Marvin wrote:painting the figures with a little pva glue beforehand.
A very good idea by Marvin.
Ender Duman I have some more suggestions:
-Remove the grease (applied during production) that covers the surface of the figure by soap or terpentine.
-Roughen the area you want to apply your putty with a needle, knife point or file.
-Use a putty mix of green stuf and magic sculpt (or a similar putty that can be cut or grinded afterwards) so you keep some of the stickyness of the green stuf but it is more easy to reshape it after hardening.