When filling gaps in floorboards it's important to use a filler that is both strong and flexible. This is to allow for the movement of the floor boards when walked on plus the shrinking and expansion of the gaps because of temperature and moisture changes. General wood fillers can crack and fall into the gaps over time. We recommend Bona Gap Master or Osmo Interior Gap Sealer. Both fillers are ideal for sealing gaps between floor boards prior to sanding and finishing.