Rough Sanding

• After you've installed an unfinished hardwood floor, you need a coarse grit like 20, 36 or 50 to level the boards. To save time, refinishers often work the floor diagonally with the coarse paper, then go over it parallel to the boards with the same paper before moving on to finer grits. This approach works for leveling cupped or crowned boards that have been damaged by moisture and may also be appropriate for removing a tough finish from a hard material, like maple or Brazilan walnut. The coarse paper leaves deep scratch marks, but subsequent sanding usually removes them.
  
  

Finish Sanding

• After a floor has been rough-sanded, you have to do one or two passes with a finer grit paper to prepare it for finishing. The two grits most commonly used for this are 80 and 100. When used one after the other, they remove the scratch marks and polish the floor in preparation for the finish. A further pass with a finer grit paper like 120 may create a smoother surface, but the difference probably won't be noticeable enough to warrant the extra work. The exception is sanding parquet with an orbital sander when a fine grit is necessary to remove cross-grain scratches.
  
  

Screening

• Screening is an alternative to sanding if you just want to knock down the old finish and recoat it rather than remove it entirely. It's also a procedure that refinishers use between applications of the finish coat. The best way to screen is to remove the buffing pad from a floor buffer and replace it with a 120-grit sanding screen. A buffer is lighter than a floor sander, and, with the fine paper, there is little danger of it wearing through the finish. Instead, it knocks down bumps and bubbles to ensure a smooth, glassy surface when you apply subsequent coats of finish.
  
  

Engineered Flooring

• Sanding an engineered floor presents the undesirable possibility of wearing through the veneer of the flooring boards to expose the core. Some engineered boards have a veneer of 3/16 inch, but many are thinner, and attacking them with a coarse grit can ruin the floor. As general rule, you shouldn't use a grit coarser than 80 on an engineered floor when operating a drum sander or edger. If you aren't making progress, switch to an orbital sander with a coarser grit. If your engineered floor has already been refinished once, think twice before doing it again.