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Waterproofing Plywood. Sealing Plywood for longer life.

Plywood is an important and stable building material, comprised of many different veneers glued together at right angles. These layers provide dimensional stability, but leave the edges, which expose all of the laminate sections, particularly vulnerable to water damage.

Many different grades of plywood exist. These use different glues to hold the veneers together. The veneers themselves will be made of different materials, hardwood or softwood species for instance. These offer varying degrees of water resistance, but waterproofing plywood edges, and to a lesser extent the board faces themselves, will significantly extend the life of plywood in any environment where water is present.

This technique for waterproofing plywood edges is especially valuable for treating exposed cut edges of commercially finished phenolic faced plywood boards, such as Buffalo Board™. These boards are normally used for extreme duty exterior applications like trailer floors. Sealing the edges of these boards can significantly extend their useful life. Smiths CPES does not attack the phenolic surface of such boards, and offers high quality weather protection to the not protected surfaces of them.

How much water will plywood absorb?

siding plywood waterproof impregnated with dyed cpes

Plywood is available in a variety of different qualities. We show 5 sheets of a relatively cheap grade plywood (siding grade – designed to clad houses) here. , which is designed for exterior use to sheathe the outside of a house.

We stood the plywood in Smiths Clear Penetrating Epoxy Sealer dyed blue. The bottom edge of the plywood sheets were placed in a shallow tray, and the dyed CPES was allowed to be wicked into the wood.

The CPES was allowed to cure and then the sections were cut open and sanded to show these internal sections.

The dyed regions show just how far the CPES has penetrated into the plywood, and clearly demonstrate areas of abnormal porosity (areas that are significantly darker than those around them). These areas of abnormal porosity will be the first to fail in exterior use, as they will readily wick up water and encourage the formation of fungal growth (rot).

Can we waterproof plywood with Clear Penetrating Epoxy Sealer?

We can significantly enhance the resistance of plywood to the ingress of water with Smiths CPES.

waterproofing plywood siding ply impregnated with dyed cpes after sealing

This photograph shows another 5 sheets of siding plywood. These sheets were first saturated with Smiths CPES – which is naturally clear.

48 hours was allowed for drying, and then the sheets were placed in a shallow tray with dyed CPES like the samples in the photograph above.

The CPES was then allowed to cure once more, and the sections were cut and sanded as above.

The dyed regions are far smaller than before. The wood has far less porosity and will be far more water resistant.

edge sealed plywood after 4 wet years

Here is a real world example of plywood that has been sealed with CPES and exposed to water. It’s a bath panel, fabricated by me in 2015, and this photo was taken in 2019. The construction is detailed in Edge sealing plywood – waterproof your ply.

This shows the bottom edge, which gets wet every time the bath is used (I have children – so the wetting is pretty much guaranteed). Two plywood panels are here, there is the cheap ply which forms part of the engineered walnut flooring, and the mid grade hardwood ply from my local timber merchant that I assembled the boards onto.

The CPES may have significantly added to the cost of the plywood, but it has increased the life of the product beyond belief. Application took little time, and no top coat was required as the area is not exposed to sunlight.

If your plywood is going to get wet, and you need it to last, there is a solution.

What about Buffalo Board, or other Phenolic coated ply boards?

Phenolic surfaces are added to plywood to give massive durability for trailer floors and similar purposes. The edges are at best covered with paint, and in the real world the boards need to be cut to fit the destination object.

Those exposed edges are the weak part of any phenolic board. You must seal them too to prevent water ingress. See Sealing Phenolic Boards for more details.

This explains why sound wood treated with Clear Penetrating Epoxy Sealer resists rot so very well.

The CPES also makes the final varnish or paint finish last much longer. See  here for how to waterproof external timber and make varnish finishes last on it: Waterproofing Exterior Wood For Varnish.

This is the technique used by many sailors with wooden boats – who can achieve varnish finishes which last up to 20 years.

waterproofing plywood exterior exposure smiths cpes


The Rot Doctor in America has just published his results on the preservative effects of CPES on plywood. Performance is impressive under exposure to the most extreme elements. The results are astonishing, and demonstrate how well CPES acts as a waterproofing preservative on plywood. Click here to view the results.

We have our own Penetrating Epoxy Resin Performance Tests Here.


Want to have a go:

Smiths Clear Penetrating Epoxy Sealer is the product you need. Buy it here to waterproof your ply and extend it’s durability.