How To Seal Plywood For Exterior Use

How To Seal Plywood For Exterior Use

We’ve all used plywood for some project or another in our woodworking shop or in a home DIY project.  It’s real wood but much less expensive than solid wood, and for many uses is a perfectly acceptable alternative.  Plywood comes in a variety of types, grades, thicknesses, and sizes, and the project will determine the type of plywood you use.  

We’ve written about plywood so often in the past, and you’ll find a number of our articles throughout the Obsessed Woodworking website.  One of the articles speaks to the types, grades, thicknesses, and sizes of plywood directly, and you will find it here.

Key Points

  • Choose the right plywood rating for outdoor projects. It will help point you in the right sealing method to use.
  • Sealing methods include the use of paint, polyurethane, or epoxy.
  • Consider banding the edges of the plywood to add further protection against the elements.

Plywood uses include both interior and exterior applications, and which of the types and grades you choose will depend on where it will be used, as well as how.  We’ll cover all of that in this article, and when you’ve finished reading it, you will know what you need to know to make smart decisions for the use of plywood outside.

These and other points will be addressed, so read on for the whole story.

What is Plywood?

Plywood is a manufactured wood product, real wood, made by gluing and compressing veneers, or “plies,” that are rotated 90 degrees as each ply is added.  The grains are rotated like this for 3 reasons:

  • It adds strength to the sheets consistently in all directions;
  • It helps avoid expansion and contraction due to changes in the environment, and, 
  • It helps prevent splitting when nailed.

The rotation of plies makes the plywood stronger than if you hadn’t turned them.

The types of plywood usually refer to the number of plies, whether 3-ply, 5-ply, or multi-ply, with 3-ply being the most common.  We choose it for interior use primarily because of its appearance as more decorative than thicker sheets with more plies.

Plywood Is Real Wood

Yes, it’s a manufactured product, but it is manufactured with real wood.  The wood fiber is glued together with a resin, and different glues are used depending on the rating of the plywood being manufactured.  As mentioned earlier, the plies are turned 90 degrees during assembly to enhance strength, frustrate expansion and contraction, and make it strong enough to handle nailing.

The most common size of plywood is a 4′ x 8′ sheet.  The top ply, the face veneer,  will usually be of a higher quality than the core plies in order to keep the cost of the sheet low.  However, it is possible to purchase sheets where the core is also of the same high quality as the face veneer but at a higher cost, also.

Common Plywood Uses

Plywood is a versatile wood product and is an attractive material to use because of its cost.  Some of the more common uses for plywood include:

  • The exterior of wall framing when building a new home
  • Floorboard over the joists
  • Furniture, cabinets, desks, shelving, and more

Because of its strength, it’s a common choice when the project requires a high stability factor.  The benefits of using plywood are many:

  • Stability – rotating plies add great strength and resistance to changes in the surrounding environment
  • Strength-to-weight ratio – plywood sheet shear is twice as strong as solid lumber, has good high impact resistance, and stability against changes in the environment
  • Stands up to chemicals – plywood does not corrode, making it useful in chemical works and concrete forms work.

If you work in construction, you already know this.  But for outdoor projects, the elements can be dangerous.  We’ve said many times that water is the enemy of wood, and if you’re using plywood for an outdoor project, you need to consider ways to prevent water intrusion into the plies – you must waterproof plywood for your project.

The Grades of Plywood

When talking about the grades of plywood, we are referring to the quality and appearance of the sheets. These grades determine the price of a sheet of plywood, too.  The better the quality and appearance, the higher the price will be, as you’d expect.

  • A grade.  The highest quality and with the highest price.  Sanded smooth, blemish-free, and suitable for painting.  
  • B grade.  Solid surface, sanded smooth, but with some visible defects in appearance.  Repairs will have been made, most often showing as football-shaped patches.  You might see some knots, but no wood will be missing.  Less expensive than A grade.
  • C grade.  This grade is not sanded and will show defects and flaws in appearance that have not been repaired – you’ll have to do that yourself.  This grade will be used in ways where it will not be seen, such as floorboards.
  • D grade.  This grade, also, is not sanded and will show even more defects and flaws in appearance that you’ll have to repair yourself.  There will be some discoloration, and some wood will be missing.  Again, this grade will be used where no one will see it.  

The Ratings of Plywood

This is where we begin considering what plywood to purchase for exterior use.  The rating is as important a consideration in what plywood you will use as is the grade of plywood, especially when it comes to exterior use.

Exterior.  Plywood sheets with an Exterior-rating will have been waterproofed and are suitable for use in outdoor projects.  Waterproof plywood is manufactured with a different glue than plywood that would be used for interior projects.

Exposure 1.  These plywood sheets, also exterior plywood, have been waterproofed and can withstand the elements during construction but are not to be used for prolonged exposure to the elements.  It’s okay if they get wet for a while but eventually will need to be either sealed or covered.

Exposure 2.  These plywood sheets will have been partially water-proofed – the glue used can withstand some exposure to water, but using this plywood should be limited to interior use.

Interior.  This plywood is not waterproofed, and its use should be limited to interior use.

Structure 1.  This plywood has been manufactured to withstand earthquakes.  The vast majority of us will never have to use this rated plywood – only those who live in areas that are prone to earthquakes will consider it.

Other Things to Know About Plywood

Using Plywood Outside

Within these grades and ratings, there are also features, categories, and uses that are good to know, and some of them relate to the question of this article.

Softwoods.  Softwood plywoods are manufactured with softwoods like pine and cedar.

Hardwoods.  Hardwood plywoods are, as you would expect, manufactured with hardwoods like birch, oak, walnut, and maple.  Baltic Birch is probably the best known of these plywood categories, and we like birch, especially the sheets where just the face ply is birch, with lesser woods in the core.  Birch finishes well and presents a beautiful appearance on the plywood surface.

Building aircraft.  Yes, airplanes are made partially of plywood.  The early planes, biplanes, and even military planes were made with plywood.  Strong, durable, able to stand up to the elements, and inexpensive – a perfect material for such use.

Marine plywood.  It’s not specifically used to build boats – other woods are better suited for that.  But it refers more to the grade of the plywood.  Marine plywood is well-used for exterior uses like benches, decks, and other features in your gardens because of its high-quality construction.  Marine-grade plywood will serve you well in these uses, but read on for additional measures you should take.

There are grades within grades of plywood, also, but for our purposes today, we will not dig any deeper into all things plywood.  Perhaps in another article, because today we are talking about waterproofing plywood for exterior use.

Preparing Plywood For Exterior Use

Even if you have planned to use exterior plywood, waterproof plywood manufactured for exterior use, you will still want to take additional measures to protect the plywood from moisture. There are several ways to do this, each of which is not much more than you would finish plywood you were using for interior projects.

Clear Sealants For Plywood Waterproof Exterior use

When working on an interior project, whatever it might be, we often turn to a polyurethane varnish as a top coat.  It’s clear, allows the wood to show through, and when working with Baltic Birch plywood, we do want to see the wood and its grain.

Even though water-based polyurethane varnishes will tell you they are good for outdoor use, we suggest you choose instead an oil-based poly.  It’s more durable and offers greater protection.  You can get away with a water-based poly as a topcoat for paint since it’s the “suspenders” to the paint’s “belt” – an added bonus level of protection rather than the sole source of protection.

A spar varnish, higher in solids than a regular varnish, is another good choice to seal plywood for exterior use.  It will offer more protection against the elements – rain, snow, ice, large temperature swings – than regular varnish.

An oil-based product like Thompson’s WaterSeal is another excellent product.  We know Thompson’s products are used to treat boat decks at marinas, where it can help extend the life of wooden boat decks for years with regular re-application.  It forms a protective layer that will keep boat decks, and your outdoor plywood products, from all harmful elements and weather.

If you start with waterproof plywood, exterior rated, and treat it with one of these clear sealants, you will be able to protect it from water damage and the intrusion of natural moisture for a long time.  Occasional re-application will extend the life of the plywood for years.

Even untreated plywood-rated exteriors will eventually succumb to the elements, but the addition of sealants such as these will protect against that from happening.  Go the extra step, don’t rely upon the exterior rating completely, and you will be happy you did.

When using one of these clear sealants to waterproof plywood for exterior use, be sure to follow all standard safety protocols, including wearing gloves, a mask, and safety goggles.  In the case of an oil-based polyurethane varnish, be sure to use it in a well-ventilated area or a dry and clear day for outside application, and wear a respirator.

Follow the manufacturer’s recommended dry and cure times before applying each additional coat.

Painting to Waterproof Plywood for Exterior use

Paint is also a good sealant product for plywood to be used in outdoor projects.  The plywood should be treated first, though, before applying paint.  A water-based paint applied in several coats can be effective, but pre-treatment with a sealant like Thompson’s WaterSeal (maybe two coats) is a good idea, so long as you use an oil-based paint on top of the oil-based Thompson’s product.

Why use both a sealant and paint?  If you have used a clear sealant rather than a sealant that also contains stain, you might want to match colors with the rest of the project.

If you are relying entirely on paint to waterproof plywood, you will want to use a primer first.  This will give the paint something to adhere to well and thus add to the level of protection the paint can provide.  

A primer like Zinsser’s Oil-Based Primer does a good job and gives the paint a solid bonding with the plywood for an extra measure of protection.  Use an oil-based paint over an oil-based primer, too, just as you would use an oil-based paint over an oil-based sealant, as noted earlier.

Zinsser 03504 Cover Stain Interior/Exterior Oil Primer Sealer, 1-Quart, White
  • Cover stain, QT, Oil based stain killer primer/sealer
  • Adds extra shine to your product
  • Manufactured in United States
  • High-hiding formula blocks most stains and helps seal water, smoke and nicotine stains

Epoxy Sealer to Waterproof Plywood for Exterior Use

We know epoxy resin to be a really thick and waterproof finish, and it is also a suitable choice to waterproof plywood for outdoor use.  It, too, is a clear sealant and allows the beauty of the wood to show through.

The application, though, can be a bit tricky unless you are simply using it on the plywood’s flat surface – you pour on the epoxy sealant, not brush or wipe it on.  Epoxy sealant, though, is an ultra-waterproof product and will protect the plywood for a very long time.

Epoxy needs to be mixed – two components – and the ratio of the two components is important to produce a usable sealant.  You might want to practice with it first before applying it to the actual project, just to be sure you know what you are doing and will be happy with the results.

Sealing Plywood Edges

No matter which of the methods of sealing and water-proofing plywood, you must deal especially well with the plywood edges.  They are obviously the most porous surfaces of plywood and an easy entry point for moisture and water elements.

We recommend a couple of coats of whatever sealant you choose, except maybe for an epoxy coat.  Whether polyurethane varnish, spar polyurethane, or a Thompson’s WaterSeal, brush it firmly, pushing it well into the wood fibers along the edges.  Allow the first coat to dry according to the manufacturer’s recommendations before applying the second coat.  

On Sale
Thompsons Water Seal 24111 1.2-Gallon Clear Low VOC Water Sealer For Exterior Use
  • Prevents water damage
  • Beads and waterproofs a variety of surfaces
  • Works on masonry, brick, concrete, decks, fences, etc
  • Exterior use only

If painting for waterproofing plywood, you will still want to pay close attention to the edges with a sealant or a couple of coats of primer.  

Another option is to iron on the banding.  We wrote about edge banding plywood in an earlier piece, and you can refer to it for the how-to and the why.  Edge banding plywood will enhance the appearance of the project and give it a more professional look to it.  It, too, is made of real wood and will take sealants, paints, and epoxy just like plywood and solid wood does.  

Another Option To Waterproof Plywood

Liquid rubber.  Yes, there is such a product.  We must admit we have not used it before, but we are aware of it as a viable option to waterproof plywood for outdoor use.  

Choose the right type of plywood, the right grade, and the right rating; choose your method of sealing the plywood and its edges.  Do this, and your outdoor project using plywood should last for a very long time.

Last update on 2023-06-06 / Affiliate links / Images from Amazon Product Advertising API

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