MDF (medium density fiberboard) is a composite wood made of wood fibres glued together under pressure. It is a popular choice for manufacturing furniture as it is strong, cuts leaving smooth edges, insulates against sound and has an even finish with no wood grain. However, MDF is very heavy and can wick moisture. Taking the time to seal an MDF project will increase the board's resistance to water.
Prepare the glue size. Glue size is a concentrate; mix with water according to manufacturer's directions. Alternately, you can make a homemade version by mixing polyvinyl acetate (PVA) wood glue with water, using 1 part glue to 10 parts water. Mix the glue size until it has a smooth consistency.
Pour the glue size into a bowl and brush it on all sides of the MDF board. Make sure to brush into any screw or nail holes as well. Allow 30 minutes between coats so that the glue size can soak into the wood. Apply three to four coats, sanding lightly between.
Prime the board with oil-based primer. Roll on two coats of primer, allowing each coat to dry according to manufacturer's directions. Sand lightly between coats.
Finish the board with exterior latex paint. Exterior paints are more resistant to moisture than interior paints. Roll on two coats of latex, allowing each coat to dry completely before adding the next coat. Follow the manufacturer's directions for drying time.