It seems to be a well-established fact that sharkskin was used as sandpaper. This includes the use of the skin of rays and dog sharks.
My question is:
A) The earliest documentation I've found regarding the use of sharkskin as sandpaper goes back to the British Empire in the mid 18th century. Sharkskin was apparently only used to finish very fine work:
Cabinet makers would use the more accurate honed edges of planes to get a smooth surface, and the finest work was finished by burnishing with a cow's rib-bone or a piece of agate. Only the most delicate work was further smoothed by the only abrasive sheet material available - the skin of a shark or dogfish.
Nonetheless, I would guess that sharkskin has been used as an abrasive since ancient times, but definitive documentation may be hard to find. I assume that sharkskin doesn't tend to stick around to be discovered by archaeologists, nor is its use notable enough to receive much attention in historical records.
Abrasives are certainly ancient though: sandstone as an abrasive goes back at least to the Egyptians, and sandpaper goes back at least to 13th century China and 17th century Europe. For example, here's some speculation that Viking woodworkers used sharkskin. And here's some speculation that sharkskin sandpaper was a common trade item in bronze age Indian Ocean trade routes. Maybe someone else can find a definitive mention of the practice before the 18th century.
B) The skin of the coelacanth (not a shark) has also been used by Comoro Islanders like sandpaper--but given that the skin is primarily used to roughen the interior of bicycle tires before gluing on a patch, one can infer that the practice isn't exactly ancient.
One thing I should clear up up-front: While sometimes classified together, Sharks (and the related Rays) are not really fish. They in fact are less related to fish than we humans are. For instance, they don't have bones like we and fish do. There are numerous other physical differences too, but more importantly for your question, their skin is also very different. It is a material similar to their teeth in fact (having skin teeth doesn't seem so weird, once you realize that sharks shed their teeth). This is why the rough skin of a Chondrichth might be useful as an abrasive, where the scales of fish typically are not.
I found a lot of references to Polynesians using it as sandpaper. However, they were not a literate people prior to the colonial era, so I can't find anything prior to colonial times documenting this. It seems possible it was an old practice though. Their seafaring culture dates back roughly 2 millennium, and they certainly had plentiful access to sharks.
Another interesting case is the Chumash of California. A huge amount of their diet was sharks and rays, and they report a cultural practice of using the skins to smooth their seacraft and other woodwork. They certainly would have had plenty of it lying around, as archeologists found it to be their #2 source of protean (sardines were #1). This dietary practice appears to have gone back at least 1000 years.
The practice goes back at least to the early centuries of the common era, since the Mishna (Kelim 16:1) mentions "rubbing with fish skin" as the typical way of finishing wooden utensils. Doesn't say anything about whether it was sharkskin or some other type, though.
