http://CollisionBlast.com – Thia Q&A video answers the question of what grit of sandpaper to use when sanding body filler and when removing paint.
Body Filler Grit Chart
http://www.collisionblast.com/wp-content/uploads/2012/07/sand-body-filler-pdf.pdf
Remove Coatings Chart
http://www.collisionblast.com/wp-content/uploads/2012/07/remove-coatings-pdf.pdf