sand paper origins and uses

This unexciting but necessary product was thought to have been invented in 13th century China by using natural gum to stick crushed shells or sand to parchment. Moving forward several centuries to the 1800’s crushed glass was used to make a product still in use as today’s glasspaper.
Shark skin is a natural sandpaper, Dermal denticles on the skin give a rough texture which imitate tiny grooved teeth, but are in fact placoid scales. These scales do not increase in size as the shark grows, more of the same are produced. A small shark / dogfish could produce skin that is 80 – 220 grit which will cut in one direction and burnish in the other.
Commercial or Industrial grades of sandpaper are available. The main difference being, Industrial grade uses higher quality materials for all three of its components, these being the material used as grit, the backing material (paper) and the glue to adhere the other two together.
There are five main types, with a large range of grit grades available.
Glass/flint paper, Garnet paper – probably best finish for wood, but slower than some others, Aluminium Oxide    ,Silicon Carbide, Cerami.c
Whilst this mind boggling array of information may have gone over your head (as it did mine) the reality is much simpler in the hands of someone who knows what they are doing.
In Short – To avoid your fine finish going gritty – get the right grade !