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How to Waterproof Plywood

Plywood is one of the most-used materials for DIY projects, new constructions, and DIY home renovation products. It’s durable, versatile, and easy to work with which makes it ideal for many projects, but like any other material, it requires certain care for longevity.

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Why Waterproof Plywood?

Since plywood is made of bonded layers of wood, it’s absorbent and if exposed to moisture for a long period of time begins to swell and eventually become brittle, breaking apart under pressure. This is undesirable for a couple of reasons:

  • Swollen materials will warp a construction. Even if it’s able to dry out before becoming brittle, this will distort whatever you’ve worked hard to craft, leaving bows and gaps everywhere.
  • If the plywood is subjected to repeated swelling, it will eventually begin to crack and break apart. The bonding that holds the layers together will break down and the plywood will crumble. 
  • Even if the plywood doesn’t fall apart, there’s more likelihood of it molding than there is with regular wood. The multi-layer quality of plywood is a great breeding ground for all kinds of fungus without proper prevention.
  • With repeated small exposure to moisture, plywood will eventually rot. Areas will become too soft to bear a load.

This is all avoidable, however, as plywood’s absorbent qualities means it will absorb and hold onto waterproofing sealants like a champ. So let’s look into several ways we can waterproof our plywood to prevent the disastrous side effects of moisture.

How to Waterproof Plywood

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There are several methods of waterproofing plywood, all of which have different benefits and levels of difficulty. While you can DIY a plywood waterproofing job, if at any point these methods exceed your level of capability don’t hesitate to call in a professional for help. It’s better to ensure it’s done right than risk the hazards of improperly waterproofed plywood. However, if you have the time, energy, and tools these DIY methods should be easy to complete for anyone with minimal DIY project experience.

Prep the Plywood

Regardless of your chosen method of waterproofing, your plywood needs to be prepared first. Plywood comes with a rough texture, so it needs to be smoothed to best adhere to whatever you use to do your waterproofing. 

First make sure to fill any gaps, holes, or dents in the plywood with wood filler. This is essential because any cracks, etc. in the wood can result in gaps in waterproofing which allows moisture to settle and cause any one of the aforementioned side effects of moisture in plywood.

Choose a sanding surface – a sanding sheet, block, or electric sander will work – and smooth the entire surface of the plywood. Make sure to clean the plywood off after using a clean brush or vacuum so there’s no sawdust debris remaining. Otherise, the debris will be trapped under your waterproofing product. It’s also a good idea to take a wet cloth to the surface after dusting or vacuuming to catch any remaining dust. Make sure to let the wood completely dry after prepping if you decide to apply any wet cloth to it.

Epoxy Sealant

This is the most popular method of sealing plywood against moisture. Epoxy is a polymer clear coating that strengthens the surface of the plywood while also providing a waterproofing barrier. It comes in spray or paint form, and great care needs to be taken to apply in thin, even coats so that bubbles, bumps, and cracks don’t form in the epoxy. 

This goes on clear and shiny which enhances the look of the natural wood, but to add a stain or color on top of the epoxy you can sand the surface of the epoxy and apply your colored finish on top. 

Epoxy dries hard which strengthens the plywood, which is desirable for most projects, but if your project requires you to maintain a level of flexibility in the plywood consider using one of the following waterproofing methods instead.

Latex Paint

Latex is flexible, long-lasting, and more expensive than other methods of waterproofing plywood. It can be sprayed or painted, but painting latex onto the plywood is thicker and more long-lasting, as well as easier to make an even application. 

For interior plywood, a single, even coat of latex spray paint may be enough, but for exterior applications, it’s best to cover the plywood with two or three layers of latex paint using a paintbrush. With spray-on latex you have less control of how evenly the coat is applied, but with a paintbrush you have total control of the application, allowing for a thicker coat of latex protection against the elements.

While latex is the longest-lasting waterproofer for plywood, it’s also the most difficult to apply so it might be a good idea to ask for help from a friend, family member, or professional if this is your first time working with the material.

Penetrating Oil

Oil waterproofing is another option, mainly for interiors as it doesn’t provide a thick layer on top of the wood to protect it from ice, wind, and heavy elements. Instead, penetrating oil waterproofs by soaking into the wood itself and treating it against humidity.

This product is applied much like oil stain and can be painted onto the surface and then rubbed in with a clean, dry cloth. Once the stain dries, another layer (up to 3 layers in total) can be applied for extra waterproofing, and colored stain can be applied on top of one or two layers of the waterproofing for an aesthetic touch.

If you’re a beginner with these types of products, penetrating oil might be your best option as it doesn’t require a high level of skill and can be cleaned up easily. Because it’s so thin and soaks into the wood, drips and uneven layers aren’t much of a concern.

Fiberglass Resin

Fiberglass is impenetrable, and that’s why many showers and wet rooms utilize the material. This is ideal for outdoor projects that don’t require flexibility on the part of the plywood, as fiberglass resin dries rock hard and doesn’t allow any bending or flexing. If you will need to bend or mold the plywood, it’s best to do so in assembly before waterproofing with fiberglass resin.

When using fiberglass, a paintbrush will be your best tool. It prevents dripping that can occur with a roller and allows more control in application since fiberglass is very unforgiving once it’s dried, and it dries pretty quickly.

Another important note about fiberglass resin application is that it contains strong, hazardous fumes that should be avoided at all costs. So this should be done in a large, open area with good airflow using protective equipment like masks, gloves, and safety goggles.

PVA

PVA is a bonding agent usually used as a glue, but can also be used for off-brand purposes like waterproofing since it is completely water repellant. This material is safe and easy to work with, and is ideal for indoor and outdoor use for beginners and experts alike.

This glue is thick, however, and will be easiest to work with if diluted to a thin liquid with water. Several coats can be applied for a 100% waterproof finish, but each coat needs to completely dry before another is applied, otherwise, water will remain trapped inside the plywood with no way to escape, completely negating the purpose of the PVA.

It’s also vital to store the plywood somewhere completely devoid of dust and debris while drying each coat, as PVA is a glue at the end of the day and is very sticky until it’s completely dry. This method takes longer than most others due to the drying time involved, but it’s also the most beginner-friendly and leaves the plywood flexible after waterproofing, unlike fiberglass resin and epoxy.

More Plywood Waterproofing Tips

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When waterproofing your plywood it’s essential to make sure your application is flawless and that you perform necessary upkeep on the waterproofing.

On top of the already hazardous effects of water on untreated plywood, water that seeps in through imperfections in waterproofing can be more damaging because the waterproofing layer allows less airflow which means the water will stay inside the plywood without drying out. So let’s explore just a few useful tips and tricks to ensure a smooth, even, long-lasting layer of finish on your plywood.

  1. Know your limits. If you aren’t handy or don’t have much experience with many DIY projects, it might be a good idea to seek out help or do additional research into the product you decide to use. Otherwise, you risk faulty application that can lead to hazardous mold if exposed to moisture. Even if you’re skilled in handy-work, it might be a good idea to ask for help from a friend who’s worked with the materials before if you haven’t worked with the materials in the past.
  2. Check and double-check your tools. Make sure you’ve assembled all your tools beforehand. This will free you up to focus on your method rather than stopping mid-project to search for a paintbrush, sander, or dry rag, etc. Working with most of these products is time-sensitive and they don’t leave much room for timing errors.
  3. Seal your edges. Since plywood is made of thin wood layers sealed together by bonding adhesives, the edges leave those layers exposed. You’ll see what we mean if you look at the edge of a piece of plywood: you can see many stripes of wood, which when exposed to water will allow moisture in between the layers. It’s ok to add extra layers of waterproofing on the edges of your plywood, assuring moisture has no way in.
  4. Read labels carefully. Know what you’re working with. There are many stains, sealants, and paints available for woodworking, and they all perform different tasks. Some of them, though, look remarkably similar so if you aren’t confident in what product to buy, ask for help from a sales associate to ensure you’re using the right waterproofing material for your task.
  5. Keep more product on hand. Once you’ve finished waterproofing, make sure you keep more of the same waterproofing on hand for the future. Waterproofing eventually wears away, especially if you use oil or latex, and it’s important to add more waterproofing periodically to make sure your wood stays protected against moisture.

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