Milliput is versatile epoxy putty which can be sculpted and is highly adhesive to most materials.
At Gnomish Bazaar, we like to use Milliput as an alternative to Green Stuff, especially for sculpting basing decorations and filling gaps on minis.
It's not as sticky a Green Stuff, so many people find it easier to work with, and it can be temporarily thinned with water to help squeeze it into tiny cracks when gap filling.
Once cured, it's rock hard and can be drilled, sandpapered, filed, and painted.