Medium-density fibreboard (MDF) is often selected by builders and do-it-yourself homeowners for building bookcases, cabinet boxes, workbench tops and other household items that need a sturdy base. MDF is far less expensive than quality hardwood lumber, but has other limitations, such as weight and a complete intolerance to water. MDF is porous, so you must prime it before painting, both to protect the MDF from liquids and to provide a smooth, even finish for the paint. Priming MDF is something most homeowners can manage without hiring a professional.
Place a drop-cloth in a well-ventilated area. Place wood blocks or paint props on the drop-cloth to hold the MDF above the floor so that none of the edges touch the ground.
Wipe the MDF with a clean cloth to remove dust.
Paint a coat of primer on the MDF. Use a smooth-finish roller or sponge brush for a smooth finish. Paint the edges of the MDF, making sure you get a good coat of primer on them. The edges are raw MDF and far more porous than the facing. Allow the primer to dry completely.
Sand the primer with fine-grit sandpaper to smooth any paint lines. Be careful when sanding that you do not sand below the layer of primer or you can damage the MDF.
Wipe the MDF with a clean cloth to remove sanding dust.
Paint a second coat of primer on the MDF as in Step 3. Allow the primer to dry completely.
Sand the primer as in Step 4. Wipe the MDF with a clean cloth to remove sanding dust. If the primer looks even, and the finish smooth, then you do not need another coat. If the primer looks thin and the color splotchy, then paint a third coat of primer on the MDF and sand afterwards.