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Cworkshop's How To

How to waterproof plywood

The basics

Why waterproof plywood?

Plywood is a brilliant material to work with. Compared to solid wood, it's much cheaper and more durable than chipboard or MDF, and plywood is visually pleasing enough to be used for furniture.

Plywood is made by glueing and compressing layers of veneers in opposite directions. This helps create a structurally strong board and helps keep the board as flat as possible.

Unlike other wood-based materials, plywood isn't fully waterproof. Still, the good news is that you can take steps to stave off moisture from penetrating the board, thereby giving your plywood furniture a longer life. You must seal your plywood, especially if used in high moisture areas such as bathrooms, kitchens, or outdoor use.

So, why should you waterproof plywood?

Like other natural materials, untreated plywood is susceptible to rot caused by moisture and water, creating a breeding ground for fungus or mould within the board. As the wood rots, it affects the board's integrity, which can cause it to break down over time.
The plys that make up the board are made by glueing and pressing. Although good at adhesion, the glue used is not generally waterproof, which means that if the glue disintegrates, it will cause the plys to pull apart, causing irreparable damage.
Weak spots
Weak spots
When you cut plywood, you expose the edges. The plywood edges are a weak spot because they will absorb moisture much quicker than the surface. It is essential to make sure you seal plywood edges to protect the board. A good seal on the edges is one of the most crucial steps when waterproofing.

Prevents warping
Prevents warping
Another situation you'll find yourself in if you don't correctly waterproof plywood is that there is a possibility of the board warping. Water damage causes plenty of materials such as timber or MDF to warp, so make sure your plywood sheet is bone-dry before sealing.
Longer lasting
Longer lasting
Sealing the plywood edges and surface properly also gives your material a longer life, and continuous application throughout the life of your furniture or project will keep it looking good and protected, not matter what life throws at it.
Applying a good sealer will also make your plywood surface look remarkable and can transform birch plywood into something similar to oak or walnut.

Before you begin

Preparing plywood for waterproofing

It is essential to make sure your plywood is prepared to be worked on. Ensure that the surface is first lightly sanded with coarse sandpaper to remove blemishes and provide a surface that will help any varnish or sealant properly adhere to the surface. Then make sure the surface is wiped down and free from dust particles. We haven't mentioned filling any holes in the surface - if you purchase your plywood from Cworkshop, you will be assured that you will receive high-grade plywood with the best veneers possible.

There are many ways you can accomplish this when waterproofing plywood. More often than not, you will have to apply the waterproofing agent using spray-on or paint-on methods.

When it comes to sealing options, here are a few methods of waterproofing you should consider.

A hardened finish for protection

Latex paint

Latex-based or polyurethane-based paint is one of the most accessible forms of waterproofing plywood. There are plenty of colours to choose from, which can be applied using a spray gun or painted with a roller. You must apply two to three coats of paint for maximum effectiveness. Latex paint is also one of the cheapest methods. However, you will need to repaint it often. You can use latex-based for indoor or outdoor projects. Coating your board with latex-based paint can give your board a lovely, complex finish, but you will lose the natural aesthetics.

Simple and effective

Lacquer and varnish

Lacquer and varnish as a sealant are perfect for both indoor and exterior use projects. It can be applied using a brush, roller or spraying to achieve a finish to your liking. The brilliant thing about polyurethane varnish and lacquer is that they let the natural veneer grain become more prominent, giving you that natural look. Using varnish and paint to seal plywood provides not only an enticing colour but also water protection and UV protection; however, there is a possibility that this type of sealant can yellow over time. 

When choosing your lacquer or varnish, find one that matches the plywood veneer you're working with to give you the best results after sealing.

Deeper protection

Wood oils

If your project is used indoors, you could try using wood oils. Due to the porous nature of wood, the oil penetrates and soaks into the surface veneer. This type of sealer gives the wood a natural look and can be applied by painting on or using a cloth to rub the oil into the wood. You will need to apply 2-3 coats of oil, as the plywood will fully absorb the first coat.

Shelter from the elements

Epoxy sealant

This type of sealant works best for outdoor projects as it offers up a bit more protection. The best thing about an epoxy sealant is that you only need one layer to get the best protection. Depending on the type of epoxy you use, you will have an outcome that retains the natural colour of the veneer, or it will slightly tint it. You can apply this sealant using a paint roller or paint brush, ensuring that you keep the strokes even throughout.

Cheap and easy

Water-based paint

Another cheap method of sealant is a water-based paint. This sealing method is not so standard as it's not as long-lasting as the other methods. It is also possible that this paint will need to be touched up more often, but it is cheap, quick and will still protect your plywood from moisture.

It's worth mentioning that it is not advisable for water-based paint to waterproof plywood for outdoor use, as the harsh elements, either UV rays or continuous rain, will wear down the paint within a few months.

More than one option

Choose the right plywood

If you're looking for the plywood that you're confident, even after sealing, will have the most protection against water and moisture, then perhaps you should consider marine-grade plywood. Marine plywood is a type of plywood that is more water durable than regular plywood. The glue used to stick the veneers, or ply, is waterproof, which means it won't de-laminate when exposed to moisture or water. Marine plywood also boasts that there are no gaps between the plys. Gaps weaken the overall structure of plywood and create a path for water or moisture to get inside the plywood sheet, which can cause the board to weaken from within. The surface veneer of marine plywood is also from a wood that can withstand the elements, thereby giving your plywood a longer life.

As the name suggests, marine plywood is often used in boat building. So, not only does it have excellent waterproofing capabilities, but it's also hardwearing and can take on impacts like a pro boxer.

One of the reasons why plywood is often used is because the edges provide a lovely effect. If this is the main reason for using plywood, then you should consider getting melamine-faced plywood, which means you will only need to seal the edges rather than the entire plywood sheet, thereby saving you time.

Make it easier for yourself

Get professional help

Suppose you don't have the time or expertise to seal your plywood to make it water-resistant professionally. In that case, our spray coating service provides varnishing, lacquering, and polyurethane paint to help waterproof your plywood.