How to Make Plywood Waterproof

How to Make Plywood Waterproof

Last updated Sep 08, 2021

Posted in Roofing

Plywood is a versatile building material. It belongs to the engineered wood boards family, and it is often confused with MDF (medium-density fiberboard) and Chipboard (particle board). However, plywood is quite different from the two in several aspects. It is often used for construction inside the house and outdoors. Plywood is a popular choice of wood because it is readily available and economical.

Builders use plywood to erect internal walls, lay foundations, and roof sheathing in the northern hemisphere, where wood is the primary material used to construct homes. Detached garages, sheds, and sub-floors inside the house can be built using custom-built plywood. Panels can have tongue and groove edges, which fit into each other to construct a firm and durable surface to support the required load. Inside the house, it can be seen in the shape of furniture, fitted cabinets, wardrobes, dressers, chairs, bookshelves, and so much more. It is a popular choice because it is easy to use and transforms effortlessly into solutions for your home. It has become the go-to material for home DIY enthusiasts because you can choose the veneer finish and the length of the board. DIY projects like constructing work benches, stools, storage bins, dog houses are just some of the things you can make using plywood yourself.

Whether you use it in construction or for DIY projects, plywood proves to be a strong, durable, and handy material. Plywood is manufactured by gluing together plies of wood veneer. The veneers are placed one on top of each other, so the veneer grains are placed at right angles to each other, and then they are fused with glue, pressure, and heat. Before plies of veneer can be acquired, logs are prepared by steaming or dipping into hot water. The water is soaked up, and the wood expands. The wood is then put on a lathe, and thin plies of 1mm and up to 4mm thickness are peeled off the logs. Afterward, the layers are pressed and heated, and the water is squeezed out of the layers, which further binds the layers tightly. It is due to this that water and plywood make for a bad combination.

When wood gets wet, it absorbs water and expands. However, pure wood will eventually return to its original shape, and it takes a long time for the wood to lose its original integrity. On the other hand, when plywood soaks up water, each layer begins to expand again and pull against the adhesive used to hold the layers together.

Water damage can also take the form of dry rot and warping. Rot is fungal growth, which gives the wood a yellowish or brown appearance, and the wood either crumbles or splinters apart when you touch the area. Fungal spores usually grow in places like the basement, when plywood is exposed to moisture and cannot dry entirely because of poor ventilation or lack of sunlight. Continued exposure to moisture causes the glue holding the plywood veneers together to crack up and allows the panels to store more water. Until the source of moisture is checked and treated, the rot continues to spread. Warping results from exposure to moisture and heat, which results in plywood shrinking and taking on a cupped or bowed shape.

Methods for Waterproofing Plywood

To preserve plywood from damage, the wisest course of action is to waterproof the plywood. There are five primary techniques you can use to protect plywood. The method you adopt will depend on where it is used and how much exposure to water it will have to deal with. It also depends on what kind of look you want.

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Oil

Oil is a great way to protect your plywood. With minimum effort, you can provide water and stain resistance. Oil is easily absorbed into the wood's pores and fills the cracks and spaces to create a homogenous surface. After entering the pores, it does not allow water to enter, mainly because oil and water don’t mix. A significant advantage of using oil is that it brings out the grain of the wood. Dry oil is ideal for outdoor use as well as indoor plywood surfaces. Tung Oil and Linseed oil are the two best oils for waterproofing plywood. Treating plywood with dry oil allows the wood to retain its natural moisture and flexibility. Since it works by becoming part of the wood, it adds strength to the wood. Dry oil hardens to a thin film after it is exposed to air because the chemicals in dry oil polymerize through oxidation. The coating that seals the wood is transparent, waterproof, and flexible.

The method is simple. First of all, you need to clean the surface of the wood. Prepare the wood by sanding the plywood to remove rough edges, loose particles, and dirt. A cleaning agent can help remove ingrained dust and stains but make sure to wipe off all traces of the cleaning agents afterward.

Oil is applied evenly over the surface of the wood and left to soak it into the wood. Heating the oil can improve the results of the process. You should heat the oil to 50C-60C and then apply it to the board with a brush or a roller. Special care needs to be taken around the edges of the sheet to ensure the oil has fully penetrated the wood. Next, you can leave the board to dry or use an iron heated to 200C or use a hairdryer. Afterward, repeat the process twice or thrice for the best results. Apply the oil carefully and then dry it. The process only stops when the wood can no longer absorb any oil.

Danish Oil and Hard Wax Oil are two blended oils. Danish Oil is a mixture of oil and varnish. The presence of varnish makes it a good choice if you want to add a sheen to the surface and protect against chemicals. The application follows the same principle as other oils. You generously pour the oil onto the surface and give it fifteen minutes to penetrate the wood. Next, add a second coat and then let it dry for a day. Subsequent coats can be added to heighten the finish, but limiting one coat per day is best. Hard Wax Oil is a blend of tung oil and carnauba wax designed to give high-quality protection and finish to furniture and wooden flooring. Oil strengthens the plywood while wax forms a durable, exceptionally resistant barrier against water, stains, heat, and dirt. It should be allowed to dry for four to six hours and preferably overnight after application. If it is used to protect the floor, no one should walk on it for sixteen hours.

Sealer

A sealant protects by forming a layer on top of the plywood to stop the water from penetrating the wood. There are several water sealants available in the market to condition plywood. The rugged, durable exterior that a sealant builds up protects from moisture, water, and other damage, ideal in outdoor and indoor settings.
The method for applying a sealant is to begin by preparing the plywood for the sealant. Clean the surface using a damp washcloth and a cleaning agent or a mixture of vinegar and water to wipe away any dust, debris, and stains. Mix the sealer or varnish in a container to reach a uniform consistency. Use a paintbrush to apply the sealant to the surface of the plywood. Alternatively, you can pour the caulking out into a paint tray and use a roller to spread the sealant. Coat the entire surface evenly, ensuring no bubbles or streaks compromise the sealant’s finish. A significant part of the process is to give the chemicals enough time to dry and form a complex layer to protect the plywood. An oil-based polyurethane sealant takes 24 hours to dry, while a water-based sealant takes four to six hours. Some sealants require multiple coats to effectively waterproof the plywood.

Polyurethane is a synthetic compound that is strong and durable, and one of the most popular varnishes used to provides a clear topcoat finish. They can be oil or water-based, but out of the two, water-based ones dry faster. Epoxy and latex are also sealants.

Epoxy

Epoxy Resin is a prepolymer that laminates the surface of the plywood to protect it from the ravages of water. Epoxy is available in the form of spray or paint. It soaks into the surface of the plywood and forms a tough, protective coating with excellent hardness. This outer layer prevents water from reaching the sensitive parts of the wood. It not only makes the plywood waterproof and more robust, but it also makes it resistant to scratches and dents. Since it is a transparent laminate, the texture of wood remains visible.

The method for applying epoxy begins with a thorough cleaning of the surface. First, clean the plywood, sandpaper it to get a smooth surface. Fiberglass or carbon fabric works well with liquid epoxy resin. Liquid epoxy resin provides the best long-term protection against water. Dilute a small amount of epoxy resin and apply the first coat. Insert the fabric and apply it and then smooth the surface with a brush. A second coat can provide double the protection, so it can prove quite helpful to apply two coats of epoxy before applying any finish like paint. After the epoxy has dried, you can sand it and apply your choice of paint. Epoxy sealer is a popular method for sealing plywood against water.

Latex

Latex is an emulsion made up of complex substances that solidify when they are exposed to air. It is a material that creates a shell around plywood to protect it from water and mold. Liquid latex is the easiest method to waterproof plywood. Once you’ve applied latex onto plywood, you can be sure that there are no threats to plywood from water. The challenge, however, is to ensure that the latex has covered every part of the plywood completely to seal it. If you are using liquid latex, the solution is to generously spray every section of the plywood, and then after it dries, apply a second coat to be safe.

Latex can give adequate coverage, and it sticks very well to plywood. Once you know it is fully covered in latex, you need not worry about it for the next ten to twenty years. You can quickly get a spray or paint latex from a hardware store.

The method for applying latex is the same as any other and begins with a thorough cleaning of the plywood surface with a cleaning agent or a water and vinegar mixture to scrub off dirt and debris. When the surface is dry, apply the paint latex with a paintbrush or paint roller. If you are using spray-on latex, then spray it on smoothly and evenly. It takes about an hour for the liquid latex to dry, and it will form a layer over the plywood to protect it. If needed, a second coat should be applied to ensure all parts are sealed.

Paint

Paint is the least perfect method for waterproofing plywood, and it can in no way compare with epoxy or oil for long-term benefits, but waterproof paint does protect your plywood from succumbing to moisture, mold, and warping. You may need to retouch or paint every two years, even though water-based paints don’t crack or peel easily. The primary advantage of using paint is that your plywood can be the color you want. It is best to choose paints that refine and waterproof plywood. In residential areas, water-based paints work better because they don’t have chemicals and strong odors that can be difficult to live with. Apart from being less poisonous, they dry faster. Exterior plywood walls and structures in open-air spaces are better options for enamels.
The method of applying paint is straightforward. Use sandpaper to smooth down the surface of the plywood and remove dirt and debris. Sanding helps create a suitable surface for protection from water. Using a roller or brush, apply an acrylic primer to prepare the surface for paint. After two or three hours, once the primer has dried, use putty to fill micro-cracks and irregularities. Grind the surface again to remove excess putty and sand over with a primer. Dry the plywood completely, and then apply the paint.

Benefits of Waterproofing Plywood

The reason to keep your plywood waterproof is to keep it looking great and prevent wear and tear from the adverse effects of weather. Plywood waterproofing repels water, prevents absorption of water, and stops the wood from swelling up. It becomes safe from insect invasions and molds. Additionally, if adequately treated, exterior walls will continue to look fresh, delaying aging.

Prolong life

Waterproofing can extend the life of plywood phenomenally depending on where and how it is used. Wood lasts only for two years if it is not treated and plywood lasts for a lot less if exposed to the outdoors. Inside a home, water leaks, and the buildup of moisture inside plywood can eradicate it. However, waterproof plywood can last for 20-40 years.

Costly to replace

Waterproofing is a simple step that can save you worry and a lot of money in the long run. If any part of the plywood is damaged, you will end up having to replace it entirely, for instance, entire walls or the floor of your deck. Any damage to the foundation or the walls will mean that they cannot bear the weight for which they were fitted out, and if the wall or a subfloor were to collapse, it could result in further damage, and the roof could also cave in. Replacement of compromised panels can be very costly.

Prevent Rot

Waterproofing plywood is a necessary preventative measure for preventing structural damage due to dry rot caused by fungi. Microscopic fungi are hard to see, and by the time you recognize the signs of rot on the wood surface, it is too late. A tiny amount of moisture can provide a breeding ground for fungi, and then even after the plywood has dried out, the fungi continue to grow within.

Moisture & UV Rays

Waterproofing can prevent the degradation of the panels due to continued exposure to moisture and UV rays. Untreated plywood will lose color, become de-shaped and show surface cracks over time.

Reduce Splitting

Waterproofing can stop the plywood from splitting apart. Again, the damage may go unnoticed until the essential integrity of the plywood has been compromised. Constant exposure to moisture that seeps into the pores will eventually harm the glue, and then the thin layers of veneer will begin to pull apart.

Waterproofed Veneers available

If you are building something new or planning additions to your home, it is better to invest in plywood that has already been waterproofed. This will save you a lot of hassle trying to waterproof it later. Waterproofed veneers available in the market are ADX, ACX, BWP, MR, ABX, CDX.

  1. Moisture resistant (MR)
    Indoor: kitchen & bathroom
  2. Boiling water plywood (BWP)
    Outdoor & indoor: kitchen & bathroom
  3. Marine plywood (ABX)
    Outdoor: home, ship & boat decks
  4. High quality A surface C category back (ACX)
    Outdoor: shed & indoor: furniture and cabinetry
  5. High quality A surface D category back (ADX)
    Indoor: flooring
  6. CDX sheathing (CDX)
    Outdoor: foundation & roof sheath, subfloors