The number on the back of your sandpaper doesn’t always provide the apples-to-apples comparison. You might think you're “running through the grits,” but without understanding the grit-grading systems, you might accidentally use a coarser paper.
Years ago, most of the sandpaper sold in the United States was graded on the CAMI scale. This standard was so commonplace that sandpapers simply listed the grit number on back. In Europe, the FEPA had its own metric grit-grading system. FEPA-graded paper carried a P prefix in front of the grit number.
Today that’s not always the case. Now that most manufacturers have switched to the FEPA-grading system, the letter P isn’t always stamped on the paper. Although most abrasives now sold use the FEPA grading system, you may still run into problems if you’re using up stockpiled sandpaper. Although similar, the cross-continental grit- grading systems are different enough to create sanding problems, especially when finish sanding. As shown in the scale at right, the grit-ratings line up to 220. From there, the FEPA- grade gets increasingly coarser than the similarly- numbered CAMI paper. For example, the 400-grit CAMI paper is finer than 600-grit FEPA.