When I did my screen, I was using the "Orignal Formula" only.
The one lesson I learned as far as eliminating cracks and pits go, is sanding down the Bondo slowly. Nice and easy strokes, especially if you're using a sanding block.
It also really depends on the density (grit):
I used 80 (dry only), 400 (dry & wet), and 800 (wet only).
I remember that 80 (dry) and 400 (dry) left me with pits and small cracks/lines.
Oh yeah, and my so-called "applicator" was a chop-stick I cut to make it look like a mini-shovel.