The best way to treat plywood for outdoor use is with a particular type of sealer. This will protect the wood from water damage and make it more resistant to insect infestation. The sealer should be applied in a thin layer and allowed to dry for at least 24 hours before applying the fiberglass. This will ensure that the plywood does not absorb too much of the sealer, which can cause it to expand. The best type of plywood for outdoor use is marine-grade or exterior-rated plywood. These are made with a special resin designed to resist water damage. Treating the plywood with insect repellent for long-term use is also a good idea. This will help prevent the wood from being attacked by termites and other insects that can cause damage.