I build all my masters from scratch using epoxy putty, mostly A+B and DURO, and other materials. To save time, however, I produce basic resin mannequins that I can use as a base for new masters.
Shown here is an early anatomy mannequin made of A+B epoxy putty (I now use almost exclusively DURO), and some partial anatomies cast in polyurethane resin, all derived from the same original mannequin.
These partial resin anatomies, sometimes in turn slightly modified, are the starting point for creating the masters of all my miniatures.
Note the holes in the mannequin’s torso, where the head and arms will be inserted. I always keep a master’s head and arms separate from the body. This is to reduce undercut issues when producing the silicon rubber molds, as well as to allow for multiple poses.