Let's clarify something else here, exactly what kind of sander have you used? Because a "DA" like this, is a horrible tool for cutting filler of any kind. I still have a couple as they are THE tool for metal finishing (using no bondo in a repair) as I was taught many years ago. It's my tool of choice for door skins and the like.
But an ORBITAL sander like this is perfectly fine for roughing the panel into shape, it is effectively a "round block".
And the larger version a National Detroit 800 is my work horse, I am sorry, but after years of using this tool getting a panel long board flat isn't a dream, it's reality.
And this baby, a Hutchens 3800 is pretty hard to beat, an orbital long board.
Cutting filler with that NEW, SHARP, QUALITY paper on one of these tools is pretty hard to beat. You can hold it off the surface and apply as much or as little pressure as you want.
They are damn nice to have in the tool box.