For sculpting larger areas of a figure you need to use an epoxy putty, like milliput or A+B, Tamiya etc.
If you try and use a normal solvent based putty like Squadron green, you will find that in thicker aplications it will not set for a very long time (many days). Epoxy will cure no matter what thickness you apply it in.
The best stuff I have come accross is milliput superfine white.