What is 400 grit sandpaper used for?

180 to 220 Grit Sandpaper: Finer grit sandpaper is great for removing the scratches left by coarser grits on unfinished wood and for lightly sanding between coats of paint. 320 to 400 Grit Sandpaper: Very fine grit sandpaper is used for light sanding between coats of finish and to sand metal and other hard surfaces.