How to Waterproof Plywood: Polyurethane vs. Epoxy

Certain types of untreated plywood used, like Marine Grade and ACX plywood, are water resistant and the best choices for exterior use. However, they are not 100% watertight and should still be treated before their use externally.

Likewise, any other water-resistant, untreated exterior plywood product, including OSB, can be made waterproof with two of the most popular sealants: polyurethane and epoxy coating.

Keep reading to learn how to make waterproofing plywood sheets on plywood and the differences between using polyurethane vs. epoxy for waterproofing untreated plywood.

How Do You Keep Plywood From Rotting?

Plywood boards and sheets are not naturally waterproof, but some types of plywood sheets and plywood boards are more water-resistant than others.

Marine-grade plywood offers the highest level of moisture resistance. This is partially because of the way it’s manufactured but also because of the way it’s sealed after construction.

Firstly, marine-grade plywood is built without splits or knotholes, which prevent water-based paint’s natural moisture from getting trapped between plywood pores, and the core layers protect the plywood. Secondly, it’s treated to withstand high moisture environments. This makes it perfect for exterior use or in the construction of boats.

Using Marine Grade plywood is the best option to keep untreated plywood for outdoor use and marine-use plywood from rotting.

However, you can seal any plywood to make it waterproof. The best way to get plywood structures to do this is to apply an epoxy or polyurethane coating over finished plywood.

How Do You Weather Seal Plywood?

Most plywood sheets, like cabinets, wall panels, subfloors, and wall sheathing, are designed for interior use. However, you can use plywood outside for shed construction, external window frames, and exterior wall and roof sheathing. The best type of plywood to use outdoor plywood outside is Marine Grade plywood, but you can also use OSB or ACX plywood outdoors.

When using any plywood structure outdoors, it’s important to seal it properly to protect it from wind, rain, and sun.

Shellac-finished plywood is particularly vulnerable to high sunlight exposure, so this plywood structure should be adequately sealed to keep moisture damage and avoid fading or bubbling of the surface finish.

The best way to weather seal plywood is to apply a polyurethane or epoxy finish to seal plywood edges to the wood. Epoxy is often better and more durable than polyurethane because it’s thicker, but only if it’s applied correctly.

The mixture of resin flakes to denatured alcohol needs to be perfect, and you must apply the epoxy properly to the plywood edges to prevent imperfections.

A polyurethane finish differs from a lacquer or varnish despite often being referred to as a type of varnish. You can use lacquer or oil-based varnish to waterproof plywood, but it isn’t the best option for exterior use.

Polyurethane involves a similar application method but offers better protection against the elements without disrupting the natural grain finish. 

What is the Cheapest Way to Waterproof Plywood?

You have several options to affordably waterproof plywood for outdoor use. You can purchase waterproof plywood (Marine or Mr. Grade plywood).

This MR-grade waterproof plywood offers the most protection against moisture because it’s constructed without knots that let water seep into the core curved plywood.

Painted, plywood structures designed for interior use in high-moisture areas (like kitchens or bathrooms) can be finished with an oil-based varnish wood stain or lacquer to be moisture-resistant and waterproof.

External plywood projects are better finished with polyurethane or epoxy, which are more weather-resistant.

The cheapest way to waterproof plywood depends on whether you’re painting the wood and intend to keep it inside or outside. Generally, Marine Grade plywood waterproofing can cost between $26 for a 1/8” sheet and $112 for a ¾” sheet. It’s best to use this type of marine plywood waterproofing method on plywood sheets for exterior construction.

If you’re painting plywood and need it to be waterproof for use in a bathroom, it’s best to seal it with polyurethane or epoxy.

Epoxy sealant costs between $50-120 per liter, and polyurethane sealant is around $50 per tub. The cheapest option depends on how much plywood you need.

Purchasing low-grade plywood and applying polyurethane is the cheapest way to seal plywood for outdoor use, but it may require more maintenance long term.

Investing in Marine Grade plywood and coating it with epoxy or polyurethane will offer maximum protection against all weather and last the longest. However, installing seal plywood for outdoor use will likely cost more overall.

The Best Thing to Seal Plywood With

Plywood can be water-resistant, but it isn’t watertight, so you should seal it after painting and staining, or before using it for exterior construction.

Some water-resistant plywood, like Marine Grade, can handle deep-water penetration and has a higher water resistance than other types of protected plywood. But it should still be sealed to protect it from the elements, especially after painting.

You have two main options for sealing plywood to make it watertight. You can either apply an epoxy resin or paint/spray-on polyurethane.

Epoxy is more durable than polyurethane, but it is harder to apply. It’s also slightly more expensive. Polyurethane is long-lasting, and waterproof glue offers better protection against abrasion and chemicals but will need reapplication every 5-10 years.

How to Waterproof Plywood

The two most effective ways to put waterproofing material, a plywood or wooden structure, are to apply polyurethane or epoxy. Both finishes offer protection against dampness and moisture and can be used for interior (like bathrooms) or exterior projects (like sheds and outhouses).

Sealing plywood with epoxy is best if:

• You have some experience applying epoxy
• You want to keep the natural grain
• You’re happy with a glossy, plastic-like finish
• You want to minimize routine maintenance
• You want it to last between 20-30 years

Whereas polyurethane (which can be applied as a paint or as a spray-on layer drying oil) is best used if:

• You require an inexpensive solution
• You need the sealant to be a quick and easy application
• You are happy to reapply the sealant every 5-10 years
• You need the finish to withstand abrasive foot traffic
• You need the surface to be protected against chemical exposure

Apply polyurethane

Polyurethane is an extremely thin varnish you can apply to plywood to make it waterproof. You can apply it over the top of a stained or painted surface with ease. Generally, plywood only requires 1-2 coats of polyurethane for maximum protection against all weather. Polyurethane is a quick, cheap solution to waterproofing plywood and can last between 5-10 years.    

Two types of polyurethane exist: water-based and oil-based polyurethane. Using an oil-based sealant is better than a water-based solution.

Oil-based polyurethane lasts longer, requires fewer coats, and enhances the color of the natural wood much better than water-based products.

You can apply paint polyurethane or a spray-on product. There is virtually no difference between painting or spraying polyurethane on plywood in terms of the finish provided. However, spraying is quicker, easier, and can offer a smoother, stroke-free finish compared to paint. 

Coat with epoxy

Epoxy sealer is strong, thick, and highly weatherproof. It’s mainly used on garage floors, as a resin tabletop, or as a type of glue. Epoxy can be used as a finish to plywood products, like wooden tables, cabinets, benches, or chairs.

Applying epoxy sealant as a waterproof coating to plywood requires some skill. Epoxy sealer is thick and fast drying, so you need to work quickly to ensure it coats the surface of the plywood evenly.

To get waterproof paint on plywood, you should only apply between 2 and 4 coats of epoxy. Never apply more than four coats of epoxy resin, as this can cause the painted/stained finish to fade.

When applying epoxy outdoors, it’s a good idea to cover it with a clear or colored UV-resistant topcoat, like liquid resin, to protect it from the sun. This will minimize long-term maintenance and ensure your finish lasts longer.

Polyurethane vs. Epoxy: Which is Better?

	Polyurethane	Epoxy
Which is More Affordable?	Slightly cheaper	Slightly more expensive
Can it Be Used for Exterior or Interior?	Both	Both
Which Lasts Longer?	It can last between 10-20 years	Long-lastingKeeps a grain finish that can be reapplied without removing the previous layer
Pros	It can last between 5-10 years	Needs to be removed entirely before reapplication in future offers some protection against UV rays
Cons	Requires some skill to apply needs 2-4 coatsIs not naturally UV resistant	Needs to be removed entirely before reapplication in the future offers some protection against UV rays

How to Waterproof Plywood: A Step-by-Step Guide

What you’ll need:

• Wood putty (stainable wood filler is best if you plan on staining after)
• Medium grit sandpaper
• Paint/stain (optional)
• Polyurethane or epoxy finish

1.    Fill holes

o waterproof plywood, you must ensure any holes and imperfections are sealed. Otherwise, water will penetrate the holes and damage the core veneers. This can cause the plywood to bode, bend, and warp over time.

Use a putty knife and apply wood filler evenly to fill the holes.

Apply more than you need so the surface is slightly raised. Wood putty shrinks as it dries, so it’s best to apply slightly more than you need and sand it away once dry. You can skip this step if you’ve purchased Marine Grade plywood, as these sheets will have minimal or no knot holes.

2.  Sand

Once the wood filler has dried (which should take 30 minutes to 2 hours at room temperature), lightly sand the area until it’s smooth. Once smooth, lightly sand the rest of the plywood. This will create a slightly rough surface that allows the paint or stain to adhere to the wood easily. It’ll also prevent chips from occurring after several coats of finish. You can use sandpaper or an orbital sander at a slow speed with 180-grit sandpaper.

3.  Apply paint or stain

Next, clean the surface of any dust, dirt, or sawdust using a slightly damp microfibre cloth. Then, lightly sanding between coats, apply your chosen paint or wood stain using a paintbrush or paint roller. Painting plywood should require 2-3 coats (plus primer), while staining can be done in 1-2 coats, depending on how dark you want the stain to appear. Remember, when staining, the longer you leave the stain to set, the darker the color will be.

4. Finish with waterproof sealant

Once you’re happy with the color and finish of your plywood product, you can seal it with your choice of waterproof sealant (epoxy or polyurethane). Before application, lightly sand the edges and surface with low-grit sandpaper.

Applying epoxy requires working quickly after mixing the resin flakes with denatured alcohol. Pour the mixture onto the surface and use a paintbrush to spread it evenly. When brushing, go in the direction of the grain.

Once dry (which should take 3-5 hours), you should only apply a further 1-2 coats to create a protective layer suitable for outdoor use.

To apply polyurethane, use a paintbrush with a thin coat of the sealant in long strokes. Be careful to avoid brush marks as best as possible. Before applying a second coat, ensure the surface has dried completely (which can take between 2-3 hours).

You can sand lightly between coats to remove brush marks or dilute the polyurethane with mineral spirits on the last coat to ensure a smooth final finish. Don’t apply more than two coats in 24 hours, as this can make the coating thick, tacky, and blotchy.

Summary: Things to Remember

• Some paints claim water resistance, but they don’t offer maximum protection against prolonged moisture, so you should still seal the finish with epoxy or polyurethane to make the plywood waterproof
• Epoxy lasts longer than polyurethane
• Polyurethane is easier and quicker to apply than epoxy
• Preparation is critical before applying either product to plywood
• Permanently seal plywood edges carefully, especially once cut, to ensure the entire plywood sheet is protected from moisture