Personally, I'd avoid filler that's "rock hard". It should flex.
I've used the Abatron filler and often found it very sticky to sand.
One of my favorites is System 3 QuikFair. It's easy to mix, apply, and sand. Unfortunately, it's pretty much doubled in price recently.
To reduce sanding, on large areas, lay sheet plastic over the filled area then, place a smooth board on top secured with clamps or a couple screws.
Clean off any excess filler before it completely cures. The filler won't stick to most plastics.
Use a rasp to quickly level any high spots.