depends on how big of an area, if its large i start with 36grit on an 8inch orbital sander, then when its roughed in, wipe another coat on, then shape it with 80grit on the 8in sander, then wipe another coat, and block it with 80, when its stright and smooth i wipe it wiht glasing putty then block with 180, and ur ready for primer.

if small i srart with 80 on a 6in DA (dual action) sander, then block with 80, and block with 180.