How to Waterproof Plywood (& Make It Last 10x Longer)

Researched & Written by Craig

Plywood is such a great product for home and outdoor use.  It’s cost effective, versatile and easily available.  Like any wood based product though, it’s vulnerable to damage from water or moisture. 

We’ve put together a handy guide for you to protect and waterproof your plywood so that it lasts MUCH longer – whether you’re using it near wet areas like bathrooms or using it outdoors where it’ll be exposed to the elements.

Note:  When using products like sealants or varnishes, always wear the appropriate protective gear, like gloves.  Work in a well ventilated area to avoid inhaling excessive fumes, and always follow the manufacturer’s directions very closely.

Also, before working on any store bought item, make sure that by treating it yourself that you’re not voiding your manufacturer’s or retailer’s warranty.

Waterproofing Your Plywood

Stack of plywood before being waterproofed

There are a number of products and techniques you can use to waterproof plywood effectively:

Penetrating Hard Wax Oil

Hard Wax Oil is a great product to use on plywood and other woods because it makes the wood very durable, resistant to wear, and water repellent while keeping the natural moisture of the wood sealed inside.  It’s also an easier process than using a sealant or latex coating.

Here’s how:

1.)  Clean the plywood thoroughly to remove any debris and dust.  Use a clean rag or cleaning cloth and add a touch of dish soap or cleaning agent if you need to remove ingrained dirt/stains.  Wipe off any traces of the cleaning product afterwards. 

You can also sand the surface of the plywood very lightly if some stains are very hard to remove.

2.)  Once the plywood is entirely clean and you’ve removed any cleaning product traces and left over sawdust, apply a layer of the Hard Wax Oil evenly, following the manufacturer’s instructions closely.

3.)  Allow to dry for at least 4 – 6 hours, or preferably overnight.  If you’re using the Hard Wax Oil on the floor, you must wait a minimum of 16 hours before people walk over that area.

If you want to be super sure about durability or are going to use the plywood outside, you must apply 2 or more coats of Hard Wax Oil, waiting for the needed drying time between coats.

 

Liquid Latex

Spray – on or paint – on Latex is another effective plywood protector.  It’s available at hardware stores and forms a waterproof layer on the surface of the plywood to protect it from moisture.  Liquid Latex products are easy to apply and relatively inexpensive.

Here’s how:
1.) Using a clean cloth and some detergent, remove all the dust and debris from the surface of the plywood. You can also use a 50/50 mixture of water and vinegar for a streak free finish if you’d prefer.


2.) Once the plywood is clean and dry and without traces of cleaning agent, you can either spray the Liquid Latex on or apply it with a clean paint brush or paint roller. Whether you spray, brush, or roll it on, make sure that it’s applied in a smooth, even layer.


3.) After applying the Liquid Latex, give the plywood a drying time of an hour or so, it dries very quickly. The Latex will form a layer on top of the plywood and protect it from moisture and things like mold.
If you think it might be necessary, you can add another coat of Latex once the first coat has dried.

 Waterproof Wood Sealant

A sealant is perfect to use for your plywood whether you’re using indoors or outdoors.  A good sealant will provide a hard, durable layer on the plywood surface that will protect it from moisture, water, and wear and tear damage.  Your local hardware store should offer a range of sealants – speak to the sales clerks for expert advice on your needs and always follow the manufacturer’s directions on the package!

Here’s how:

  1.  Using a clean cloth and a cleaning agent OR a mixture of water and vinegar, clean any dust and debris or stains off the plywood.  The cloth for cleaning should be kept damp, it shouldn’t be soaking wet.
  2. When you’re sure that the plywood is free from dirt, apply the liquid sealant, following the manufacturer’s directions closely.  The easiest way to apply sealant is with a clean paint brush.  Try to get a paint brush that is a size appropriate for the plywood, it’ll make the job quicker and easier.  Make sure that the whole surface of the plywood is evenly coated and that there are no streaks or bubbles in the finish.
  3. It’s important to allow the sealant to dry thoroughly on the plywood as this allows the compounds to harden and form a protective layer.  Drying time for an oil based polyurethane sealant is 24 hours, and for a water based sealant is 4 to 6 hours.  Follow the recommendations on the sealant packaging for best results.

Often more than 1 coat of sealant is recommended for greater durability – apply 2 or 3 coats of the sealant allowing drying time in between coats.

Is Exterior Grade Plywood Waterproof?

Exterior grade plywood is made of many layers of wood held together by waterproof glue, because it’s being used outdoors and will be exposed to the weather it is more water resistant and tougher than ‘ordinary’ plywood that you would use indoors. 

It is not automatically waterproof though, and here are some easy D.I.Y techniques to get your exterior plywood fully waterproof!  This project will take you a day or 2.

For the following techniques, ALWAYS start off with plywood that is both clean of dust and debris, and fully dry.

Polyurethane Varnish

Polyurethane varnishes are made to save materials like plywood from moisture and tough weather conditions, so save yourself time and money wasted on plywood damage and protect it!  You will need:

  • A small paintbrush
  • Masking tape
  • Polyurethane Varnish

All of these items as well as drop sheets and items for protecting your hands and other surfaces can be found easily as your local hardware store.  For this treatment you’ll be focusing on the plywood edges, because that’s often where water gets in.

  1. Apply a coat of the varnish to the edges of the plywood with a brush.  Avoid getting the varnish onto the surface of the plywood – if varnish gets onto the plywood surface, it will stop it from absorbing any future wood treatment/product. 

    To prevent the varnish from getting onto the surface of the plywood, cover it fully with the masking tape, leaving only the edges bare.
  2.  Once you’ve painted the varnish onto the edges and you’ve followed the manufacturer’s directions for drying time, add another 1 or 2 coats to the edges, allowing for drying time in between coats.

Water Based Stain

Water based stains for wood are often very light in color and will not show any bright shade.  They are good for your plywood because not only are they waterproofing, but they protect the plywood from UVA sun damage too, just like your favorite sun screen!  You will need:

  • Water based pigment stain
  • Cloth rags
  • A pump up spray bottle
  1.  With your spray bottle or a disperser provided by the manufacturer, cover the entire surface of the plywood in an even layer of the staining product.

After spraying the whole surface, mop up any excess or dripping stain with a clean cloth rag.  Water based stains are only meant to provide a light finish so don’t expect to see any bright or noticeable change in color.

  • Following the manufacturer’s instructions on the stain packaging, allow the product to dry thoroughly and don’t be tempted to skimp on drying time! 
  • For best results you should leave the treated plywood for 24 hours after you’ve sprayed it to allow it to dry fully.  Once you’re certain that it’s completely dry, you can proceed and add another coat or 2 to your liking.

Water Sealant

Water sealants on plywood are a good idea for any householder because these products guard against water penetration.  Sealants also condition your plywood without blocking all the pores in the material itself.  You will need:

  • Water sealant made for plywood
  • A spray bottle or brush
  1.  Using your spray bottle or brush, apply a layer of sealant to the plywood.  Make sure that you apply it evenly and that you have no bubbles or streaks in the finish.  Check the packaging of the sealant for the drying time – some products take only an hour or 2 to dry, others may take longer.  Always follow the manufacturer’s directions.
  2. Once the initial coat of sealant is dry, you should re – apply it another 1 or 2 times for maximum water protection, allowing time to dry between coats. 
  3. For water sealants being used outdoors on things like decks, it is recommended that you reapply the sealant every 2 years. 

What’s the Best Water – Resistant Plywood?

The best plywood to look for if you’re going for waterproof capability is BWP or ‘boiling water proof’ plywood.  It is also called Marine Plywood so double check with your hardware retailer if you’re unsure. 

This type of plywood can be used both inside and outside – it’s treated with undiluted resins and has a higher quality of veneers that make it water resistant.  This would be a good product to use in moist areas like a deck, kitchen, or bathroom.  Also, it’s the perfect solution if you live in humid areas or in areas with a high annual rainfall.     

Not only is BWP plywood totally impervious to water, it’s also borer and termite proof, which is why it’s even used on the outside of buildings!

Building made out of BWP plywood

   

Tips for Waterproofing your Plywood

  • When fitting or making your own drawers, cupboards or furniture from plywood, make sure that you mount the piece onto feet or spacers (these are placed underneath) so that the plywood won’t come into contact with water on the floor.
  • Use the right type of plywood for the right purpose – if you know that the plywood will see severe weather conditions and heavy usage, stick to BWP graded plywood, it’s made for the toughest of conditions and you can use it and have confidence in its ability to stay dry without rot.
  • If using plywood to build outside, use an exterior grade of plywood – talk to your hardware guy about what you’re using it for, and the performance you expect.  Also, remember to CAULK all your joints, inside and out.  Design your construction with no vertical seams or joins that will collect water. 
  • Use a high grade epoxy coating both on the edges of your plywood and on any exposed area of it that touches the ground.
  • Where possible make sure that the top surface of your plywood if it’s going to be standing outside is pitched, like a roof.  This will stop water pooling on top of it.
  • For plywood structures like dog kennels that sit outside, you can use a sheet of aluminum as a layer on top of the plywood.  Simply fold or bend it down over the edges of the plywood, then tack it on about 1.5 inches below each edge.  This will add another very sturdy layer of protection from water and insect damage.
  • Another great solution is to stick tar paper underneath the plywood to protect it from water seeping in through the edges or from underneath.   

Plywood – it’s one of today’s ‘miracle’ materials and so versatile for both indoor and outside projects.  Also, easy on the pocket which makes it a ‘go to’ for home Do It Yourselfers!

Treated and maintained correctly, it will last you for many years.

Happy D.I.Y folks!