What I do .... Autobody filler goes right on to a clean grounded bare metal surface.. 36 grit or 40 grit grounded surface ,make sure you feather back the edges first back far enough so you have only a filler to metal surface application only.. make sure when you apply it should be a tight skim for to make sure you have no trapping of airpockets.. then apply a thicker application over top. sometimes in case both directions. this keeps any airbubbles in the filler to a minumal.. .. go the length of the panel.. not up and down.. the wider the damage to fix.. the wider the Squeegee to apply . Make sure its completely dry prior to blocking.. so you dont create any chance of pin holes from tearing the filler.. Board length wize for straightness then cross hatch block for Compound curves.. 40 to shape, or 80 to shape pending on the size of the area.. if you at any chance reach metal on either side of he area.and see a pocket of damaged area. dont keep blocking.. apply to the area the same way you did the first run.. try to keep the filler tight in the area you have to fix.. then you use 80 to feather into the previous filler.. once you have blocked straight.. smoothin out lightly with 120 grit.. any divots you can use a 2 part Putty..dont over use.. Its not intended to fix dents,to clean up just imperfections through the repair..
Thats what I do on a daily base.. every one has a different take on this ,, so you have to decide..
