What do you want to do with it? Are you looking for a filler, or for a sculpting medium?
I ask, because I use Aves Apoxie Sculpt 2-part epoxy putty, but I use it for sculpting, not for filling seams or gaps. I have used Miliput, and Sculpey as well. Aves is fine-grained, and can be shaped or smoothed with water while it's still soft. It cures rock-hard and can be sanded well.
For filling gaps or seams, I use Squadron's white putty, thinned with acetone, or I use stretched sprue and bits of styrene card, and liquid styrene cement, depending on the size of the seam or gap.