Plywood was never meant to withstand the forces of nature alone. Although it is designed to hold up to the elements for a short period of time, it must be weatherized in order to prevent degradation. There are several things that can be done to plywood to extend its life, such as weather treatment and even colour treatment that will leave the plywood grain intact while lightly tinting the wood. The average do-it-yourselfer can complete this project in one to two days, depending on the size and scope of the project.
Seal the edges of your plywood using exterior polyurethane varnish. Use a small stain brush to apply the varnish to the plywood. However, keep it off of the surface of the plywood sheet by taping the surface with masking tape. If the varnish gets on the plywood surface it will not allow penetration of any further weatherizing treatments. Apply several coats of varnish to the edges of the plywood, allowing 30 minutes to one hour between coats. This treatment will prevent water from penetrating the laminated edges of the plywood. Once water enters there, the plywood will separate.
Coat the plywood surface with an exterior water-based pigment stain. These tints are light and almost undetectable. The stain works by slightly tinting the plywood surface, thus keeping out harmful UV ray damage. Use a pump-up garden sprayer to apply the stain, and follow the container directions for drying times. Once you spray on the stain, wipe off the excess using a cloth rag. This will remove runs in the stain. Again, this type of stain is not intended to completely colour the plywood, but tint and protect the plywood from the sun. Allow the plywood to dry for 24 hours.
Apply water seal to the surface of the plywood. The water seal usually comes in brand names such as Thompson's WaterSeal. However, there is a wide variety of weather sealants found in any hardware store paint department. Use a pump-up garden sprayer to apply the sealer. While this sealer will not completely close the pores of the plywood, it will condition the wood and prevent water from penetrating the surface. Apply several coats of sealer, and reapply every two years.