Question: What is it meant by the term "grade of abrasive paper," "raising the grain," and why is it done?
Answer: The "grade" (or grit) of abrasive paper (sandpaper) refers to the size of the abrasive particles in the sandpaper.
Given the same number of passes and the same amount of pressure, paper with larger particles sands deeper (and rougher) than paper containing smaller particles. A lower number indicates the paper is for rough sanding where a high number indicates the sandpaper is meant for finish sanding.
Usually 30-and 60-grit papers are used for rough sanding, 100-to 150-grit sandpaper is for medium sanding and 220-grit sandpaper is used for finish sanding. Of course, this changes with the type of wood and whether sanding is done by hand or with a machine.
Sanding a soft wood with rough sandpaper could tear the wood fibers (the grain). Sanding perpendicular to the fibers could tear the wood fibers.
Another way of causing the "grain to raise" is to saturate wood. The best way to determine what grit to use is to test sand. Keep in mind that rough grits of sandpaper leave deep scratches, so start with finer grits (150 to 220) and work up to the rougher grades.
Raising the grain is what painters must contend with after the first coat of paint is applied. Once the paint or varnish has dried, the first coat and the raised surface must be smoothed.