What do sandpaper grades mean?

Q:

At a woodworking show I bought some sandpaper in the grit numbers I normally use, but the abrasives didn’t match the texture of the products from my local supplier. I think the sandpaper I bought is imported. Does that make a difference?
—Neal Harrison, Blacksburg, Virginia

A:

Coated abrasives (the technical name for sandpaper) follow one of two common grading systems used in North America: CAMI (Coated Abrasives Manufacturers Institute) and FEPA (Federation of European Producers of Abrasives).

FEPA products are sometimes called “P grade” because that letter prefixes the grit number on the back of the sandpaper, as shown above. If there’s no prefix, you can assume that it’s a CAMI-graded product. As you can see in the chart at left, particle sizes in the two systems closely parallel each other up to about 220 grit; then FEPA numbers increase rapidly. If there is a letter after the number, it refers to the weight of the paper or fabric.

Manufacturers may use one or both systems for various lines of abrasives. For example, Klingspor uses the FEPA scale exclusively, while 3M uses both scales. 

A second key difference is that the CAMI standards permit greater variation in particle sizes used within each grade. That may be the texture difference you noticed.