Marine Plywood Grades: 5 Types But None Are Waterproof (X Glue Explained)

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Woman on a boat dock looks at its construction and wonders about the grades of marine plywood, and is marine plywood waterproof and what are the types of marine plywood and rot resistant plywood.

Did you know that there are various grades of Marine plywood available for your construction needs?

Marine plywood is different than other forms of this type of building material for one reason…quality.

But, despite the name “marine,” the plywood itself is not actually waterproof. Yes, it provides moisture and weather resistance thanks to it’s exterior waterproofing and glues, but it’s not completely waterproof.

But, you might be wondering about CDX plywood…isn’t it good enough? There are some major difference in the rating of building materials, and the “X” glue doesn’t stand for exterior.

However, marine grade plywood has unique features and delivers high performance, especially for outdoor use and boat construction.

But before you go out and buy the materials for your next project, learning about the grades of Marine plywood and the types of glues that are used to make them can help you choose the best option.

This guide explains everything you need to know.

What Is Marine Plywood?

Plywood is a building material that is manufactured using sheets of wood, compressed and glued together in alternating layers, called plies.

The layers give the flat surface more strength.

Marine plywood is different that standard plywood in the way that it is manufactured, and the grades of marine plywood available.

It is designed to have a resilience against moisture damage (rot) and other outdoor elements, but it is not ‘treated’ specifically. Some plywood is compressed and treated with chemicals that make it more durable to resist damage, but there’s another factor involved in the marine grade.

That is the crucial qualification for plywood to be considered marine grade.

Thanks to this feature, the material is suitable for the boat construction industry, outdoor furniture and home building projects, as it withstands long periods immersed in water, snow, wind and other exposure.2

But in addition, marine plywood must be constructed from Douglas Fir boards, and a minimum of five layers, bound together with waterproof glue.7

OSB plywood sub-flooring structure material stamped with the brand name advantech.

Marine plywood grades also have fewer air pockets, unlike standard plywood types, and are finished (the top and bottom layers) to a specific grade as well.

What Are the Marine Plywood Grades? How Is Marine Plywood Rated?

There are five primary grades of Marine plywood, and letters denote the “finish” quality of the top and bottom layers.

The best finish is “A” and this means that there are no surface flaws, such as knotholes rough areas or other imperfections that would make finishing the surface difficult.

Because of this, grades of marine plywood are easy to sand and paint for a smooth, waterproof surface.

1. A-A

These types are ½ an inch thick, and manufacturers produce them in 5×12 feet or 4×8 pieces, commonly, but you can also purchase smaller sizes at many hardware stores.

The “AA” means that both the top and bottom piles of the piece are the best finish possible.

2. A-B

This grade has similar dimensions as the A-A, but the difference is in the thickness. Instead of ½ inches, it comes in ¾ inch dimensions, making it thicker and sturdier.

The “B” indicates that one side is perfect, and the other is slightly less than “A” grade finish, which means there might be filled in knotholes that have been sanded smooth to create a smooth surface.

3. B-B

This common type of marine-grade plywood has a face-back design and is often referred to as Face-Back plywood.

The “B” grade can have knots, but no holes, so the knotholes are filled. The medium density overlay, or high density overlay are options for both A and B grade plywoods.

4. MDO

The Medium-Density Overlay is a unique construction used on marine plywood that features thermosetting and resin fiber overlays, providing a weather proof surface.

It is also resilient to scratching and can withstand chemical and weather damage.

5. HDO

This High-Density Overlay is also commonly known as Steel-Form plywood and is created with more resin.

It is more of an industrial-grade design and comes in various thicknesses to cater to different needs, such as boat construction.

What About X After the Letters?

You may be confused after seeing other plywood grades, such as CD, in a lumber yard.

These letters also refer to the exterior plies used in plywood, but they are not marine grade. The C and D indicate that the surface can have flaws such as knot holes and gaps. But, the X refers to the type of glue used to attach the plies.

The “X” actually refers to exposure. This indicates that the material can withstand some exposure to water and elements, but not as much as marine grades.

How Is Marine Grade Plywood Made?

Manufacturers create various grades of Marine plywood by stacking at least 5 Douglas Fir layers perpendicularly such that the grains alternate vertically-horizontally. This design is critical to ensure that each ply strengthens the structure.

The veneers stick together using a special waterproof glue under extreme pressure and heat then the design finishes off with a water-resistant layer on the exterior. The outstanding feature of the marine grade plywood is its waterproof glue and the absence of voids and air pockets.

Note: if danger of rotting is a concern, it’s better to choose pressure treated plywood.

Additionally, this board features thinner high-quality wood layers, not thick, low-quality wood, unlike other standard plywood types.

Many confuse it with pressure-treated versions, but the difference is that Marine plywood doesn’t involve soaking in chemicals to increase its durability and water resistance.8

Manufacturing Process of Grades of Marine Plywood

Industries create plywood differently, but the goal is to make them durable and high-performance, able to withstand moisture exposure.9

They use the best timber quality for thin veneers, and the layers undergo pressure treatment to create a smooth exterior.1


Next, they press the plies using strong waterproof adhesives and use quad-press technology, which guarantees powerful bonding and no gaps in between.

Using the best wood and adhesives results in the famous high-performance Marine plywood.

Features of Marine Grade Plywood

You may ask what makes Marine plywood marine, and the following are the main qualities of the material.

  • It can maintain its structural integrity even after prolonged contact with water.
  • Its surface is moisture-resistant thanks to the special phenolic plastic used in its construction.
  • The sleek surface without pores keeps off the water, which causes delamination in standard plywood.
  • It is very flexible and ideal for interior and exterior use or where there is frequent contact with moisture.
  • Experts can easily shape it since it can maintain its structural integrity, and the upside is that it doesn’t break under minimal pressure.
  • Manufacturers use quality adhesives and modified resin to keep it water-resistant.
  • Marine plywood is best for boat construction where there is excessive water exposure, and you can also use it in making kitchen and bathroom finishes.
  • It is the sturdiest and most durable plywood and maintains its integrity regardless of application.
  • Given the above impressive qualities and the manufacturing process, marine-grade plywood is generally more expensive than standard versions.

What Is the Grading System of Plywood?

The grading of various Marine plywood is based on three main factors, the waterproof glue’s quality, the wood type used in the stacking, and the absence or presence of voids. The material comes in grades A to D, A being the highest quality.

The plywood has a dual-grading system that describes the sheet from each side. Therefore, A-B has a perfect side but is knottier and looser on the other face. The A-A, A-B, and B-B follow this principle, although there are two more versions, the MDO and HDO.6

The two grades boast high-resin waterproof fiber layers, and the outstanding feature of the High-Density Overlay is it features higher resin content, hence a shinier and tougher surface.

How Are Grades of Marine Plywood Different?

Marine plywood entered the market in the 1930s with the discovery of waterproof adhesives to bind the wood layers.10 There are various exceptional versions in the market, but the grades of Marine plywood stand out from the rest.

From the name, this material is uniquely formulated to handle water exposure or submerging, like in the construction of boats.

The initial designs came in handy in making boats during the second world war, but experts found more applications in the later years.

Marine plywood became common in the construction of outdoor furniture where there is excessive exposure to the elements, which applies to the construction of coastal homes. You can tell that they are not ordinary thanks to the impressive structure and use of quality materials.

Is Marine Plywood Waterproof? What Grades Are Waterproof?

Marine plywood has grown more popular in recent years as a construction material for the boat sector and the construction of outdoor furniture, thanks to its resilience against moisture. It is the most durable in the industry, and many believe that it is entirely waterproof.

However, Marine plywood is water-resistance, not completely waterproof. Its construction material and manufacturing process guarantees this, given that it features more than five layers of Western Larch and doesn’t have imperfections of other plywood, like air pockets.

It also boasts waterproof adhesives, which, together with the quality wood, makes it a marine grade.

However, note that the MDO and HDO are generally better at water resistance thanks to the tech used in their construction.

Does Marine Plywood Need Sealing?

Many believe that the grades of Marine plywood are waterproof, hence don’t need sealing, but this is a common mistake. First, the plywood is water-resistant, not entirely waterproof, and second, it is still wood, meaning it needs proper care and maintenance.

It will only be 100% safe from moisture if you seal it. It will absorb water from the air or constant immersion and eventually give in to the pressure. Sealing is critical if your boat is made of Marine plywood since it will always stay submerged.

The sealant is the perfect material that blocks the plywood from harsh elements, keeping it safe from moisture exposure that causes rotting. However, the goal is to do it properly, preferably using an epoxy-based sealant.3

How To Seal Marine Plywood

There are four simple steps in sealing Marine plywood to increase water resistance.

Step 1: Cleaning with Acetone

The initial step is to dip a cloth in acetone and then use it to wipe the wood’s surface.11

It eliminates all the dirt that would otherwise prevent the sealant from completely adhering to the surface.

Step 2: Sanding

You should also sand down the Marine plywood’s surface using fine-grit sandpaper.

It is essential for all forms of sealing on any plywood since it makes the surface smoother for the sealant to attach and strengthens the bond. After sanding, clean the plywood one more time.

Step 3: Applying the First Coat

With a clean and smooth surface, apply the first sealant coat, but carefully following the manufacturer’s instructions. Attentively seal throughout the plywood surface and wait for it to dry.

Most epoxy sealers will require more than one coat and require a high level of safety consciousness.

Step 4: Re-Apply the Sealant

Once the initial coat dries up, you can reapply the sealant for a perfect finish.

Experts advise applying as many coats as the veneer layers on the plywood; if your Marine plywood has five layers, you will need five sealant coats.

Are Grades of Marine Plywood Rot Resistant Plywood?

Marine plywood is a durable, high-quality material but is not waterproof.

It is a unique design that resists water damage since it features waterproof glue and advanced manufacturing technology using the best wood.

Piles of plywood with some exposed to weather conditions.

However, Marine plywood is not rot-resistant, given that its design doesn’t involve chemical treatment. Pressure is used during the manufacturing process, but the material itself is not soaked in any chemical that resists rotting and molding.

Therefore, it is critical to use a sealant on the surface to increase its water resistance and dangerous exposure to moisture.

What Is Marine Grade Plywood Thickness?

You can find all the grades of Marine plywood in various specifications depending on the company, and you can choose one based on your needs. While the standard dimensions are 4 by 8 feet, the layering or thickness varies.

Based on the grade and manufacturer, you can find Marine plywood in 3/8, ½, ¼, ¾, or 5/8 inch thicknesses.

Note that the thicker the Marine plywood, the more durable it is, and by extension, the higher its cost.

What Are the Disadvantages of Marine Plywood?

There are two main downsides to opting for grades of Marine plywood. First, it is not readily available given its high-quality nature; hence it may be challenging to find if you live in a remote or rural area.

Secondly, Marine plywood is costly. Its unique design uses the best tech to make it durable and high-performance, which translates to high costs, unlike more affordable plywood like MDF.12

Why Use Marine Grade Plywood?

  1. This wood is of durable veneers that protect it from fungal attacks and damage from harsh weather.
  2. The plywood is dense and tough, able to withstand powerful impacts, which is an excellent feature supporting its use in boat or marine vessel construction.
  3. It is high-performance and long-lasting for all your needs.
  4. One of the most impressive features of marine-grade plywood is its water resistance, thanks to the glue used for bonding the layers.
  5. Using the modified plywood guarantees stability and maximum protection from damage.
  6. The wood is easy to bend yet sturdy enough to resist deformation under extreme conditions, making it ideal for structural use.
  7. Besides the high quality and performance, Marine plywood is also a beautiful material for interior and exterior finishes, resulting in a soft and sophisticated look.

What Are Grades of Marine Plywood Used For?

One advantage of Marine plywood is its versatility.4 It is easily bendable without compromising its structural integrity, making it suitable for various uses.

  • It is perfect for boat making
  • It is handy in building docks
  • You can use it to construct your deck
  • You can use it to design a platform for your fishing
  • It is suitable for kitchen and bathroom floors
  • You can use it to construct a gazebo
  • It is perfect for creating outdoor furniture
  • You can use it to create planter boxes

Marine Plywood vs Plywood, What Is the Difference?

Wood is a crucial construction material, considering how many trees are cut down each year. The conventional plywood is of thin, soft, or hardwood veneers from pine or cedar, but grades of Marine plywood are of Western Larch or Douglas Fir, selected for their refined grains and high durability.

The plies are also comparatively thinner than ordinary plywood, making them more durable, unlike standard versions whose layers are fewer and thicker. Besides the difference in wood type, the marine grades feature waterproof adhesives, making them water and heat-resistant.

The compounds in the adhesives make the board water-resistant, ensuring that all the plies don’t split and nothing can penetrate the attached layers.

Another feature is that the Marine plywood is high quality and expertly arranged by the manufacturers such that there are no crevices in between.

Therefore, the material can effortlessly resist de-lamination and other damages.

What Are the Applications of Marine Grade Plywood?

Since the 1930s, marine-grade plywood has become a standard construction material. Experts know its exceptional properties and implement them in their projects.

It can be used to build a number of projects, but it’s also often used as a concrete forms, providing a smooth surface for the concrete.

Pieces of wood and a pile of plywood in a construction site.

The board is also ideal for constructing birdhouses, gazebos, fishing platforms, and outdoor furniture.13

It is water-resistant, and your property will be safe from the elements, whether rain or snow. It can also withstand submersion, explaining why it is used in boat construction.

It also works indoors, and you can install heat-resistant versions in your kitchen and bathroom where you expect frequent water contact. A plus is that the wood looks sleek and elegant for all your finishing touches.

Marine Plywood Performance

Trees reduce CO2 emissions and are vital in the construction sector. The marine grade is a top choice for such needs, thanks to its high performance. Although not fully waterproof, it boasts water-resistance properties, making it best for boat and outdoor furniture construction.

Unlike other materials, the wood used in its creation is not chemically treated, a technique that makes plywood rot-resistant. Unless there is wood treatment, the exterior ply may suffer degradation, but the impeccable manufacture protects the internal layers.

Pressure or chemical-treated wood is better at protection from rot than Marine plywood grades. You can always enhance your board’s performance by applying a sealant. This protective layer increases its water-resistant properties and its lifespan in general.

Grades of Marine Plywood Price

Grades of Marine plywood come in various shapes, sizes, prices, and qualities. Manufacturers rate them according to their construction process and features, with some having better performance and being more expensive than others.

Several factors come to play in determining the cost of Marine plywood or any other board. For one, it will be more expensive if the manufacturing process involves high-quality wood at the core.

The face veneer quality and the nature of the manufacturer also dictate the board’s pricing. If the plywood needs a lot of processing and involves more intensive work, it will generally be more expensive than other versions.

Other price determinants include the layers and the quality of the adhesive used; the thicker the board, the more expensive its cost. Similarly, waterproof glues used in Marine plywood lead to increases in prices.

The thickness is the primary determinant of Marine plywood cost, where a standard 4 by 8 feet board costs differently depending on its size.

While a ⅝-inch piece will cost about $84, a ¾ inch alternative can go for around $150. The pricing will also depend on the grade and other particular product features.

How To Install Marine-Grade Plywood

The grades of Marine plywood are effective for various projects, both indoors and outdoors.

However, its installation varies according to its usage and location of the project, but the best part is that you don’t need special equipment; you set it up the same as any other plywood.

Shot showing how to install Marine-Grade plywood properly.

Initially, you cut it accordingly using a jigsaw and ensure that you follow the grain lines for a smooth cut.

If undertaking a massive project and are using thicker boards, you can use 3-inch screws, but the trick is to use one for at least every 12 inches.5

How Long Do Grades of Marine Plywood Last Outside?

Marine plywood is preferred for outdoor use, given its durability and resistance to the elements. Its multiple layers and high-quality wood make it last 15- 25 years, depending on the type and several other factors.

The quality A grades perform better than lower grades since they are made of better materials that make them more durable.

Additionally, plywood lasts longer under proper care and maintenance, surpassing 20 years before physical changes start showing.

Is Marine Plywood Best for The Outdoors?

The trickiest part about picking wood for outdoor use is that it must be weather-resistant. Luckily all the grades of Marine plywood are specially formulated to withstand rain, snow, and extreme sunlight.

They are made heavy and durable, lasting more than two decades under proper care. This board is the best for use outdoors, whether you are building a deck, gazebo, or outdoor furniture.

What Wood Is Used in Plywood?

Plywood construction involves bonding multiple wood veneers from hardwoods or softwoods, meaning that various tree species make different boards. The wood type determines how many trees it takes to build a house, and it applies to other aspects of construction.

The most popular softwoods used include Pine, Cedar, Spruce, Redwood, or Douglas Fir but the most common hardwoods in plywood construction are Mahogany, Maple, Teak, and Ash.

However, composite plywood (partical board and OSB) will involve lumber-piece chunks.

House construction site using plywood flooring on 2nd floor.

Compared to other materials, Marine plywood is one of the most durable and versatile boards for all your construction needs. Since the invention of waterproof glue for bonding, it has become a popular material for boat construction and designing outdoor projects like decks and docks.

You can also use it indoors in your bathroom where there is excessive water contact. It is an aesthetic board that also works in the kitchen, although most prefer using it on their porch or outdoor furniture.

By understanding the grades of Marine plywood available, you’ll be able to choose the best one for your specific project.

Frequently Asked Questions About Grades of Marine Plywood

Can You Paint Marine Plywood?

You can always paint your Marine plywood to enhance its look. You can use a quality primer to fill the wood’s pores, then finish off with your preferred paint color. The additional layer will also serve as a sealant to further protect the board.

What Are the Types of Marine Plywood?

Marine plywood comes in five main variants based on quality and special modifications. The A-A is ½ an inch and comes in 5×12 feet or 4×8 feet pieces, while the A-B comes in ¾ inches, and the B-B version is also known as the face-back design. Lastly, the MDO features thermosetting and resin fiber overlays, while the HDO is mainly used for industrial purposes and comes in various thicknesses.

Read More About Grades of Marine Plywood


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