Sanding a wooden floor is a gradual step-by-step process. We recommend three steps or more as shown below:

1. Coarse sanding. The purpose here is to remove dirt, old finish and marks on the wood. The most common practice is to start with grit 36 or 40. In exceptional cases, with very uneven floors or pine coated in old finish, you may decide to start with grit 24 or even grit 16.
2. Medium sanding. Next, we recommend medium sanding with grit number 50 to 80, which will remove traces of coarse sanding
3. Fine sanding. The fine sanding is usually done with grit 120. If you use solvent-based finishes, you can use grit 100 for the final sanding and still obtain a well-bodied surface. However, with a water-borne finish, the final sanding should be done with grit 120 or finer
   
   Note: After fine abrasion, deep grooves remain if you miss out the medium sanding. To obtain good sanding results, never jump from a coarse to fine grit size