How to Waterproof Plywood? (Explained!)

Plywood is made from veneer sheets that are glued together using adhesives. The layered structure of plywood makes it porous and not waterproof. So how do you make plywood waterproof?

You can make plywood waterproof by covering the plywood surface with either a waterproof paint, epoxy treatment, water sealer, or polyurethane varnish.

Each of these finishes has good moisture resistance and will waterproof the plywood to protect it from moisture damage and mold.

However, before you can apply any of these finishes on the plywood, the plywood must have been prepped using the right tools and materials. Here’s a guide to that.

Why It’s Important to Waterproof Plywood?

There are many benefits to waterproofing plywood. The main benefit is that it protects against water damage. Plywood due to its synthetic makeup and layered structure consists of veneer sheets that don’t do well when exposed to water.

If the plywood is exposed to moisture constantly, the water will soak into the plywood core where it will cause the sheets to swell. The presence of water between the sheets will also weaken the adhesives in the boards causing the sheets to become delaminated.

Eventually, the plywood will become waterlogged and will start to rot. You may also notice white or dark water marks on the plywood surface.

Here are other important reasons to waterproof a plywood surface:

1. Makes The Plywood Last Longer

Nothing damages plywood faster than water. When your plywood is constantly exposed to water, it will get damaged but waterproofing the board is a way to make it last longer.

Experts estimate that waterproof plywood lasts 3 times as long as regular plywood, especially when used outdoors.

2. Makes Exterior Plywood More Weather Friendly

The good thing about waterproofing plywood is that the sealants that work don’t only make the plywood waterproof but they also make the plywood resistant to the elements.

For instance, polyurethane varnish makes the plywood waterproof but it also makes the wood more resistant to heat, dust, UV rays, and the like.

So, asides from being waterproof, the plywood will also be weather-friendly when it is covered with a waterproof sealer.

3. Improves Its Beauty and Luster

Depending on the waterproof option that you go for, you can either get more luster and sheen or you get more color and beauty.

If you use polyurethane varnish to waterproof the plywood, the waterproof coating when dry will reveal a glossy appearance.

If you use waterproof paint, the waterproof coating when dry will be colorful giving the plywood an improved finish. So, when picking your waterproofing choice, ensure to go for one that also has a nice finish when dry.

4. Waterproofing Plywood Prevents Mold

Like all wooden surfaces, plywood can also develop mold when it is exposed to water. The mold growth is facilitated by the presence of water, contaminants, dust, and air around the plywood board.

However, if there is a waterproof coating on the plywood, then you don’t have to worry about mold or fungi growth on the wood.

Now that you know the importance of waterproofing plywood, let’s check out products that you can use to waterproof plywood.

Products You Can Use To Waterproof or Seal Plywood?

Products You Can Use To Waterproof or Seal Plywood?

While waterproofing plywood is a good idea, certain products do a better job at waterproofing the plywood than others. Here are the top 4 products to use to waterproof a plywood surface:

1. Epoxy Sealant

Epoxy sealant, also called epoxy resin is a thick and oil-based coat that penetrates the wood pores and makes the plywood waterproof from the inside. To apply the epoxy sealant, you need to first mix the content in equal parts. Then pour the epoxy on the plywood or wipe it on with a cloth.

The epoxy resin binds together with the wood and seals the pores so it becomes impossible for water to penetrate the plywood. As an added advantage, epoxy sealant also protects the plywood from heat. Some epoxy resins have UV blockers that protect the plywood from UV deterioration and discoloration.

Pros of Epoxy Sealant

  1. An epoxy sealant protects the plywood from high moisture levels and rainfall.
  2. You can go for a natural wood look with transparent epoxy.
  3. Epoxy makes the plywood weather-resistant in addition to being waterproof.
  4. You can get a colorful plywood finish as epoxy also comes in tinted colors.

Cons of Epoxy Sealant

  1. Since it needs to be poured or wiped on, epoxy is difficult to apply on vertical plywood surfaces like fences and walls.
  2. Epoxy sealant takes a long time to dry.

2. Water Sealer

You can also seal plywood with a water sealer. Water sealers are usually transparent coats made from synthetic resins, solvents, plasticizers, and additives.

These sealers can be sprayed, brushed, or wiped on the plywood and they dry quickly. Some water sealers penetrate the plywood board to make it waterproof from the inside while others stay on the top layer and form a waterproof film. The best water sealer to waterproof plywood is Thompson’s water sealer.

Pros of Water Sealers

  1. Water sealers like Thompson water seal can be used anywhere (indoors or outdoors).
  2. Water sealers are easy to apply and you can choose from 3 applicator methods.
  3. Water sealers dry faster than other waterproof sealers.

Cons of Water Sealers

  1. It’s difficult to get a dark color or look with a water sealer.

3. Waterproof Paint

Another way to waterproof plywood is to apply waterproof or exterior paint on the plywood. Waterproof paints mostly come in spray cans which means you need to spray them directly on the plywood.

The best part about waterproof paints is that they come in a variety of colors and finishes. You can get any color and any type of finish with waterproof paint. Liquid latex is the best waterproof paint option for plywood.

Pros of Waterproof paint

  1. You get to choose from different colors and finishes.
  2. Waterproof paints dry quickly, usually in less than 2 hours.
  3. Waterproof paints are easy to apply.

Cons of Waterproof Paint

  1. Some waterproof paints can be expensive due to the additives and special formula of the paint.
  2. Waterproof paint can’t give you a natural wood finish because of the color pigments in its formula.

4. Polyurethane Varnish

Polyurethane varnish works great to protect plywood from water. Polyurethane varnish contains essential plasticizers, resins, and solvents that all repel moisture.

The main strength of polyurethane is the presence of urethane compounds in its formula. Urethane compounds are liquid plastic substances that when dry form an ultra-thick and waterproof film on the plywood.

Exterior polyurethane protects the plywood from dust, UV rays, and heat. You can also tint the polyurethane paint with wood dye or acrylic paint to give it some color.

Pros of polyurethane

  1. Polyurethane protects plywood against moisture and rainfall.
  2. Polyurethane prevents mold growth.
  3. Exterior polyurethane is heat and UV resistant.
  4. Polyurethane is easy to apply.

Cons of Polyurethane

  1. Polyurethane takes several hours to fully cure and can take up to 3 days.
  2. Polyurethane doesn’t have colors and you have to tint the coat if you want a colorful finish on the plywood..

Next, let’s check out some non-recommended products to make plywood waterproof.

Non-Recommended Waterproofing Methods For Plywood

Though waterproofing your plywood is a great way to protect it from water damage, there are some waterproof methods and products that aren’t recommended for use on plywood. Here are some of them:

1. Rust-Oleum Spray Paint

Rust-oleum spray paint is a waterproof and rust protective coat designed for use on metal and not wood. Though the finish does a great job at repelling moisture, it won’t be a good fit on plywood because it’s not designed for wood.

2. Wood Oils

Wood oils are waterproof and durable finishes that also protect the plywood from water. They include Tung oil, Linseed oil, Danish oil, and the likes.

These finishes are not great on plywood because they are more suited to natural wood like Oak and Maple than faux wood like plywood.

Applying wood oil on plywood won’t get the perfect finish because the veneer sheets and glue in the plywood will not accept the oil as natural wood will.

3. Sanding Sealers and Shellac Primers

Sanding sealers are applied and sanded on the wood to fill the wood pores and prevent water from penetrating. Shellac primers are also used for the same purpose.

However, both products are preparatory coats used before applying a top coat. So, you shouldn’t use these products to waterproof your plywood because they won’t offer much protection without a top coat.

How to Waterproof Plywood?

Here is an expert guide on how to waterproof plywood and what products to use. But before we dive in, you’ll need the following tools and supplies:

  • A drop sheet
  • Masking tapes
  • Polyurethane
  • Paintbrushes or a paint sprayer
  • Wood filler
  • A plastic putty knife
  • A pair of work gloves
  • Breathing protection or a face mask
  • Sandpaper
  • Clean rags
  • A portable vacuum or duster

When you have gathered all the needed tools, you can now waterproof the plywood by using any of these 3 methods.

1. Use Polyurethane (For Interior and Exterior Plywood)

Use Polyurethane (For Interior and Exterior Plywood)

The best method to waterproof plywood is to use polyurethane, this works for both interior and exterior plywood surfaces. You should use exterior polyurethane for outdoor plywood and regular polyurethane for indoor plywood.

Here is how to waterproof plywood with polyurethane varnish:

  1. Prepare your work area by removing items and furniture on and around the plywood surface. Then fill holes in the plywood with wood filler.
  2. Put on a pair of gloves and a face mask for protection.
  3. Wipe down the plywood surface with a lint-free rag and mineral spirits to remove grease and oily scum – if it’s a new plywood structure, you can skip this step.
  4. Leave the plywood to dry and sand down with 180-grit sandpaper
  5. Remove dust and sand again with 220-grit sandpaper.
  6. Vacuum the plywood surface and use masking tape to cover the plywood edges.
  7. Apply an undercoat or paint if needed and allow to dry (you can skip this step if you want to apply the polyurethane directly on the plywood).
  8. Cover the plywood surface with 3 coats of polyurethane. Wait at least 4 hours between coats of water-based polyurethane and 10 hours for oil-based polyurethane.
  9. Cover the edges and bottom of the plywood with equal parts of polyurethane. This ensures the entire plywood is waterproof and not just the surface.
  10. Allow the polyurethane to dry for 3 days before normal usage is resumed.

Tip: Polyurethane only gives a transparent finish so if you want a color finish, ensure to mix the polyurethane with wood dye or acrylic paint before application.

2. Use Epoxy Sealant (For Indoor and Exterior Plywood)

Use Epoxy Sealant (For Indoor and Exterior Plywood)

Epoxy sealant can be used on indoor and outdoor plywood and it also protects against water damage. Since epoxy is very thick, you can only wipe it on wood, use a roller, or use a bristled paintbrush.

Here is a guide for applying an epoxy sealant on plywood:

  1. Prep the plywood surface by removing all objects.
  2. Repair the plywood by filling holes and gouges with wood filler.
  3. Sand down the plywood surface and edges.
  4. Apply primer paint on the plywood.
  5. Apply the first coat of epoxy sealant using a paintbrush, cloth, or roller.
  6. Leave the first coat to dry for 2 hours.
  7. Scuff the first coat with 400-grit sandpaper.
  8. Apply two more coats and allow the epoxy sealant to dry fully.

Tip: Epoxy is very thick so you should only apply light coats on the plywood and leave at least 2 hours between coats so the sealer can soak in the plywood.

3. Apply Water Sealer

Apply Water Sealer

This is the third method and it works well. You can use Thompson water sealer on the plywood but if you don’t have the Thompson water sealer, you can use waterproof paint. The same application method works for both products.

Here is how to waterproof plywood with water sealer or waterproof paint:

  1. Wipe down the plywood with mineral spirits to ensure it’s clean.
  2. Sand the plywood with 220-grit sandpaper and remove dust.
  3. Prep the water sealer – usually, you need to shake the water sealer container to prep it. For waterproof paint, you’ll need to shake or stir the content depending on the type you have.
  4. Use a paintbrush or paint sprayer to apply the first coat of water sealer and leave it to dry.
  5. When the first coat is dry, apply the second coat and leave it to dry. Usually, you only need two coats of water sealer but for waterproof paints, you can apply up to 3 coats.
  6. Test the finish when dry to know if you need another coat.

What Types of Plywood Can You Waterproof?

You can waterproof all types of interior plywood boards. Interior plywood boards are designed for looks and not durability or moisture resistance so these types of plywood can be waterproofed using any of the methods explained above. The most common types of plywood for interior designs are BWR/BWP ply, Marine ply, and MR-grade plywood.

Are There Types of Plywood That Are Waterproof?

Exterior plywood is designed with durability and moisture resistance in mind. Unlike interior plywood that shows designs and beauty, exterior plywood reveals blemishes and knots but the board is reinforced with waterproof adhesives and thicker boards. This makes the plywood much more resistant to water damage.

The most common types of exterior plywood are CDX-grade, Pressure-treated grade, and some Marine-grade plywood. These plywood surfaces are designed to thrive when exposed to moisture, but you can still waterproof them to improve their moisture resistance.

Final Words

In summary, you can waterproof plywood by using a sealant, polyurethane varnish, or waterproof paint. Any one of these finishes is guaranteed to waterproof your plywood board but the best option is down to you and the type of surface you are working on.

Remember to always prep the plywood before applying a sealer coat. For exterior surfaces, use a sealer coat that has weather resistance and UV blockers.

Finally, ensure that the product you pick is recommended for synthetic wood like plywood. If you use the wrong product, you’ll end up with the wrong finish.

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