Easy Ways to Seal Plywood for Outdoor Use

7 Easy Ways to Seal Plywood for Outdoor Use

In Exterior Paint, Exterior Wood, Tips by Jamie

Plywood is an excellent versatile building material that can be used for outdoor projects. However, it must be properly sealed to protect it from the elements. This article will help you choose the right plywood for your project and seal it so it lasts for years to come.

Is Plywood Waterproof?

There is no simple answer here because there are different types of plywood with different levels of water resistance. Plywood is graded by the veneer on the front and back, and it is also given an exposure rating. The exposure rating looks at how waterproof the bond used to stick sheets together is, and how well the veneer will stand up to moisture.

By looking at the exposure rating, you can tell how waterproof the plywood is likely to be (if at all). The four exposure ratings are:

  • Exterior – Fully waterproof bond. This type of plywood is designed to withstand constant exposure to moisture and can easily be used outside.
  • Exterior Exposure 1 – This grade of plywood also has a fully waterproof bond, but it is not recommended for constant exposure to moisture outside. Although it may hold up to some moisture, it will not be durable enough to handle constant rainfall.
  • Exterior Exposure 2 – This plywood is designed for interior use and uses an intermediate glue that is not fully waterproof. It can withstand a small amount of moisture (slight splashing or spraying) but constant exposure to water will destroy it very quickly.
  • Interior 2 – This plywood is designed for interior use only and cannot deal with any moisture at all. If it gets even slightly wet, the layers may begin to come apart and the wood will warp.

If you are using plywood outside, you need to look at the exposure rating and use an exterior grade plywood where possible. However, bear in mind that this doesn’t mean it is completely impervious to water. You will still need to seal it properly if you want it to last for years to come in the rain.

Does the Type of Plywood Used Outdoors Matter?

Does the Type of Plywood Used Outdoors Matter

Yes, the type of plywood you use outdoors absolutely does matter and it’s not just the exposure rating that you need to look at either.

Firstly, you need to understand how plywood is constructed and what veneer grades are. Plywood is made from thin layers of wood called veneers. The stronger the veneers, the stronger the end product will be. The veneers that are used to make softwood plywood are graded A, B, C, or D, with A being the strongest and D being the weakest.

  • A-grade veneer – This is the highest quality wood you can get. It has a nice smooth finish and takes paint very well. It’s also the most expensive option.
  • B-grade veneer – This is still good quality but it is not quite as smooth and it may have some small imperfections.
  • C-grade veneer – A and B grade veneer do not have knots in them but C-grade may have some. They will be no larger than 1.5 inches in diameter. You may also notice other flaws and a rougher surface.
  • D-grade veneer – This is the cheapest option and it can have knots up to 2.5 inches in diameter. It will also be much rougher and have lots of imperfections, so if aesthetics are a concern, avoid using D-grade plywood.

Each piece of plywood is given a separate grade for the front and back. For example, it may be graded AC, AA, or BD. Often, the front veneer is a higher quality so it looks better and is easier to paint.

The grading system for hardwood plywood is slightly different. The lettering system for the front veneer is the same but it uses a number system (1 to 4) for the back panel. This is essentially the same as the lettering system that softwood plywood uses, with 1 being the highest quality and 4 being the lowest.

You will also see some plywood graded with an X after the first two letters or numbers. This means that it is an exterior plywood that is designed to be more water-resistant. The most common examples are CDX and ACX.

In general, hardwoods are better for exterior use than softwood because they are more durable and moisture resistant. So, it may be better to go for a hardwood plywood if you are using it outdoors. Consider the thickness of the wood too; plywood with more layers will generally be tougher and stand up to the elements better.

Thin plywood, on the other hand, will easily soak all of the way through and the core will begin to rot.

Most people don’t realize how many different types of plywood there are out there, and it’s incredibly important that you choose the right type for outdoor projects.

What Type of Plywood Should You Use Outdoors? Plywood Sheet Board, A Grade, 22 x 12 inch, 1.5mm Thick, Pack of 10 Unfinished for Crafts Basswood by Craftiff

In general, you should be looking for any types of plywood with an X grading because they are designed for outdoor use. Look at the grading of the veneers too and consider the presence of knots. If there are knots in the wood, it is easier for water to collect and the wood is more likely to rot.

Veneers that are completely knot-free provide a stronger barrier. Marine plywood, graded AA, AB, or BB, is the most water-resistant and if sealed properly, will easily stand up to the elements. 

You can also consider pressure-treated plywood. This wood has been treated with chemical additives that protect the wood from moisture and make it naturally water-resistant. Pressure treated wood is one of the most popular options for exterior projects because it gives great protection but it is cheaper than a lot of the hardwoods that are naturally moisture resistant.

Finally, you can use overlaid plywood. This is very similar to ACX or CDX plywood but the overlay gives it a nice smooth finish, making it ideal for projects where aesthetics are very important.

Overall, when looking for plywood for outdoor use, choose the following types:

  • Marine plywood (AA, AB, or BB)
  • Exterior plywood (CDX or ACX)
  • Pressure-treated plywood
  • Overlaid plywood (CDX or ACX)

What Types of Plywood Should Be Avoided for Outdoor Use? 

There are some types of plywood that should not be used outdoors. Firstly, anything that is not exterior rated and has a low grade (CC, CD, DD, etc). Low grade plywood like this has lots of knots, which let a lot of water in.

Anything with an MDF core should be avoided too. MDF is made with lots of wood fibers that are mixed together with resins and glue before being pressed into sheets. When dry and protected, they are durable and can be great for interior applications.

However, as soon as they get wet, all of the fibers swell and warp. This is quite common in cheap kitchen worktops with an MDF core. If you put anything containing MDF outside, it won’t hold up.

 

OSB (oriented strand board) is another option that people think is good for exterior projects. It is slightly different from MDF and plywood because it is made from lots of strands of wood that are layered up and pressed together. Although more weather resistant than MDF, it is designed to be semi-exposed to the elements (covered by siding or a roof, for example), so will not last long if constantly exposed to moisture.

T1-11 plywood is a popular choice for siding, but it may not be the right option for your project. When it has some protection from the roof, and it is sealed with a high-quality exterior paint, it can be effective. However, if you are building something that will be in direct rainfall, it will require more maintenance than exterior grade or marine plywood if you want to avoid decay.

How to Seal Plywood for Outdoor Use?

1. Use an Exterior Wood Paint

Rust-Oleum 3104 Zinsser 1-Qt. PermaWhite Exterior Satin

Exterior wood paint is the ideal way to protect your plywood and make it look great too. If you are building a gazebo or decking, or even some wooden signs, a coat of paint will bring it to life. Acrylic latex paints are usually the best option for exterior plywood as they go on nicely and when applied properly, can last up to ten years.

A great sealing paint is Rust-Oleum Zinsser PermaWhite Exterior Paint. It will resist dirt and moisture making the plywood sealed and ready to go. This option is great for roofing and many other building projects. 

Follow these basic steps to apply exterior wood paint:

  • Clean and lightly sand the plywood – This will remove dirt and grime and give you a nice surface for the primer. If the plywood is new, a simple wipe down with some sugar soap and warm water should be enough to clean it.
  • Apply two coats of primer –  Latex-based primer will seal the plywood and give you a better finish on the paint, making it look nicer and creating extra durability. Plywood absorbs more paint than other woods too and the primer prevents this.
  • Apply paint – Your paint needs to be applied between 2 days and 2 weeks of applying the last coat of primer. Both latex paint and oil-based paint will go on top of your primer. Apply two coats, giving enough time to dry between, and you will have a strong protective layer for your plywood.

See our best exterior paints for wood here in our top 10 guide.

 

2. Liquid Rubber Smooth Polyurethane Deck Coating Liquid Rubber Smooth Polyurethane Deck Coating - Solar Protection Deck Sealant, Non-Toxic Multi-Surface Waterproofing Membrane, Easy to Apply, Misty Gray, 5 Gallon

The Liquid Rubber Smooth Polyurethane Deck Coating is an incredibly hard-wearing top coat that works brilliantly on plywood. When dry, it creates a very hard top layer that protects against moisture and UV rays. It comes in a large range of colors too so it’s ideal for most outdoor plywood projects.

To apply this excellent coating, follow these simple steps:

  • Clean and dry the surface – Use a brush to sweep away any debris and, if necessary, you can use a deck and patio cleaner to remove stubborn stains and dirt. You can also wipe it down with warm water and sugar soap.
  • Prep joints and seams – Use a paintable wood filler to smooth out the seams so you get an even finish all over.
  • Apply primer – A single coat of all-purpose primer will create an extra moisture barrier and give you a better finish.
  • Apply Liquid Rubber Polyurethane Coating – Using a roller is the easiest way to get an even coat. Always avoid applying the coating in temperatures below 10 degrees Celsius or in direct, hot sunlight. It will take 4-6 hours before you can recoat and you need 3-4 coats for full protection. Once the final coat is finished, leave at least 3 days before walking on the plywood or moving it around, etc.

You can find more detailed information in this video on YouTube.

3. Marine Spar Varnish TotalBoat Gleam Marine Spar Varnish, Gloss and Satin Polyurethane Finish for Wood, Boats and Outdoor Furniture (Satin Low-Sheen Quart)

Marine spar varnish is designed to protect wood that is in constant contact with seawater, so it is more than capable of protecting your exterior plywood projects. Spar varnish is a clear coat that creates a hard protective layer on top of the wood.

This TotalBoat Gleam Marine Spar Varnish is one of the best products you can get for protecting outdoor plywood. It contains tung oil so you’ll get a nice natural finish alongside the protective clear coat.

Follow these steps to apply a marine spar varnish to plywood:

  • Clean and sand lightly – Before starting, clean the plywood and then lightly sand it to give the clear coat a better surface to bond to.
  • Apply spar varnish – There is no need for any primer before applying spar varnish. Simply use a good quality brush with natural fibers to apply the first coat. Alternatively, you can use a sprayer for ease and uniform coverage. For most applications, you will need 4-6 coats to get a good protective layer. If you are using a satin finish spar varnish, apply 3-4 coats and then sand lightly with a 320 grit sandpaper before applying 1 or 2 more coats.

4. Apply Minwax Helmsman Spar Urethane  Minwax Helmsman Water Based Spar Urethane, Quart, Gloss

The Minwax Helmsman Spar Urethane is very similar to a spar varnish and gives you the same top layer protection. However, this is just a basic spar urethane, so it doesn’t contain things like tung oil, which alter the color of the wood. This will give you a completely clear gloss finish, and can be used over the top of other paint or varnish.

The application process is similar to spar varnish:

  • Clean and dry the surface – A quick wash with sugar soap and warm water will suffice.
  • Apply paint or stain (optional) – If you want to paint or stain the plywood first, do this according to the manufacturer’s guidelines before applying your spar urethane.
  • Apply your clear coat – Once the paint or stain is fully dried (if using), you can apply your clear coat. Use a brush or sprayer to do 4-6 coats, giving plenty of time to dry between. There is no need for sanding between coats as this is a gloss finish. However, if you want to take some of the shine off it, give it a very light sand with fine sandpaper once the final coat is dry.

5. Thompson’s WaterSeal THOMPSONS WATERSEAL 21802 VOC Wood Protector, 1.2-Gallon, Clear

Thompson’s WaterSeal is one of the most popular wood sealers out there and it has a well-earned reputation for quality. It’s excellent for protecting all kinds of wood against moisture, and it’s easy to use. Unlike spar urethane or polyurethane,

Thompson’s WaterSeal doesn’t form a layer on top of the wood, it soaks right into it. This means that you can’t use it over the top of paint because the paint will already seal the wood, so the WaterSeal cannot soak in.

Application of Thompson’s WaterSeal is quick and easy: 

  • Clean and dry surface – Before applying the WaterSeal, you need to clean any dirt and grime off the plywood. Thompson offers their own deck cleaning product, which they recommend, but any similar cleaner will do just fine.
  • Apply a coat of WaterSeal – The great thing about Thompson’s WaterSeal is that you only need to apply a single coat for full protection, in most cases. Using a brush or roller is the best option. They do say that you can use a sprayer, but in my experience, a small garden sprayer is likely to get clogged and it’s best not to thin the WaterSeal. If you have a sprayer that is a bit more heavy-duty, go ahead and use it. Otherwise, stick with brushes and rollers. Make sure that it soaks into the wood and you get a thin, even layer. Wipe away any puddles quickly before they start to dry.
  • Paint over your WaterSeal (optional) – If you do want some color on the wood, you can actually paint after you have used the WaterSeal. If using oil-based paint, you need to give the WaterSeal at least 3 days to dry. If using latex paint, you need to wait 45 days.

6. Wood Oils Tung Oil - 500ml

Wood oils are good for finishing plywood and giving you a nice natural feel. They penetrate the wood and protect from moisture. Tung oil and boiled linseed oil are both good choices for plywood. Tung oil gives a harder, more water-resistant finish but it is a bit more expensive. Mineral oil is also a popular choice because it is quite thin, so it soaks into the wood nicely.

If you are using a wood oil, follow these basic steps for the best finish:

  • Clean the surface and dry – The surface of the plywood needs to be cleaned with warm water and possibly sugar soap or wood cleaner if it is particularly dirty.
  • Cover the surface with oil – Next, apply a heavy layer of oil with a cloth and let it sit on the surface for ten minutes. This allows it to soak in, but you will have some excess.
  • Wipe away the excess – After around ten minutes, take your cloth and wipe away the excess, working the oil into the grain of the wood as you do so.
  • Sand and repeat – Let the oil dry and then sand the surface of the wood, creating dust. Leave this dust and apply more oil. It will mix with the oil and clog the pores of the wood, making it more water-resistant. Then, repeat the process of leaving the oil to soak in and wiping away the excess. Do this 2-3 times and you should be protected from moisture.

7. Epoxy Finish  AQUASEAL 2 Quart Marine Epoxy Resin Kit | Fast Set Clear Epoxy Resin | Bar Top, Countertop, Table Top Epoxy Resin | 2-Part Combined Casting Resin Epoxy Kit Includes Cups, Spreader, Stirrer & Gloves

Epoxy is a solid substance that can be used for gluing wood together, but is also excellent as a finish for plywood. The bonding nature of it means that it will create a solid, impenetrable seal against moisture and improve the general durability of the plywood. Products like this AQUASEAL Marine Epoxy Resin are ideal for repairing and protecting wood.

To use epoxy finish, follow these steps:

  • Prepare the surface –  You need a clean surface to work with because any dirt will be trapped in the epoxy and can ruin the finish.
  • Mix your epoxy finish – Epoxy is a two-part finish that needs to be mixed with a hardener before you use it. Bear in mind that you get about 20 minutes of work time before it starts to harden, so only mix what you can use to avoid wasting it.
  • Apply epoxy – When it comes to applying the epoxy, there are a few ways to go about it. You can brush it on or use the supplied spatula to scrape on a thick layer. It’s a thick product so brushes or rollers can be tricky to use. Alternatively, if you are only coating small pieces of plywood, you can dip them and then hang them somewhere to dry.
  • Let it cure – Finally, you just need to let it cure. Most epoxy products take around 24 hours to fully cure. The AQUASEAL, on the other hand, should take around 10 hours, as long as you keep the temperature stable.

You can also buy epoxy varnish or epoxy paints, which give you a solid finish while also coloring the wood.


What are Some Uses for Plywood Outdoors?

There are so many excellent uses for plywood outdoors:

  • Decking 
  • Walkways
  • Furniture
  • Outdoor signs
  • Gazebos
  • Sheds
  • Planters
  • Outdoor shelving
  • Birdhouses

Will Untreated Plywood Last Long Outside?

Yes, untreated plywood will last a while outside. If you use a durable exterior grade plywood, you can expect it to last from anywhere between 2 and 5 years before it starts warping and decaying. However, if you protect it properly, it will last much longer.

Waterproofing Plywood Related Questions

What’s the Cheapest Way to Seal Plywood for Outdoor Use?

Using a simple clear coat like spar urethane is usually the cheapest option for sealing plywood. If you are more concerned with aesthetics, you may need to go for a more expensive option like paint or spar varnish.

Is Plywood Vulnerable to Moisture?

Yes, low grade plywood is vulnerable to moisture. If not protected properly, it will soak through and rot very quickly. Even exterior grade plywood is still vulnerable to moisture after a while, so it must always be protected when used outdoors.

Can You Use Plywood as a Deck Floor?

Yes, plywood is a great affordable option for decking. For it to be safe, the plywood should be at least an inch thick, if not more. Otherwise, it won’t hold your weight when walking around and sitting on it. Placing the joists closer together will also give you more stability.

Can You Buy Pressure Treated Plywood?

Yes, plywood is very porous and absorbs protective chemical treatments very well. So, pressure treated plywood is very effective.

What is Marine Plywood?

Marine plywood is any plywood that is graded AA, AB, or BB. This means that it has no knots and is very durable and smooth, making it more water-resistant.

How Can You Seal the Edges of Plywood for Outdoor Use?

Sealing the edges of plywood is crucial if you are using it outdoors. If you don’t, water will easily seep in and the sealer on the front and back will be rendered useless. Make sure that you do multiple coats and get a brush to get into all of the edges. This is especially important if you are using a roller or a sprayer to apply clear coats because you can easily miss the edges. It’s a good idea to do an extra coat around the edges just to make sure.

Do You Need to Seal Plywood that is Used for Roofing?

Yes, the roof takes the full brunt of the rain, so it needs to be sealed properly. COnsider extra coats of sealer if you are building a plywood roof.

Final Thoughts

Although good quality plywood can last a while outside without protection, sealing it is always a good idea. Any one of these methods can be used to increase its lifespan and stop it from rotting due to excess moisture.