Rope

• Large gaps, whether between boards or in the middle of a single plank, can be filled with rope. Rope is good to use because, like wood, it's a natural material that can expand and contract with atmospheric conditions and therefore won't be ejected with temperature fluctuations. Use a tool like a screwdriver or even a butter knife to scrape any loose debris up out of the gap. Vacuum the area to remove any dust. Choose a piece of cotton rope or string that is a similar width as the crack. Tint the rope with wood stain to match the floor if you wish, but let it dry before proceeding. Push the rope in between the cracks or gaps using a putty knife or screwdriver. Apply a layer of finish if necessary to smooth the surface.
  
  

Sawdust

• Mix wood glue together with sawdust saved from sanding the floors or another project to make a filler the consistency of toothpaste. Push the paste into the cracks using a putty knife or even your hands if you're wearing gloves. Roughly sand the floor and then apply a latex wood filler to the entire area. Sand it flush with the surface of the floor when it dries. This method helps to keep new cracks from forming as the floor moves as well.
  
  

Wood Putty and Other Fillers

• Wood putty, also known as wood filler, is a paste that can be purchased ready-mixed and used to fill cracks. It comes in a variety of colors, so it can be used on a finished floor and does not need to have a layer of finish applied over it. It does dry very slowly and is best used between coats of oil-based polyurethane--it should not be used under water-based polyurethane. Lacquer-based and epoxy wood fillers are also available.
  
  

Sand and Refinish

• If the cracks in your floor are not very deep or they are more like gouges, sand them out and refinish the floor. Use a hand or palm sander fitted with 80-grit sandpaper on individual problem spots, or a floor sander to refinish the entire floor. After sanding, vacuum the dust away and make sure the cracks are gone. Use a latex wood filler to fill in any remaining cracks. Sand the area again with a finer-grit sandpaper, then stain and finish the area or floor.