How To Waterproof Plywood For Boat: 4 Steps That Work

Plywood is an affordable boat material. It also comes in marine plywood versions with better moisture and outdoor element resistance. But as much as marine plywood is durable, you should still make it waterproof to enhance its longevity on your boat. In this post, I will discuss how to waterproof plywood for boat so that it will last long on your vessel.

Take note that plywood is prone to wear and tear. If you don’t seal it with a waterproof coating, it will start to absorb moisture and damage many parts of your boat. With the right coating, even the cheapest marine plywood will last for years.

Preparing wood for waterproof coating

Before applying your choice of waterproof coating, it’s important to prepare the plywood first. Here’s what you need to do:

  • Sand the surface. The first step is to sand the plywood to remove any bumps and nibs. This will also allow the coating to be absorbed into the wood for efficient waterproofing. I recommend using 80-grit sandpaper to prevent damaging the material.
  • Clean up. After you’re done sanding the plywood, you should vacuum-clean it to remove dust and dirt. Some woodworkers will pressure-wash the plywood. If you did so, wipe it, then let it dry first.

Once you’re done with these steps, you can now proceed with the methods below.

Steps to waterproof plywood for boat

Step 1: Use a wood filler

Once your plywood is also smooth and sanded, you can apply a wood filler as a primer. This is very helpful for plywood pieces with rough and uneven surfaces. It will smooth out your plywood further for better coating absorption.

Make sure that you know your wood fillers well. Some types can be applied directly into the wood, while others must be mixed with a hardener to adhere to the workpiece properly.

However, you don’t need to perform this step if you purchased premium marine plywood. Most of the time, high-quality plywood is already smooth and ready for coat application.

Step 2: Apply a wood sealant.

Next, it’s time to apply a wood sealant. This sealant will prevent moisture from seeping into the plywood material. A wood sealant is crucial to apply on the edges of the plywood since it has exposed grains where water can enter.

The sealant will prevent the plywood from harboring molds and mildew. It will also increase the lifespan of your marine plywood, especially if you’re often boating.

Aside from moisture resistance, a wood sealant will also provide a UV-resistant benefit to the wood. This is a very crucial benefit as boats are always exposed to harsh sunlight.

Step 3: Apply an epoxy coat.

Once the wood sealant is dry, the next step is to coat the plywood with an epoxy finish. Epoxy is an excellent choice for plywood because it penetrates deep into the grains. Aside from that, epoxy will also enhance the waterproofing benefit of the wood sealant.

Make sure that you spread the epoxy up to the edges of the plywood for a perfect seal. Take note that epoxy coating dries fast, so you should apply it quickly and efficiently.

Also, you can apply multiple epoxy coatings, but you should let each layer dry faster before putting on another.

Consider using transparent epoxy since it’s easier to apply and will require less maintenance in the long run.

Step 4. Finish up with paint

Lastly, you need to apply your choice of paint to match your boat. I swear by spray-on paint since it provides a smoother and even finish without the mess of using a brush.

Always use a generous amount of paint evenly spread throughout the plywood material. The same with epoxy, you should let each coating dry before applying another one.

Added tips in making plywood for boat waterproof

Aside from following the steps above, you can also add these tips to your arsenal for the best waterproofing results.

  • Use high-quality plywood. Aside from using the best coating material, you should also invest in premium marine plywood. Regular plywood will work, but it will not provide the nest durability that you need for such taxing environment boats are exposed to.
  • Consider using latex paint. While it’s not a requirement, latex paint is an excellent choice for marine plywood waterproofing. You can use latex paint in place of spray-on paint. This will enhance the waterproofing characteristics of sealants while adding an aesthetic touch to your boat.
  • Utilize tar paper. For added waterproofing, you can place a layer of tar paper underneath. This will stop the water from seeping through the edges of the wood.
  • Dry the wood first. Before applying a waterproof coating on the plywood, make sure that it’s 100% dry. Wet plywood will not absorb sealants and coatings well. Worse, it will be susceptible to water damage once installed on your boat.
  • Use multiple coats. Since boats are often exposed to high levels of moisture, you should consider putting multiple waterproof coats on the marine plywood. The general rule is that the number of sealant coats should be the same as the number of plies. For example, three-ply plywood requires at least three coatings.

How can you tell if it’s marine plywood?

For the beginner woodworker, it can be challenging to identify marine plywood from a regular one. To check if your material is marine plywood, you should look for the following:

  • Smooth finish
  • Heavier weight than regular plywood
  • Fewer knots on the surface
  • Hardwood veneers
  • Less bending capacity

Aside from these, you should also check the label of the plywood. Trust only legitimate sellers to ensure that you’re getting premium and real marine plywood for your boat.

How long will marine plywood last?

Marine plywood is more durable than typical plywood. It can last between 10 to 25 years, depending on the quality, maintenance level, and exposure to various elements.

If there’s a higher moisture level in the environment, marine plywood will wear out faster. Still, you can stretch its lifespan by waterproofing the plywood before using it on your boat.

As much as marine plywood is extremely resistant to moisture, repetitive and prolonged exposure to it will soon affect the material’s condition.

How do you seal the edges of marine plywood?

When waterproofing plywood for a boat, it’s crucial to seal its edges. The edge of plywood reveals its grains, where moisture will enter and be absorbed by the pressure-treated wood material.

To seal the plywood edges, you need to sand it first to remove any dirt and uneven surfaces. After that, you can pour an epoxy moisture sealer along the edges. Let the first coating dry before applying another for the best moisture protection.

How can you tell if plywood is waterproof?

The Splash Test is the easiest and fastest way to test if your plywood is waterproof. Simply pour a small amount of water into the plywood surface and let it sit for a few minutes. If the wet spot darkens, it means that the water is being absorbed into the material.

However, if the water you poured beads and rolls on the surface, it means that the plywood is waterproof.

You can still apply a sealant and epoxy to this material to ensure that it’s 100% waterproof. This is crucial for plywood being used on boats and other marine vessels.

How do you dry out marine plywood?

Since plywood is made of multiple softwood layers, it should be kept dry at all times. Otherwise, it will start to warp, and the layers will come off.

To fix wet plywood, you can place them in a heated room, with each one separated by blocks. You can also run a dehumidifier to help suck moisture out of the air and the plywood material. Lastly, you can put the plywood under the sun for a short period to remove the excess moisture.

Depending on how soaked the plywood is, there’s a tendency for it to warp and swell. At this point, the material may not be possible to repair.

Is it necessary to seal marine plywood?

Marine plywood is made to be more durable and moisture-resistant. Still, it doesn’t erase the fact that it’s still pressure-treated wood. This means it requires sealing to prevent damages due to moisture and other elements.

Sealing marine plywood will make it less susceptible to rot. A wood sealant is an easy solution here. You can also apply multiple coats, ensuring that you’re brushing it horizontally.

Can you use regular plywood to build a boat?

Regular plywood isn’t a good material choice in building boats. Woodworkers prefer marine plywood since it’s more durable and resistant to outdoor elements. By using this marine plywood, you can avoid too many patches and large voids.

Aside from that, marine plywood has hardwood veneers compared to regular types with softwood veneers.

Conclusion

Knowing how to waterproof plywood for boat will save you from the hassle of wear and tear. It will protect the wood from decay due to the wet conditions boats are exposed to. Just make sure that you do it right for the best results. I hope my quick guide here helps you with your woodworking project!

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