If plywood is to be exposed to the elements, waterproofing is a neccessity. It is a material very vulnerable to nature, especially water, and if left outside, plywood will only survive with a good coat of waterproofing. Applying a waterproof top coat is easy and affordable, and can last from four to six years if applied properly.
Mix a can of sealant by shaking the can vigorously. Open the can and stir the contents with a stirring stick.
Pour some sealant in a painting tray and generously dip a brush or roller in the sealant. Apply the sealant in even strokes, covering the plywood with a thin, uniform layer. Let it dry for 35 to 45 minutes.
Apply a second layer in the same manner. Some sealants must be applied multiple times; consult the sealant's can for brand-specific instructions.
Many brands and types of waterproof sealant exist. Consult a paint store for an overview of your options. You can choose a sealant based on price, quality, durability or colour.
The wood surface must be dry when waterproofing is applied.