Magic Sculp
Magic Sculp is a two-part epoxy putty that is designed especially for modelers and sculptors alike. It has a really fine grain that allows you to make this stuff as smooth as glass.
I find that it isn't as sticky as Milliput and it is easier to shape. It also air dries to a rock-hard finish that will not crack.
Stop by their webpage and give them a try. I won't use any other putty for my kits. Here's a tip for quickly mixing the putty: Roll equal amounts of part A and B into two 'snakes'. Twist them together like a pretzel and start kneading the putty.
If you have any questions, please feel free to write: me