Plywood sheet is a versatile, strong, and durable material that’s commonly used for numerous woodworking projects. It is durable and cheap and gets the job done, but it’s not completely waterproof.
There may be specialty boards, but there are more economical ways to waterproof them. If you want to know how to waterproof plywood easily, here’s our guide:
Properties of Plywood: Is it Waterproof?
Plywood is manufactured in various forms and produced from different species of trees. Depending on the use, some plywood is smooth, whereas others are rough.
Generally, plywood is considered water-resistant but not waterproof, unlike marine-grade plywood. Prolonged exposure to moisture can destroy it easily.
Why is it Prone to Water Damage?
Plywood structure is the reason why it is prone to water damage. Plywood is manufactured from thin wood veneers layered with glue. These veneers are layered and glued together using pressure heat.
This layering results in durable material. But because of this structure, moisture can seep into the layers, which can come apart, leading to their separation or eventual splitting.
How to Choose Plywood to Use Outdoors + Recommended Options
Plywood is graded on an A-D system where A is the highest and D is the lowest. These grades, however, have no relation to waterproofing but more to the quality of the wood.
The first letter refers to the front face and the second letter refers to the back face. The third letter refers to the type of glue used. In some instances, PVA wood glue is used on plywood.
Here are some different types of plywood to help you choose:
ABX is also known as marine grade plywood. This type of plywood is pre-treated for waterproofing. It is intended for outdoor and marine use but can perform equally well for any outdoor projects.
It has a greenish, smoothest, and most appealing overall appearance, signified by the “AB” grade. ABX is the most waterproof type of plywood available on the market today, which makes it one of the best-graded exterior plywoods.
ACX is waterproofed. It does not have a marine-specific treatment. Nonetheless, it is considered extremely durable and great outdoor plywood. It is used in decks, roof sheathing, exterior walls, and renovations.
This is a cheaper version of the ACX. The front face is high-grade, whereas the back face of the sheet is rough. It is best to use in exterior construction where you can strategically place the front face of the plywood to where it is visible.
BWP is an acronym that stands for “Boiling Water Proof.” BWP plywood is highly durable and can withstand moisture and even boiling water. This plywood is used in kitchens and bathrooms.
MR is an acronym that stands for “Moisture Resistant.” MR grade plywood is recommended for indoor use. It is not completely waterproof but can resist humid and damp areas of the house.
BWR is an acronym for “Boiling Water Resistant.” It can resist steam and hot water, but it is not considered waterproof. It is occasionally used for areas where steam is often present, like cabinets by a stove.
How to Make Your Plywood Waterproof and Ready for Outdoor Use
Waterproofing plywood is simple and easy. Not only will this fortify the material, but it will also avoid potential damage due to various elements.
These methods are applicable if you want to waterproof or seal plywood for outdoor use or coat untreated wood boards. These will close the plywood’s pores so water can no longer penetrate them.
Make sure to use the recommended material and application and find a method that is most suitable for your project or budget.
Method #1: Paint
Applying paint on plywood is one of the most common, effective, and popular waterproofing materials. You can also choose the color of paint to apply it with.
It is best to choose a water-based paint for waterproofing plywood. It is an excellent sealant and provides UV protection as well. However, fading, cracking, and peeling often happens and thus require reapplication for a few years.
In addition, using paint will not last as long as epoxy or drying oil.
Method #2: Epoxy
A common roll-on waterproofing method is using an epoxy  sealant. A good penetrating epoxy sealant produces hard, stiff, and extremely durable plywood.
Once completely dried, the epoxy sealer is adsorbed on the surface, resulting in a tough outer shell. Epoxy dries clear so the wood can retain its color. You can apply paint to it after scuffing the surface with sandpaper.
For a detailed guide, check this post on sealing painted wood for outdoor usage.
Method #3: Polyurethane
Polyurethane is a plastic type used in various global industries. There is a commercially available polyurethane varnish or sealer for plywood structures. It can easily be applied with a sprayer or by a brush.
You may also add a wood stain, depending on your preference. Make sure to apply the wood stain before varnishing. The plastic component in the varnish results in a durable finish.
Method #4: Varnish
Using varnish can effectively do an excellent job of waterproofing plywood without leaving a noticeable protective layer. Varnish is not as thick as epoxy and is applied and dries thinly. It comes in gloss, semi-gloss, or flat finish.
Method #5: Liquid Latex
Latex is commercially available in your local hardware stores. It is easy to apply. Spray a good layer on the plywood boards and allow the liquid latex to dry.
However, liquid latex is more expensive than other materials to waterproof your plywood surface. Despite the price, it is worth every penny. In fact, latex can even be painted over oil-based primers.
Make sure to apply liquid latex to plywood when it is still wet. It requires an even application to result in smooth and even finishes. Once the wood dries, it will form a layer that can now withstand natural moisture.
Method #6: Plastic Sheeting
Plastic sheeting is also known as polyethylene sheeting. It is a cheap and handy material that can create a water-resistant barrier once installed.
Method #7: PVA
PVA has been used to hold materials together. It has been traditionally used for bookbinding. It bonds well to plywood and produces a thin and clear coat on wood once completely dried.
Dilute the PVA with water before application. The PVA should be thin enough to seep through and be absorbed into the plywood pores before completely dries up. A thin PVA is also easier to apply with a paint roller or a brush.
Method #8: Drying Oil
Drying oil can also be applied to seal plywood. Drying oil allows the plywood to keep its natural wood appearance, moisture, and flexibility. It effectively creates a thin coat of protection against moisture.
Alternative: Penetrating Oils
Penetrating oils can go deep on the surface of the plywood, which seals it and prevents moisture from seeping in. Once penetrating oil is applied across the entirety of the surface of the plywood, allow it to be soaked up to seal the wood completely.
You can apply several coats on the wood surface depending on the oil you use. It offers protection, but continuous exposure to the elements can dry out the oil over time.
5 Steps to Waterproof Your Plywood
We hope the materials mentioned above have helped you choose which one suits you and your project. If you want to know how to waterproof plywood, follow these steps below:
Step #1: Cover the Holes
Clean the surface and cover the holes, cuts, and dents that may be present on the untreated plywood using a wood filler.
On average, wood filler dries within 30 to 60 minutes. Filling up these gaps will create a flat and smooth surface that prevents holes in the waterproofing layer.
Step #2: Sand
Once you have completely covered the holes with wood filler, let it completely dry. Inspect the result if it is even or not.
If it is still rough and uneven, you can proceed to sand it out and smooth out the filler. You can use medium grit sandpaper (60-100 grit).
Smoothing out the surface can help remove rough spots that interfere with the sealer, waterproofing plywood sheets, and even paint. Make sure to lightly sand and not use too much pressure.
Step #3: Coat the Plywood With a Primer or Stain
Applying a primer or stain is optional. However, if you want to do this step, apply it right after sanding the wood. Dip a rag into the primer or stain and spread it in the direction of the drain of the plywood.
Make sure to wipe away any excess primer or stain. Staining in a clear finish will be necessary if you want to paint the plywood.
Step #4: Spread the Waterproof Formula of Your Choice
Apply the waterproof or sealer formula in even coats on plywood. Apply the waterproof or sealer formula in even coats on plywood. Add coats and seal plywood edges.
Step #5: Wait Until Dry, Then Coat Again
Different waterproofing materials have different drying times. Several coats are needed for a complete application of the waterproofing material. Wait and let the previous coat dry before applying a new one to protect plywood.
Once you have completely waterproofed your plywood sheets, make sure to monitor them every few months to check if it holds up. Keep an eye on potential water damage if there is any.
Extra Tips and Reminders
These are the general methods on how you can waterproof plywood, and here are some additional tips and reminders. When waterproofing plywood, make sure to wear protective gear to avoid inhaling hazardous substances.
Furthermore, make sure to include the plywood edges. These edges are one of the most vulnerable parts of the wood. Edge banding is a thin veneer that we recommend applying to the plywood using an iron. It is worth doing for indoor projects, but sealing plywood edges can go a long way.
Why Seal Your Plywood and Make it Waterproof?
There are many advantages to sealing plywood and making it waterproof. These benefits include the following items.
Waterproofing your plywood prolongs or extends its lifespan. It can also make plywood stronger, durable enough for building a shed door. A plank of untreated wood can last about two years. A properly treated and waterproofed wood can last approximately 20-40 years.
Combat Moisture and Heat
Continuous exposure to moisture and heat will lead to plywood warping. When plywood is exposed to the elements such as rain, a flat plywood sheet can have bends and curves. Curved plywood is unattractive. Treated plywood can prevent this.
Surface Won’t Rot
Prolonged plywood exposure to water will result from rotting, which normally happens with untreated wood. Wood rot drastically weakens the material and makes it unattractive.
Waterproofing your plywood can prevent rot from happening by stopping water absorption. It can prevent the wood from going ugly but also prevent it from smelling bad due to rot.
Repel Dry Rot
Dry rot does not require much water and occurs when microscopic fungi thrive on the surface of the plywood. Dry rot damages the wood once the wood dries and the fungi have already started growing. It is difficult to detect until it is too late.
Waterproofing plywood will help prevent dry rot.
One of the most damaging effects to wood in the event of water exposure is splitting. Water causes the veneer wooden layers of the plywood to separate.
Once these sheets separate, plywood loses its strength. Splitting is difficult to detect, thus the need to be waterproof.
If water does not enter the veneers of the plywood, then it will not split apart.
Are exterior grade types of plywood water-resistant?
Exterior plywood is considered weather and water resistant plywood. This type of plywood is made from Douglas Fir. The waterproof glue further reinforces it in between its multiple thin layers.
Is Flex Seal recommended to use on plywood?
Flex Seal is recommended for use on plywood. It is easy to use and results in an excellent finish. Once applied, Flex Seal results in a rubberized watertight barrier on the entire surface.
What end grain sealer would work best on plywood?
A good end grain sealer bond that works best on plywood is an epoxy sealer. Epoxy resin should be applied first to fill in those gaps.
How do you waterproof a plywood subfloor?
You can waterproof a plywood subfloor with paint, polyurethane, or an epoxy finish. Make sure to sand the surface before applying any of these materials.
What is the cheapest way to waterproof plywood?
Painting on plywood is the cheapest way to waterproof plywood. It is commercially available at budget-friendly prices and comes in different forms.
How do I waterproof plywood for boating?
You can waterproof plywood for boating by purchasing marine plywood first. Marine plywood is resistant to moisture and is a popular choice for boating. Then proceed to apply a waterproof paint or sealant to its surface.
Plywood is a great building material. To make it more durable, it is best to waterproof it. Doing this will help extend the lifespan of your plywood pieces and repel unwanted moisture and fungi from seeping into the surfaces.
If you want to how to waterproof plywood, there are various methods to choose from, depending on your project type or budget.
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