How Do You Seal Marine Plywood – 3 Easy Steps

how to seal marine plywood

Because of its durability and resistance to the elements, marine plywood enjoys a strong reputation.

However, because this type of plywood has not been subject to chemical treatment, it can over time become damaged by precipitation and ultraviolet rays from the sun.

So, the question becomes whether you should take the step to seal the plywood.

The answer is yes. You will need to seal the plywood despite the fact that it is strong, durable, and long-lasting.

This is because without being sealed, the moisture will get inside and do damage.

To protect its durability, you will need to seal the plywood to maintain its strength and color. Plus, it will prevent rot from setting in.

Steps to Sealing Marine Plywood

Sealing your plywood is important if you are using it for constructing something that will need to be submerged in water.

To get started, you will need the following products.

  • Acetone, Sanding Block, and Paper Towels
  • Foam Roller, Sandpaper, and Paint Basin
  • Foam Brush, Clean Cloth or Rag, and Sealer

Step One: Clean & Sand

Put a little acetone on the cloth and clean the plywood. The acetone will remove the grease and unwanted stains from the surface.

Once you have cleaned the plywood, let it dry for one to two hours.

Next, use fine-grit sandpaper and sand the entire surface along with the edges of the plywood.  

Good surface preparation is key to getting a smooth finish on the plywood, so do it carefully.

If you have a sander, you can use that to make the process go faster. Be sure to sweep or vacuum up the sawdust from the wood once completed.

Step Two: Apply the Sealant

You will be using an epoxy resin moisture sealer for this step along with the paint basin.

Dip the foam roller into the paint basin which contains the sealer. Then brush the roller against the sides of the basin to remove any excess sealer.

Next, roll the sealer onto small sections of the plywood, such as 2’ x 2’. This will make sure the sealer does not dry too quickly.

Go one section at a time horizontally until all the plywood is covered with a thin coat.

If you want to avoid having brush marks on the plywood, drag a foam-tipped brush over the sealer before it dries.

Step Three: Let It Dry & Reapply

It takes from one to two hours for the sealant to fully dry.

Once it does, add a second coat which you can apply vertically to the plywood.

Let that dry and if you feel that a third or four coats are necessary, then you can keep applying them as long as you want.

Generally speaking, four coats of sealer will be enough to protect the plywood beneath the water surface – if you are using it underwater.

Also, remember to seal and treat the plywood from both sides to get good long-lasting results.

Should you seal the plywood for interior use?

Sealing the plywood isn’t essential if it’s used in dry rooms like bedrooms and dining rooms.

But if you are using it in areas like bathrooms and kitchens, it’s better to seal and waterproof the plywood.

painting and sealing marine plywood

What Is Marine-Grade Plywood?

This is a special type of plywood that uses Western Larch or Douglas fir.

In simple terms, it’s a specific variety of plywood that’s been pre-treated to withstand moisture.

The term marine plywood or marine-grade plywood does trick many people into believing that it is waterproof.

And while it does use waterproof glue to hold the particles together, it inst completely waterproof. Also, it lacks the chemical treatment that would have the plywood resist rot and decay over time.

There are four grades of marine plywood with the higher grades being quite strong with no defects and lightweight for easy handling.

This is why you see marine plywood used in so many different projects, especially the construction of boats and doors.

Also, for long-term projects such as lining the interiors of boats, boat floors, doors for coastline homes, and so forth.

Being naturally strong also makes this type of plywood an excellent choice for use in changing conditions such as when exposed to winds or the elements.

This is why you often see marine plywood is also used for temporary projects such as sealing windows when strong storms or hurricanes are expected.

But if you want it to last, you will need to seal it from the elements. 

how to seal marine plywood for exterior use

How Long Does a Marine Grade Plywood Last?

Marine plywood is quite strong and durable, especially when it is sealed from moisture and other elements.

Depending on the overall quality and how the plywood is used, you can expect it to last from ten to twenty-five years.

However, keep in mind that knowing what factors will apply and how you use the plywood will affect how long it lasts, so you will need to educate yourself on this subject.

  • Quality
  • Exposure to Moisture
  • If a Sealant or Chemical Treatment is Used

This will help you decide the grade of marine plywood that is best for the job.

This may mean that a lower quality marine plywood might work as well as a higher quality one depending on the circumstances in which it is used.

In other words, you may be able to save a considerable amount of money by purchasing the right grade of plywood for your project.


Marine plywood comes in four grades, A, B, C, and D. The higher the grade, the stronger and longer-lasting the plywood.

Depending on its use, A D-grade marine plywood may be just as suitable as an A-grade.

Of course, the supplier plays a role in the quality of the marine plywood as well, so be sure to get your plywood from a trusted source.

And inspect the plywood for any damage such as cracks or large imperfections so you do not purchase them.

Moisture exposure:

Although UV rays can take their toll, it is the exposure to moisture that is the biggest single factor in the grade of marine plywood that you choose.

Marine plywood itself is strong and durable with a reputation for lasting a long time even when exposed to moisture.

However, if you want the plywood to last for decades, then it will need to be sealed depending on the environment.

The greater the amount of moisture, the more likely the plywood is to break down over time.

This means that plywood used on a boat is going to break down faster compared to plywood used inside your home.

You can expect marine plywood used on a boat dock to last up to twenty years with the proper care.

While marine plywood used for interior flooring may last thirty years or longer depending on the care you provide.

If you are not sure of the moisture levels, then you can use a moisture meter to determine how much is present.

This will help you make the best-informed decision about which grade of marine plywood to use.

Chemical treatments:

Even lesser quality plywood can hold up to high moisture levels if it is properly sealed or chemically treated.

Most plywood types do not have the strength or quality, to hold up long in a moist environment without warping or rotting relatively quickly.

Marine plywood is different which is why you often see it used on boats and along the shoreline.

But it is not impervious to moisture. You will need to use a good wood sealer if you want it to last in such environments.

Regardless, marine plywood is an excellent choice for moist environments.

And an even better choice for dry environments where you want the plywood to last for a long time.

how to paint marine plywood

Can You Paint Marine Plywood – Rather than Sealing?

Whether it’s for home exteriors or for marine purposes, marine plywood should be sealed in order to be fully durable.

However, if sealing with epoxy seems to be tough, you can simply paint your marine plywood properly to enjoy similar benefits.

Here are some know-hows you need to check…

  • Before applying any paint, sand the surface to remove any imperfections in the wood.
  • Prime the surface with a thin coat of latex-based primer. Allow it to dry and sand the surface lightly again.
  • Now using semi-gloss latex-based paint cover the marine plywood surface evenly with a paint roller or a brush.

For achieving the right finish and protection applying at least two thin coats of paint is recommended.

An additional third coat can be applied for greater protection if you desire.

Sanding between paint coats is not essential, but you should allow enough time to dry the surface between two successive coats.

The Bottom Line

Marine plywood has earned a solid reputation for its durability and longevity in moisture-rich environments along the coast.

This is why it is often the first choice of plywood for many different projects where moisture is present.

It is also lightweight which makes it easy to handle.

But even with its natural resistance to moisture, marine plywood will still need to be sealed.

So, if you are using it for a long-term project in a moisture-rich environment, be sure to take the time and properly seal it so that it will last.

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