games-workshop green stuff, kiddies.
the more blue you use, the harder and quicker it sets. the more green, the softer and longer it sets, allowing you to carve into it over a couple of days. works on plastic and metal. use with wet fingers or wet knife so it dont stick to you! becomes bit sticky when first drying. you can even use glue to stick it on to something first, or use it like glue to jam two bits together. or use it like a stamp. i cant sculpt for shit, but i know how to customise with parts! you can paint straight over it too.