ideally you want to remove most of the dent first, using as little filler as possible.
if its a tiny dent, then just go straight at it with the filler, if its big, then its ideal to be dropping door cards off or rear quarter cards and tapping the dent out as much as possible.
if its a small dent, then rough the area up with P180, tight skim of filler and sand it down with P180.
if its a big dent, you wanna be using P80 to rough the area up to bare metal, clean skim of filler and sand it down with P80 too.
if its a huge dent then dont be affraid to drop down to P40.
keep the rubber block flat at all times, keep changing the direction you sand according to where the filler is high.
make sure when your filler is right that you drop down the grades, ideally to P320, ready to prime.
this eliminates any risk of sinking in the future and nasty deep scratches from showing