Plywood is a great material to work with for many projects, including making boxes. The end grain of the plywood can be used to accentuate design features in the box. But if there is a risk of the box getting wet or used in a damp environment such as a bathroom, how would you waterproof a plywood box?
A plywood box can be waterproofed by applying a waterproof finish to the surface of the plywood. The finish can be an epoxy sealer, penetrating hard oil, latex, paint, or varnish. Special attention should be given to the ends where the voids must be filled before applying the waterproof finish.
Plywood can make a durable, cost-effective, and decorative medium for making boxes of any shape or form. Plywood is often more adversely affected by moisture than solid timber, but it can be as good or better than any natural timber product with the proper treatment.
Ways To Waterproof A Plywood Box
Plywood is a useful medium to use on many projects due to its strength and relatively low cost. However, plywood is notorious for not handling moisture and damp conditions very well.
The layers of the wood that make up the plywood can act as a wicking agent that will absorb moisture into the wood, especially from the end grain of the plywood.
Boxes made from plywood can be lightweight, robust, and decoratively beautiful, but how do you protect your plywood box from damage due to moisture? The answer is to waterproof the plywood to ensure protection in damp conditions and increase the longevity of the box.
There are several ways to waterproof plywood, and we will discuss each of these methods to give you an overview of each. You can then select the method that will work best for your project and the environment for which you designed the plywood box.
Can Plywood Be Waterproofed Effectively: Choose Your Plywood Carefully?
Plywood is manufactured for various purposes, resulting in plywood of different quality available for purchase from your local supplier.
The adhesive used to manufacture plywood adds some benefit to the moisture resistance of the product since the adhesive is moisture resistant. However, the quality of the plywood will affect the waterproof quality of the product the most.
Cheaper plywood has more gaps and missing sections in the ply than higher grade plywood. Consequently, if you intend to use plywood for a project where the box you build will be exposed to moisture, you should get the highest quality plywood you can for the project. This will limit the risk of moisture absorption and retention in the plywood.
Marine-grade plywood is a good choice when selecting plywood for a project that could potentially be exposed to water or used in a humid environment.
Another reason to waterproof plywood is that the wood is not immune to rot, and the type of wood used in the plywood may be prone to the effects of moisture more than a solid piece of hardwood. Does that mean you should not use plywood for your project? Certainly not, but it does mean that you need to take some extra precautions to protect the plywood from moisture absorption.
Plywood can be waterproofed relatively easily, which increases its tolerance to moisture, and if the right product and methods are used, render the plywood waterproof. This can enhance the beauty of your final product and extend its life in a damp environment.
Seal Your Plywood Edges First
If you are constructing a box out of plywood and need to waterproof the box, waterproofing must start before assembling the box.
The area where plywood is most susceptible to moisture penetration is the end grain of the board. This includes if the end grain is exposed or the end grain is hidden in a joint. Water can penetrate a joint, access the end grain, and in a short time, the plywood may swell and begin to delaminate or rot.
The solution to prevent this problem is to seal the end grain of the plywood before you begin the assembly of the box.
Cut all your pieces to size and do a test fit to ensure that your pieces all line up, edges are straight, and there are no corrections to be made. If you consider the project ready for assembly, the plywood is ready for the end grain to be treated and waterproofed.
Fill the Plywood Voids
All plywood has voids in the end grain. Better quality plywood will have fewer voids, but in most cases, you will see voids at the end of almost any type of plywood.
These voids are one of the greatest contributors to moisture accessing the plywood and causing damage. So, if you want to truly waterproof your plywood box, you should give particular attention to filling in the voids.
Wood filler is a good product to fill the voids in the ends of the plywood, particularly for the ends that will be visible in the final product. This is because you can choose different color wood fill products to fill each void to maintain the linear visual impact of the plywood end grain.
If you finish the plywood in a product that hides the wood, you can fill the voids with any color wood filler.
To fill the voids in the plywood, you can follow the steps detailed below.
- Sand the end grain. Lightly sand the end of the plywood to remove rough edges.
- Apply the filler to the voids. Use a spatula to apply the wood filler to the voids, ensuring the gaps are filled with sufficient product.
- Let the wood filler dry. Give the wood filler 24-hours to dry. There is sometimes shrinkage as the wood filler dries, and you may need to touch up some areas with more wood filler.
- Lightly sand the plywood edge. Once the wood filler has dried, lightly sand the edge once again to eliminate any high spots in the filled areas.
- Apply a waterproofing agent to the end grain. Now, you can apply your chosen waterproofing agent to the end grain. This can be the same product that you will use on the rest of the box. However, you can use any sealing product on hidden edges since it will not be visible in the final product.
- Allow the plywood edges to dry before assembly. Give the waterproofing product of the end grain of the plywood ample time to dry before assembling the box.
The products you can use to waterproof the plywood box will be covered a little later. You can use any of these on the end of the plywood to keep a uniform look and finish to the box, or you can use faster-drying products on these parts to reduce the wait time for the product to dry.
Materials To Waterproof A Plywood Box
There are several products you can use to seal your plywood and make it waterproof. The one you select will mainly be determined by the level of waterproofing you need and the final look you want for the plywood box.
Some methods will give you a clear finish that will allow the wood to show through, while others will completely cover the wood, giving color to the finish.
Sealing A Plywood Box With Epoxy Sealer
Epoxy sealer is an effective way to waterproof your plywood box while still allowing the grain of the wood and the end grain to show through the finish. This provides a beautiful natural-looking finish to the plywood box and provides excellent waterproofing.
When the epoxy is in its liquid form, it soaks into the wood fibers but then solidifies to provide a hard, durable waterproof finish suitable for both indoor and outdoor use.
Sealing epoxy can be used to seal the ends of the plywood to retain the decorative linear aspect of the plywood to enhance the visual appeal of the wood. It can also be used to seal the flat surface of the plywood.
Follow these steps to apply an epoxy sealer to the plywood.
- Lightly sand the plywood surface. This will remove high spots and raised wood fibers.
- Wipe the plywood with a damp cloth. This removes loos sanding dust from the surface to get an even coat of the epoxy.
- Mix the epoxy sealer. Mix enough of the sealer to cover the area of the plywood box you are treating. Some products dry quite quickly, so only mix enough to cover the working area. Ensure you follow the manufacturer’s instructions for mixing and applying the epoxy.
- Make sure the coat is even. Ensure the first coat of epoxy is applied evenly to the entire surface.
- Let the first coat of epoxy sealer dry. Follow the manufacturer’s instructions to give the first coat enough time to dry completely.
- A second coat may be required. Sometimes, depending on the product used, a second coat is necessary on the plywood. Follow the instructions for the product to achieve the best results for the second coat.
- Allow sufficient drying time. A second coat often takes longer to dry than the first coat since less of the product soaks into the wood. Give enough time for the product to dry completely on the final coat.
Epoxy sealer is easy to apply, and it is hard-wearing and durable. Another advantage is that it can be re-applied over time as the box becomes weathered, without the need to strip off previous coats of the epoxy.
Sealing A Plywood Box With Penetrating Hard Oil
Penetrating hard oil is an oil that is mixed with a type of varnish. The oil penetrates the wood and repels moisture, while the varnish hardens to form a protective shell over the wood surface.
The combination of the varnish and the oil serves to penetrate deep into the wood and provide a good waterproofing finish.
Penetrating hard oils is generally clear, bringing out the richness of the wood in the final product. The oils can have the effect of darkening the wood slightly. So, test the finish on a piece of scrap plywood before committing to using these products as your final finish on your plywood box.
Penetrating hard oil is a good finish choice for interior use, but it does not react well to exposure to the sun and wind. In these conditions, you will need to re-apply the finish regularly to keep the plywood protected.
Follow the steps detailed below to apply the penetrating hard oil, but there may be additional instructions for different products.
- Lightly sand the plywood surface. This ensures a smooth surface for the application of the oil in an even layer.
- Clean the dust off the workpiece. Wipe the plywood surface down with a damp cloth to remove sanding dust. Allow the wood to dry before applying the oil.
- Apply the penetrating hard oil. You can apply the oil with a paintbrush, but the preferred method is to wipe the oil onto the plywood with a soft cloth. Ensure that you cover the wood in a thin, even layer.
- Give the first coat time to dry. The drying time varies from product to product. Follow the instructions on the container.
- Apply a second coat. Multiple coats are usually required for penetrating hard oils, and some light sanding with a high grit may be necessary between each subsequent coat, except the last and second to last coats.
There are many different types of penetrating hard oils, such as Danish Oil, but you will need to explore what you have available in your location. Some penetrating oils are designed for specific types of wood, so you should look for one labeled to work on plywood.
Waterproofing A Plywood Box With Latex
Latex is another useful product for waterproofing a plywood box. This product is not a clear finish, so the wood will not be visible through this finish. Latex-based paints often come in a spray-on format which makes them very easy to apply.
Generally, a primer is unnecessary when using latex-based paint, so long as you apply multiple coats. The latex in the paint seals the pores on the surface of the wood, essentially providing no access for moisture to enter the wood.
Latex and latex-based paints can be used as an undercoat for other finishes, such as paints with limited waterproofing properties.
Applying latex to your plywood box is easy and only requires a few steps.
- Sand the plywood. Sand the plywood surface to smooth the wood. Dust the surface with a fresh, clean paintbrush, or wipe down with a soft cloth.
- Spray on the first latex coat. Spray the latex onto the wood in even strokes, trying to get even coverage across the surface.
- Allow the latex to dry. Allow the first coat of latex to dry according to the product instructions.
- Apply subsequent coats. A minimum of a second coat is usually required for these products.
Waterproofing A Plywood Box With Paint
Paint can be used as a waterproofing agent on a plywood box, but you need to use it in conjunction with a primer.
If the paint is applied directly with no preparation of the plywood surface, it will soak into the wood, and you will use a lot of paint. A primer is required to seal the plywood surface first. Select a primer that is labeled as being suitable for use on plywood.
The method to paint the plywood is as follows.
- Sand the plywood. A smooth, dust-free surface is needed to apply the primer.
- Apply the primer to the plywood. Apply the primer to the plywood, giving sufficient drying time before moving on to the next steps.
- Apply the first coat of paint. Apply the paint and give it time to dry.
- Apply a second coat of paint. You can apply a second coat of paint without the need to sand down the first coat.
Waterproofing A Plywood Box With Varnish
Polyurethane varnish makes another good waterproofing agent for a plywood box, particularly if the box will be used indoors.
Polyurethane varnish penetrates the upper layers of the wood and hardens to provide a protective moisture-proof coating on the surface of the wood.
The advantage of polyurethane varnish is that it is a clear finish that allows the natural beauty of the wood to show through.
Polyurethane varnish can be applied using these steps.
- Lightly sand the wood. Lightly sanding the wood knocks the tops off the wood fibers and gives a smooth surface to apply the varnish. Dust the surface after sanding.
- Apply the first coat of varnish and allow it to dry. Apply the varnish to the plywood with a paintbrush or a roller. Give the fists coat plenty of time to dry.
- Lightly sand the first coat. The first coat of varnish causes the wood fibers to stand up again. A light sanding knocks these fibers off to provide a smooth surface for the second coat.
- Apply a second coat of varnish. Apply a second coat to the plywood and give it the appropriate time to dry. Usually, the second coat offers sufficient waterproofing.
Waterproofing a plywood box is possible, and it opens the use of this material for numerous woodworking projects both inside and outside the home.
Waterproofing the plywood can enhance the finish of the final product and will certainly make it last longer in moist or damp environments!