Refinishing hardwood can be an intimidating task, and mildew stains only add to the frustration. When dealing with mildew, you have to worry about more than just stain removal. Before you can even begin the refinishing process, you must address the fungus problem. Failure to do so may result in permanent damage to the wood, because mildew spores feed on wood for nourishment. As long as you act quickly, you can restore your wood to like-new condition.
Dry the wood completely. A report from the University of Missouri Extension recommends using a combination of heat and air circulation to evaporate moisture from the porous wood. Use a space heater and dehumidifier, if available, to get the wood as dry as possible.
Clear the hardwood surface of any furniture or other obstacles and spray the entire area with a solution of 1 part white vinegar to 10 parts water. White vinegar contains acetic acid, a natural mildew-killing agent; it also makes a great cleaning solvent, so use it to prep your refinishing effort. Wipe the entire surface with a terrycloth mop.
Sand your hardwood surface. If you plan to refinish an entire floor, the This Old House site recommends starting by hand-sanding the perimeter of the room with 180-grit sandpaper. Attach a maroon buffing pad to a buffer and make your way across the hardwood surface, always moving in the direction of the grain and overlapping each course by about 6 inches. Keep the buffer moving at all times.
Vacuum the hardwood surface using a felt-bottomed vacuum cleaner attachment. Remove all the dust and powder left behind by the buffer. For best results, do this about 15 minutes after you finish buffing.
Apply your chosen hardwood finish to the edges of the hardwood surface, using a 3-inch paintbrush. For hardwood floors, make your way along the baseboards. Try to complete this step within 10 minutes, because it's very important to proceed before the finish begins to dry.
Pour a 1-inch line of finish along the floor and use a long-handled roller to spread it in the direction of the grain, then across it. Always overlap your swipes, and continue until the entire surface is covered. Wait three hours and apply a second coat.