from the surface with a dry natural-bristled brush or cloth.

Step 4: Apply your chosen sealer.

Any sealer you choose will have a recommended application method and waiting time between coats; ensure you follow these recommendations to the letter.

Usually, this information is available on the product label. These sealer application methods can include using a chip or foam brush, paint roller, or spray bottle. 

We recommend using a roller for applying epoxy or varnish and tipping it immediately with a foam brush or high-quality chip brush to level out the texture left by the paint roller.

It is crucial to tip immediately before the surface gets tacky and do it along the grain. You can also apply varnish by spraying. This option is even better.

Spraying is equally perfect for any remaining sealers we will recommend, so you may want to consider it whenever it is one of the options at your disposal. 

In any case, make sure you cover the surface adequately with several thin coats rather than few thick ones. And ensure each coat is dry before applying the next. 

Step 5: Let it dry and seal over the screws. 

While most of the sealers will dry in a matter of hours, you may want to give your plywood at least 24 hours of drying time before installing it. 

After installing the plywood sheet, we recommend sealing over the nails or screws to make the entire thing watertight. 

Still, you will want to keep an eye out for any holes in the sealed plywood surface that may need plugging.

This precaution can go a long way in preventing severe damages to your plywood structure, ensuring you get the most out of it.

How to Waterproof Plywood for Outdoor Use—the Methods  

Now that you know the procedure for waterproofing plywood for outdoor use, it is time to understand the methods available for you. Let us look at five. 

1. Polyurethane varnish method 

Polyurethane varnish is one of the best sealers for waterproofing your plywood. It dries into a super-tough coating that makes the surface impervious to moisture, water, and scratches. 

Polyurethane varnishes are typically used to coat wooden stairs, floors, and other surfaces in high-traffic areas.

In addition, they are an excellent way to coat chipboard, hardboard, parquet, and plywood surfaces, making them watertight.

You can apply varnish with a paint roller and pad, by brush, or by spraying it onto the surface. Ensure you apply a thin, even coat and let it dry for the recommended amount of time before adding the subsequent layers. 

Once completely dry, the resin mixture will form a robust, clear coat impervious to water, making your plywood boards resilient to the harsh outdoor elements. 

2. Applying penetrating epoxy sealant

Like varnish, epoxy sealant dries into a tough layer resistant to water, scratches, and dents. It is one of the most popular and best ways to seal outdoor plywood. 

The epoxy sealant comes in tinted paint and clear forms. However, unlike polyurethane varnish, it tends to have a thicker consistency.

This thickness makes spraying a less viable means to apply it on the surface of the plywood. To apply epoxy, we recommend using a paint roller.

Apply it within the grain and against it for the best results. Remember to tip the surface with a foam brush immediately after applying epoxy to even out the texture that the paint roller leaves behind. 

And, let each thin coat dry (often for about an hour) and sand it lightly before applying the subsequent layer. 

Recommended product 

We suggest using the clear epoxy from ProMarine Supplies for the best results. It dries into a hard glossy finish that is water-resistant and protects your plywood from the sun’s harmful UV rays. 

3. Spray-on latex method 

As the term suggests, liquid latex comes in liquid form in a spray can. As such, liquid latex is one of the easiest ways to seal your plywood for outdoor use. 

It is also available in larger containers for larger projects. You can still use a paint sprayer to apply the sealer.

For instance, you may want to consider using a paint sprayer when waterproofing an entire dozen of plywood sheets or more. 

Spray-on liquid latex should do an excellent job sealing the plywood surface, but you will need to use epoxy or polyurethane varnish for waterproofing the edges.  

Recommended product

We suggest using Krylon exterior semi-transparent liquid latex spray. It offers even coverage and protects the plywood surface from UV rays and water. You may, however, have to apply several coats for maximum protection. 

4. Applying waterproof spray paint

If you are not a fan of clear coats, waterproof paint is probably your best bet. The paints come in various colors, so you can choose something that matches your exterior décor and color theme. 

The important thing to do is check to ensure the paint is oil-based to bond with plywood. 

The oil-based paint will seep into the pores blocking them and making the surface impervious to water and resistant to the damaging effects of sunlight.

For the edges, you will still need to seal them with epoxy or varnish while coating both sides of the plywood sheet with paint. 

Also, be sure to apply an oil-based paint primer before spraying the paint onto the plywood. Oil-based paint may take a couple of hours to dry between coats, so you will need a little patience to complete this project.

Recommended product 

For this purpose, we recommend KILZ Interior/Exterior Basement and Masonry Waterproofing Paint. It forms a durable coat and offers excellent coverage. However, like any paint, you will need to touch up the surface after every few years.

5. Drying oil method 

Drying oil does not form a hard coat as most waterproofing products do. Instead, it soaks into the pores in plywood, sealing them and making the sheet impervious to water.

This means you will need to use a more robust sealer for the edges. Varnish or epoxy should get the job done, sealing plywood sheets at the edges. 

Using drying oil is advantageous as it helps the plywood preserve a percentage of its natural moisture and remain relatively flexible. This way, the oil strengthens the plywood against water damage instead of making it watertight. 

Recommended product 

Consider using Thompson’s WaterSeal for this purpose. It nourishes the plywood and forms a protective layer on either side of the board. 

Why is Plywood Vulnerable to Water?

Plywood is vulnerable to water due to its layered structure. The product is made of thin sheets of wood layered and glued together into a compact board. 

The process of creating plywood involves gluing together several wood veneers under heat and pressure. This procedure sucks out moisture and water from the wood, creating a robust sheet of tightly bonded veneers. 

The edges retain a significant level of porosity where water can seep in and cause the individual layers to expand, pull away from the glue, and separate. Such damage can be irreparable and would mean the end of the plywood. 

Why is it Important to Waterproof Plywood?

The whole idea of waterproofing plywood is to keep water and moisture from entering its core. But why is this important? 

Waterproofing protects plywood from UV rays and water damage 

We have mentioned that water could cause the plywood to warp and split. However, prolonged exposure to moisture and the sun could be just as damaging. 

Over time, accumulated moisture would have the same effect as water as it seeps into the pores causing the sheets to swell. 

When sun rays hit such plywood, it will dry out quickly, warping in the process. If this cycle continues, it may lead to terminal damage to the material. A good type of waterproofing helps prevent such damage from happening. 

It causes the plywood to last longer.  

The atmosphere always has a certain level of humidity that can rise or fall depending on the weather. 

This situation means untreated wood boards eventually absorb some moisture. As a result, they will only last about two years outdoors, even with the best routine care.

Exposure to water and other extreme elements can damage plywood much sooner, in a matter of months. 

Sealing introduces moisture resistance that keeps plywood structurally sound, extending its service life to 20-40 years.  

Waterproofing keeps the plywood from rotting. 

Like any wood, damp conditions would cause the sheets to rot and develop a foul odor. The dampness would also provide a foothold for fungi and other bacteria, leading to dry rot. 

Neither of these conditions is desirable. Thankfully, sealing plywood is a sure way to keep them from occurring.

FAQs  

Can I make plywood waterproof?

Yes, you can make plywood waterproof by spraying it with liquid latex, waterproof paint, or varnish. You can also coat it with epoxy sealant or hard-drying oil to seal it and make it impervious to water. 

What can I use to waterproof plywood?

You can use liquid latex to waterproof plywood. Alternatively, you can also use waterproof spray-on paint, clear-colored polyurethane varnish, or epoxy sealant to seal plywood and make it watertight.

What is the best waterproofing for plywood?

An epoxy sealer is likely the most effective and popular material for waterproofing plywood sheets. The clear, viscous liquid dries into a hard shell covering the entire surface of the plywood. 

Epoxy is available in both paint and spray forms. Whichever option you choose will give you the desired results, making the plywood stronger and impervious to water. 

How do you seal plywood from water?

Use a water seal to protect plywood from water. We recommend using a garden pump-up sprayer to apply a thin, even coat of water seal to your plywood after scuff sanding and prepping the surface. 

Ensure you sand lightly between each coat and allow each coat to dry completely before applying the next. Use several sealant coats to get the best resilience against water penetration.

Is plywood water-resistant? 

Plywood is not water-resistant because it comprises veneer sheets which are untreated timber glued together with waterproof WBP glue.

The sheets are porous and remain open at the edges where water can enter and damage the material. 

Thankfully, you can use one of the many wood sealants to seal plywood and make it completely water and moisture-resistant. 

Which plywood to choose for outdoor use?

The best plywood to use for outdoor projects is or marine grade or ACX plywood. These are designed to withstand the harsh weather elements outside, even though you could still waterproof them with a suitable sealer to prolong their service life further.  

Which is the best water-resistant plywood

Marine plywood is the best water-resistant plywood and the highest grade available. The product is typically made from Douglas fir or Western Larch wood and glued with WBP, a phenolic glue of the highest quality. You can buy marine plywood in either grade A-A, A-B, or B-B.   

How to make plywood waterproof with liquid latex

To waterproof plywood with liquid latex, sand the surface with 180-grit sandpaper and wipe the sanding dust, then spray the liquid evenly across the surface. Let the coat dry completely before applying the next. Repeat this process until you have applied your last coat of liquid latex. 

Is exterior grade plywood water-resistant? 

Exterior-grade plywood—often called marine plywood—is not waterproof, as many people may believe. Instead, it is a high-quality, hardwood composite comprising waterproof glue but it is not rot-resistant since it is not treated plywood.

Can you use Flex Seal on plywood?

No, you should not use Flex Seal on plywood. While Flex Seal may be water-resistant, it is not watertight. If used on plywood that will stay outdoors exposed to harsh weather elements, including humidity and precipitation, the chances are these elements may find their way into the core of your plywood, damaging it.

Flex Seal is better for sealing surfaces of materials such as piping, porcelain, and plastic, but not plywood for outdoor use. 

How long will untreated plywood last outside?

Untreated plywood can last anywhere from a few months to two years outdoors. However, exposure to weather elements such as direct sunlight and water can damage the wood in weeks.

So, always stick with treated wood or waterproof your plywood before using it on your deck or any outdoor project if you want it to last. 

How to seal plywood edges

To seal plywood edges, apply a coat of wood filler, let it dry, and sand it into a smooth, solid surface. Alternatively, cover the plywood edges with a thin strip of adhesive veneer called edge banding. It comes in a variety of species to match the plywood. Place the matching strip over the edge and apply heat by a household iron to apply this product. Finally, smooth it out with a woodblock. 

How do you seal plywood with water-based paints? 

To seal plywood with a water-based paint, start by applying a corresponding water-based primer using a high-quality chip brush or roller after sanding. Then, apply thin, even coats of the paint, letting each layer dry completely before adding the next coat. 

What is a good plywood end grain sealer?

A decent option for concealing the end grain of plywood is an adhesive veneer called edge banding. Place a thin strip of the veneer with a slight overhang on the edge you wish to seal. Next, iron it with a hot household iron and smooth it out with a woodblock. Finally, trim off the excess at each end and finish with a quick sanding to get professional-looking results.  

Can you seal plywood with PVA glue?

Yes, you can use PVA (Polyvinyl Acetate) to seal the plywood. But you may need to first thin it out slightly with water before sealing the sheets with it. In addition, PVA glue may not be suitable for large projects. For such projects, you may want to stick with epoxy sealant or varnish. 

Can you use Thompson Water Seal on plywood?

Yes, Thompson’s WaterSeal is one of the best weatherproofing sealers you can use on plywood. The sealer is safe to use on plywood sheets and offers an excellent protective coat repellent to water and moisture. 

Not only that, but it’s also the best waterproofing sealer for pressure treated wood.

Conclusion 

Plywood may appear smooth, but it is still porous on the surface since it is made of wood.

In addition, the layering of veneers to create the board also leaves it significantly porous at the edges.

So, it will require sealing all around to withstand the elements and any damp environments.

As you can see, knowing how to waterproof plywood is relatively straightforward. We hope that this guide helps you find a method that you are happy with.

Please let us know what you think in the comments section. 

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