We use 'milliput' which is a two part epoxy putty which when mixed together starts to set hard (about 1hr) and is great for filling gaps or reanimating figures and reprofiling the figure.

This is a fantastic product which I use to make new figures or sculpt with and for reanimating figures and filling to make things look natural.

Just mix the same amount of each putty then mix until the colours join into one colour. Apply the model (it does stick) pust in a sculpt to shape (it will feather almost flat to the surface. To help flatening use a wet finger. Allow to dry hard then the srface can be futher worked with a file or sandpaper to smooth off and feather in all seams between model and milliput. Undercoat and paint as desired.

I use dentist spatulars and similar tools to fashion in profiles and shapes and even use textured cloths pushed in and removed to give a variant to the surface finish.

It is a bit of an art which gets easier as you play and experiment. But once hard it can be reworked with a file to further improve the finish not acheived when 'wet'