I use the following Grit for bondo:
A very coarse file to remove all the excess, preferably while its still soft.
Followed up with a 40 Grit in a sanding block. To shape it
80 Grit in the Hand sander. To slowly take it down to the polygons mountains, all the while still working on the shape
Then at this point I determine if I need another layer of Bondo or not. If so.. Start back at the top. If not continue on.
120 Grit in the hand sander. To work out the bubbles and scratches and to start smoothing out the area
240 Grit in the hand sander. To finish the details and make it super smooth.
400 Grit Wet sand. To polish it all up and get it ready for priming.
Hope that helps. I'll post my process later on my site and on my thread.
Cheers!