If something is sandpapered, it means it has been rubbed with sandpaper to make it smooth or to remove a layer from it.
Sandpapered things are usually made to be smooth or clean.
Examples:
He sandpapered the wood until it was smooth.
The old table was sandpapered to make it look new.
She carefully sandpapered the rough edges.
The cat sandpapered its claws on the tree.
He sandpapered the rock until it was as smooth as glass.
The artist sandpapered the sculpture until it was perfect.