On curves and rounded areas, the long board may not work as well, and especially in tight spots.
I like to use long boards for big flatter surfaces, but tfor the rounded areas, and curves, go for a 5 and 9 inch rubber block, also use a soft block for curves and tightspots too.
One thing you want is smooth transitions from filler to metal, nice feather edges, that make a big difference.
I may not have got all my highs and lows, but my filler was sanded out nice and flat with good feather edges.
Joe on real rough panels, try the 2 k Polyester riller primer, shoot it on with a gun with about a 2.2 tip, you could bury a 25 cent piece with this stuff, and it does not shrink back much, nor does it have any ISOs.
Far as wasting material, filler is relatively cheap, and so is sand paper, you nkot being an expereienced body man your gonna have some ttrial and error, I found a lot of my applied filler wound up as sand dust on the flooer......good luck Fred