The coating, e.g. the number of abrasive grains per surface unit gives the paper it's structure and determines the abrasive capasity.

 

• Open coat (Appr. 40% of the surface is covered with abrasive grains)
• Semi open coat  (Appr. 65% of the surface is covered with abrasive grains)
• Close coat  (90-100% of the surface is covered with abrasive grains)

Close coat allows for deeper cutting at the start, but often increases the risk for clogging when sanding soft and resinous types of wood, synthetic materials and surface-treated materials. Therefore, an abrasive product with a more open coating may last longer even though it has a smaller number of grains per surface area.