i'd use durham putty mixed with wood dust and maybe microballons.
It's water based. It will leave a white residue that disappears when you epoxy. I fill cracks right after rough sanding and before using a fairing board. On my second boat i used epoxy and wood dust and this did not turn out as well as with the putty.

: I am at the point to where I am filling cracks in my Night Heron. What is a
: good resin or filler to mix with saw dust? Is there a better solution to
: filling cracks?