Plywood is made from veneer sheets that are glued together using adhesives. Its layered structure makes it porous and not waterproof. So how do you make plywood waterproof?
You can make plywood waterproof by covering it with either waterproof paint, epoxy treatment, water sealer, or polyurethane varnish. Each of these finishes has good moisture resistance and will waterproof it to protect it from moisture damage and mold.
However, before you can apply any of these finishes, 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 it is exposed to moisture constantly, the water will soak into its core which 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 surface.
Here are some more reasons why to do it:
1. Makes It Last Longer
Nothing damages plywood faster than water. When it 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 a non-waterproof one, especially when used outdoors.
2. Makes It More Weather Friendly
The good thing about waterproofing is that the sealants make it resistant to weather elements. For instance, polyurethane varnish makes the wood more resistant to heat, dust, UV rays, and the like.
3. Improves Its Beauty and Luster
Depending on the sealant you use, you can either get more luster and sheen or you get more color and beauty. If you use polyurethane varnish, the sealant when dry will reveal a glossy appearance.
If you use waterproof paint, its finish will be colorful giving the surface an improved finish. So, when picking the sealant, ensure to go for one that also has a nice finish when dry.
4. 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 board.
However, if its surface is sealed, then mold won’t be able to grow.
Sealants To Use:
Certain sealants do a better job at waterproofing than others. Here are the top 4 sealants that you should use:
1. Epoxy Sealant
Epoxy sealant, also called epoxy resin is a thick and oil-based coat that penetrates the wood pores and makes the surface waterproof from the inside.
To apply it, you need to first mix the content in equal parts. Then pour the epoxy on the surface 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 it. As an added advantage, it also protects the surface from heat. Some epoxy resins have UV blockers, too.
- It protects surfaces from high moisture levels and rainfall.
- You can go for a natural wood look with transparent epoxy.
- Makes the surface weather-resistant in addition to being waterproof.
- It gives a colorful finish as epoxy also comes in tinted colors.
- Since it needs to be poured or wiped on, epoxy is difficult to apply on vertical surfaces.
- It takes a long time to dry.
2. Water Sealer
You can also seal plywood with a water sealer. They are usually transparent coats made from synthetic resins, solvents, plasticizers, and additives. These sealers can be sprayed, brushed, or wiped on the surface 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. We recommend using Thompson’s water sealer.
- They can be used anywhere (indoors or outdoors).
- They are easy to apply and you can choose from 3 applicator methods.
- Fast dry time.
- It’s difficult to get a dark color or look with a water sealer.
3. Waterproof Paint
Waterproof paints mostly come in spray cans which means you need to spray them directly on the surface. The best part about them is that they come in a variety of colors and finishes. You can get any color and any type of finish.
We recommend using liquid latex for this.
- You get to choose from different colors and finishes.
- Fast dry time.
- They are easy to apply.
- Won’t give you a natural wood finish because of the color pigments in its formula.
4. Polyurethane Varnish
Polyurethane varnish contains essential plasticizers, resins, and solvents that all repel moisture. Its main strength 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 surface.
It protects the surface from dust, UV rays, and heat. You can also tint the polyurethane with wood dye or acrylic paint to give it some color.
- It protects plywood against moisture and rainfall.
- Prevents mold growth.
- Exterior polyurethane is heat and UV-resistant.
- Easy to apply.
- Long dry and cure time.
- No color (transparent finish).
Sealants To Avoid:
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?
Waterproofing plywood isn’t a hard process since its surface is porous and accepts sealants easily. However, you need the right guidance and tools.
Here are the tools you need:
- A drop sheet
- Masking tapes
- Paintbrushes or a paint sprayer
- Wood filler
- A plastic putty knife
- A pair of work gloves
- Breathing protection or a face mask
- Clean rags
- A portable vacuum or duster
1. Using Polyurethane
Polyurethane works for indoor and outdoor plywood surfaces. Use exterior polyurethane for outdoor surfaces, and regular (water-based) polyurethane for indoor surfaces.
Here’s how to use it:
- Prepare your work area by removing items and furniture on and around the surface.
- Fill holes in the plywood with wood filler.
- Clean the 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.
- Leave the plywood to dry and sand it down with 180-grit sandpaper
- Remove dust and sand again with 220-grit sandpaper.
- Vacuum the surface and use masking tape to cover the edges.
- Apply an undercoat or paint if needed and allow it to dry (you can skip this step if you want to apply the polyurethane directly)
- Cover the plywood surface with 3 coats of polyurethane. Wait until one coat dries before applying the next one.
- Cover the edges and bottom of the surface with equal parts of polyurethane. This ensures the entire plywood is waterproof and not just the surface.
- Allow the sealant 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. Using Epoxy Sealant
You can use epoxy sealant for indoor and outdoor surfaces. Since epoxy is thick, you can only wipe it on wood or use a roller or bristled paintbrush to apply it. So, you can’t spray it.
Here’s how to use it:
- Prep the surface by removing all objects.
- Repair the plywood by filling holes and gouges with wood filler.
- Sand down the surface and edges.
- Apply primer or paint.
- Apply the first coat of epoxy sealant using a paintbrush, cloth, or roller.
- Leave the first coat to dry for 2 hours.
- Scuff the first coat with 400-grit sandpaper.
- Apply two more coats and allow the epoxy sealant to dry fully.
Tip: Epoxy is very thick so you should only apply light coats and you must leave at least 2 hours between each coat so the sealer can soak in the plywood.
3. Using Water Sealer
You can use Thompson’s water sealer on the plywood but if you don’t have it, you can also use waterproof paint. The same application method works for both products.
Here’s how to use it:
- Wipe down the surface with mineral spirits to ensure it’s clean.
- Sand the surface with 220-grit sandpaper and remove dust.
- 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.
- Use a paintbrush or paint sprayer to apply the first coat of water sealer and leave it to dry.
- 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.
- Test the finish when dry to know if you need another coat.
You can waterproof all types of interior plywood boards as they are designed for looks and not durability or moisture resistance. So, you can waterproof them 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 Waterproof Plywoods?
Exterior plywood is designed with durability and moisture resistance in mind. Unlike interior plywood that shows designs and beauty, the exterior type reveals blemishes and knots but the board is reinforced with waterproof adhesives and thicker boards. This makes it much more resistant to water damage.
The most common types of exterior plywood are CDX-grade, Pressure-treated grade, and some Marine-grade plywood. They surfaces are designed to thrive when exposed to moisture, but you can still waterproof them to improve their moisture resistance.
In summary, you can waterproof plywood by using a sealant, polyurethane varnish, or waterproof paint. Any one of these finishes is guaranteed to waterproof it, but the best option is down to you and the type of surface you are working on.
For exterior surfaces, use a sealer coat that has weather resistance and UV blockers.
Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,