Removing Finish

• There are two ways to remove an old finish from a wooden surface. One is to strip the finish with a chemical, and the other is to sand it off. Chemical stripping works well for cabinets and furniture, but it's impractical for a floor because of the relatively large surface area. Sanding, however, is a time-consuming and difficult way to remove finish from furniture, but it's a comparatively easy way to strip a floor, primarily because of the heavy-duty machines available. They can remove the entire finish, even a multilayered one, in a single pass.
  
  

Leveling

• Old floors tend to warp as they expand and contract with changing moisture conditions. The edges can push against one another and curl upward. When this happens, the only way to return the floor to its original appearance is to sand the boards down. Even if the boards aren't warped, they usually sustain nicks, gouges and scrapes through normal use, and sanding can remove most of these, if they aren't too deep. A properly sanded floor is flat and smooth, so that when you apply a new finish, it develops a pleasing glass-like sheen.
  
  

Filling

• Just as warping can cause boards to expand and curl upward, it can also cause them to shrink and develop gaps between them. The gaps detract from the floor's appearance, so part of the job of refinishing is to fill them with wood filler. Because it's impossible to apply filler only into the gaps without getting any on the boards, you have to sand the floor to remove the excess. Spot-sanding with an orbital sander isn't effective, because sanded wood appears lighter. Sanding the entire floor ensures that all the filler is removed and the color of the floor is uniform.
  
  

Refinishing Without Sanding

• If your floor is in reasonably good shape and you only want to freshen up the finish, you may be able to avoid sanding it by using a process called screening. Instead of using floor sanding machines, fit a buffing machine with a sanding screen and run it over the floor surface. It will scuff up the finish without removing it, providing a suitable substrate for a new finish. This method isn't advisable if the floor has major defects, like cupping or gaps, and you won't be able to change the color, but you'll avoid the hard work that sanding entails.