A Top Tip. Like most modellers, I seem to spend about half my time in the workshop sanding and rubbing down, and I get through acres of abrasive papers of various kinds. Most modelling is small-scale stuff, so much of this is done with small, sometimes tiny, strips and squares of paper wrapped around my collection of small purpose made balsa blocks (always use a block, never just hold the paper in your hand, quickest way to ruin a model, and known as finger-f*ck*ng in some trades). All paper has the # number printed on the back, but very few of your small pieces will be so marked, and you usually want to keep on moving down to a finer grade. Can you tell the difference between two small, possibly used scraps of #320 & #400 wet & dry? Can you tell the difference between a new piece of #180 silicon carbide and a slightly used scrap of #120? These differences are important, you don't want to use the wrong one, so here's the tip. I keep a fine black permanent marker on my workbench, and always write the grade on the back of every scrap of new paper when I cut if from the sheet or roll. Then you'll always know which grade of paper you're using, and the marking is unaffected by water in the case of wet & dry.