It's a two-step process. You have to put a fast drying layer on top of a slow drying layer.
The top layer dries hard and becomes brittle. As the layer underneath slowly dries, it shrinks causing the upper layer to "crack" and pull apart. Changing the thickness or drying time of the layers can change the amount of cracking and size of the pieces.
In woodworking (my career) we traditionally use horse-hide glue underneath and alcohol based lacquer on top.
If I were using it on plastic, I would use an epoxy primer first to ensure adhesion.
A crackle is different from a wrinkle finish, which is sometimes seen on paintball markers and requires heating, and that "hammered" look is usually zinc flake in a clear urea resin lacquer.
It's a little bit chemistry, a little bit art, some luck, and a lot of experience.
Just try it on something before you go ahead with it. It's not that tough and I think they even sell kits with the stuff you need.
This post has been edited by Pyrate Jim: 22 July 2008 - 04:29 PM