Sandpaper is not made of sand, but is made of fine particles of natural or synthetic origin. The particles are sieved through a screen and sorted by size, and then bonded with adhesive to a paper, sponge or cloth backing to make an abrasive that can be used on its own. Whether you choose an “electric sander” or manual sanding, the correct choice of sandpaper type and grit is an indispensable part of perfecting the work. Sandpaper grit is classified according to the roughness of the paper, and the size of the sandpaper abrasive grinding particles is determined.
There are two criteria for the measurement of grit, one is CAMI (Coating Abrasive Manufacturing Institute) and the other is FEPA (European Federation of Abrasive Producers). In general, the particle size of sandpaper is often expressed by # and “P” plus a number. Lower numbers indicate larger grains and coarser sandpaper particle sizes, with strong grinding forces and suitable for initial sanding; while larger numbers represent smaller particles, suitable for finer sanding, thus making the surface smoother.
Extra-coarse sandpaper has a particle size range of P12 to P36 and is used to remove paints and varnishes that are difficult to shed; Coarse sandpaper is the roughing of wood and the removal of finishes such as light polyurethane coatings. Coarse grains are usually in the range of P40 to P50; Medium sandpaper, with a particle size range of P60 to P80, suitable for the last sanding before painting. Usually, we also use medium-grain sandpaper to initially sand rough wood to remove planning marks on the wood; The grain size of fine sandpaper is P100 to P120. For most home workshops, this sandpaper is enough to do the final sanding before the work is done; P220, P280 and P320 are called very fine abrasives, it can be used to sand paint; P400 to P600 are usually called Extra fine, it can be used for preliminary polishing of the surface of the object. While special grade or ultra-fine abrasives with particle sizes up to 2000 are most suitable for final finishing or polishing.
In the sanding process, it is best to use coarse sandpaper for rough processing, and then use fine sandpaper to polish. The combination of different particle sizes can save time and cost, making the sanding efficiency higher.