What is the verb for applying a sandpaper? Or sandpaper itself is a verb? Possible candidates for me is apply, imply, use. Also, may I say that I smoothen something with a sandpaper.
What you're looking for is sand. Merriam-Webster defines this verb as:
to smooth or dress by grinding or rubbing with an abrasive (as sandpaper)
So in an example sentence like "You should use sandpaper on that block," it would be:
You should sand that block.
The definition of sandpaper reveals a couple of verbs that go with it.
paper covered on one side with abrasive material (as sand) glued fast and used for smoothing and polishing.
(Smoothen or polish something using a sandpaper)
stiff paper that has a rough surface on one side and that is rubbed against something (such as a piece of wood) to make it smooth.
(Rub a sandpaper against a surface to smoothen it)
In fact, you can use sandpaper itself as a transitive verb. You can sandpaper(v) a surface and make it smooth.
Depending on the purpose of the sandpaper, you could use abrade.
to rub or wear away especially by friction
You can also scour or scrape a surface using sandpaper.