Is Marine Plywood Waterproof? (Is It the Best for Outdoors?)

Marine plywood has seen a surge in popularity in recent years as a construction material. If you’ve been anywhere near a boatyard or marina, you’ve probably seen stacks of it ready for building boats and other projects. It’s strong, durable, and resistant to rot and moisture – but is marine plywood truly waterproof?

In general, marine plywood is not waterproof, but water-resistant. It is made with 5 or more layers of Western Larch or Douglis Fir with less imperfections such as air pockets and knotholes. It also must be made with waterproof adhesive, which in combination of the type of wood its made from, makes it marine grade plywood.

In this article, we’ll take you through everything you need to know about marine plywood, from what the term means to its practical applications. We’ll also talk about some of the alternatives to marine plywood if it doesn’t work for your project.

What Is Marine Plywood?

If you are building things outside and needing different waterproof sheets I am sure Marine plywood has come up more than once, but what is Marine plywood?

Marine plywood is a type of construction plywood made from thin sheets of wood veneer, such as mahogany, douglas fir, and western larch. These harder woods resist rot and moisture better than other woods. It is also constructed using Weather and Boil Proof (WBP) glue, which is a type of adhesive that can withstand high temperatures and moisture levels.

Manufacturers glue the sheets together in layers, with each layer placed at right angles to the one below it. This gives marine plywood strength and stability.

This makes marine plywood ideal for construction that will be exposed to the elements, like docks, decks, and boats’, but marine plywood isn’t waterproof without sealant.

Fun Fact – Did you know that marine-grade plywood was invented in the 1930s by the Dutch door manufacturer Cornelius Bruynzeel? He used it to build boats during World War II. There are ships today that are over 40 years old that were made with marine plywood!

Does Marine Plywood Need to Be Sealed?

Since many people commonly believe that marine plywood is completely waterproof, they think also don’t think it needs to be sealed. That’s one of the biggest mistakes you can make!

Even though marine plywood is water-resistant, it’s still made of wood. This means that if you don’t seal it properly, marine plywood is not waterproof. It will absorb moisture from the environment and start to break down eventually. If you’re using marine plywood for boats or docks it will naturally spend extended periods of time exposed to the water and should be sealed.

So, how do you properly seal marine plywood? The best way method uses an epoxy-based sealant. The sealant creates a barrier between the marine plywood and the elements, protecting it from moisture and rot.

How To Seal Marine Plywood

We broke down how to seal marine plywood further with 4 simple steps below

Step 1: Clean with acetone.

Use an acetone-dipped cloth to clean the surface of the marine plywood. This will remove any dirt or debris that could prevent the sealant from adhering properly.

Step 2: Sand down the surface.

Sand down the surface of the marine plywood with fine-grit sandpaper. This step is crucial when applying sealants on all kinds of plywood, including marine grade. Sanding creates a smoother surface for the sealant to adhere to, which will result in a stronger bond.

Step 3: Apply the first coat of sealant.

Apply the first coat of sealant evenly across the surface of the marine plywood. Make sure to follow the manufacturer’s instructions on how to do this properly. You should also ensure that every inch of the plywood is covered. Once applied, wait for the sealant to dry completely before moving on to the next step.

Step 4: Repeat step 3.

Repeat the above step a few more times till the sealant has completely sealed the marine plywood. Here’s a tip to help you with this – apply as many coats as the layers of veneer in the marine plywood. So, if the marine plywood has three layers of veneer, apply three coats of sealant.

Your marine plywood is now waterproof and ready for all your water-based construction projects!

Alternatives To Marine Grade Plywood

Let’s say you’re not able to get your hands on marine plywood, or you simply can’t afford it. What next? Here are some of the best alternatives to marine plywood that you can use:

Ribbon Grain Plywood

Also known as cross-grain plywood, suppliers make this type of plywood by placing and gluing thin strips of wood together vertically. This results in extremely strong and stable plywood, making it a great alternative to marine plywood. It also costs less and can be managed more easily.

Fiberboard

Fiberboard is another popular alternative to marine plywood. To create it, manufacturers hold wood fibers together with resin. Fiberboard is relatively cheap and easy to find, but it’s not as strong as marine plywood. You can choose a medium-density fiberboard for better durability.

Fiber Cement Board

While most water-resistant plywood combines wood and resin, fiber cement boards are made from a mix of wood fibers and Portland cement. This gives them fantastic durability, making them ideal for outdoor construction projects. They’ll also weigh less than most other types of plywood.

If you would like to see more alternatives to marine plywood, we have an entire article dedicated to the same! Check it out here.

How Long Will Marine Plywood Last Outside?

The answer to this question depends on a few factors – such as the type of wood used, the number of layers, and how it’s been treated, among others. Sometimes, if marine plywood is not exposed to water, it can potentially even last indefinitely!

Unfortunately, marine plywood, never coming in contact with moisture, rarely happens. So, practically speaking, marine plywood has an average lifespan of anywhere between 30 to 40 years when used outdoors.

Is Marine Plywood the Best for Outdoor Use?

Yes, experts consider marine plywood the best plywood type for outdoor use. This is because it’s specifically designed to withstand harsh weather and wet conditions. Its durability makes it ideal for construction projects that need to stand up to the elements.

Even within marine plywood, there are different quality grades. The highest grade is AA, which means that it’s made from the best quality wood and has very few imperfections. You can also find marine plywood with a lower grade, such as AB or BB. These are of a lower quality, but still better than regular plywood.

So, if you’re looking for the best type of plywood for an outdoor project, choose marine plywood!

Marine Plywood vs Exterior Plywood (Pros and Cons)

Now that we’ve gone over everything there is to know about marine plywood, let’s take a look at how it stacks up against exterior plywood.

Marine Plywood	Exterior Plywood
Pros	Cons	Pros	Cons
Excellent water resistance	Expensive	Visually appealing	Requires a lot of energy to manufacture
Strong and durable	Chemically treated, which can be toxic to humans	More cost-effective and easily sourced	Low breathability
Uniform strength since there are fewer knots	Can sometimes bend if overloaded	Eco-friendly	Limited insulation
Easily available to buy		Good resistance against impact, moisture, and temperature	Poor fire resistance
Resistant to insects such as termites and ants

As you can see, there are a few key differences between marine plywood and exterior plywood.

Tips on How To Choose The Correct Plywood

• If you need plywood for a construction project that will be exposed to moisture or humidity, use marine plywood that’s been waterproofed.

• If you need plywood for a house project where the exposure to moisture will be minimal, you can opt for exterior plywood.

• Marine plywood costs more than exterior plywood. So, if you’re on a budget, exterior plywood is the better choice.

We hope this has helped clear up any confusion you had about marine plywood vs exterior plywood!

Marine Plywood Waterproof FAQs

Here are some of the most commonly asked questions regarding marine plywood.

Q.1. What is the cost of marine plywood?

A. Depending on the thickness, grade, and size, the cost of marine plywood varies. It can range anywhere from $58 (for 1/4″) to $112 (for 1″). Keep in mind that all plywood is 4 ft x 8 ft.

Q.2. Can I paint marine plywood?

A. You can paint marine plywood. You should first use a good quality primer to fill the pores of the wood, and then use a paint that is suitable for exterior use.

Final Thoughts

We hope this article has answered all your questions regarding marine plywood! As you can see, it’s an incredibly versatile and durable material that is perfect for a wide range of applications. So, whether you’re looking to build a deck, create some shelving, or even make your own outdoor furniture – marine plywood will make sure your project is of the highest possible quality! Happy building!

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