Plywood is a building material that is widely used in many types of applications. To protect exterior plywood surfaces from moisture, waterproof them with a sealant or stain. Plywood is also used to construct submerged structures, like boats; use a stronger sealant to protect these types of structures from water damage.
Open the sealant or varnish container and mix the contents with the paint stick to an even consistency.
Pour a moderate amount of sealant into a paint tray. Load the paint roller by dipping it in the reservoir and rolling it up the angled surface of the tray. Use the paint roller to apply a thin, even layer of sealant to the plywood surface.
Allow the layer of sealant to dry. Check the instructions for the sealant, some require multiple applications to fully waterproof. Follow the same application process when multiple layers are required.
Clean off any visible stains on the plywood with a coarse grit sandpaper.
Mix the resin and hardener components of the epoxy. This is usually a 5-to-1 ratio, but always follow the packaging instructions. Pour the epoxy mixture into a paint tray. Apply a thin, even layer to the plywood with the paint roller.
Allow the initial epoxy layer to cure overnight. Spread the fibreglass cloth over the plywood surface. Apply another layer of epoxy directly to the cloth. Use a squeegee to push out any extra resin trapped underneath the cloth.
Apply two additional layers of epoxy, allowing the previous coat to cure overnight between each application.
Waterproof plywood is available for purchase at many hardware stores, which can save time and provide a more consistent result.
Epoxy hardens in about 20 minutes, so only mix up as much epoxy as needed for one application at a time.