Easy Way to Waterproof Plywood

Sand each side of the plywood to create a smooth finish. Start with 80-grit sandpaper to remove the roughest spots. Sand along the grain of the wood in even, back-and-forth motions.

Repeat the sanding process with 120-grit sandpaper to achieve a more polished finish. This will eliminate some of the coarseness left behind by the 80-grit sanding job.

Wipe off the sanding dust with a lint cloth. Give time for the dust to clear from the air before applying the sanding sealer in order to achieve as much of a dust-free environment as possible.

Apply a generous amount of the sanding sealer to the wood with a paint brush. Apply with even strokes to the exposed side of the wood. Allow sufficient time for the sanding sealer to dry as per the manufacturer’s instructions, typically 45 to 60 minutes.

Sand lightly in-between coats using the 120-grit sandpaper. The sanding sealer will leave behind rough air bubbles that should be sanded down to achieve a smooth finish. Do not sand too roughly and risk removing the entire coat. Apply three coats of sanding sealer, but do not sand the wood after the last -- and lightly applied -- coat.

Turn the wood after it dries completely. Apply the sanding sealer in the same fashion to the other side.