Use rough grit such as 40 or 60 to remove old paint and level off blemishes. Then slowly use finer paper such as 100 grit for the smooth finish. Sanding pads are often more durable than normal sandpaper sheets so sanding pads may need to be even finer such as 120.
Read Homebuilding's handy guide to sandpaper grades for advice on which grit/grade is appropriate for different DIY tasks.