A plumber’s epoxy is any sort of compound that’s used to seal pipe fittings. However, many plumbers epoxies make great sculpting mediums as well. Kneadatite above is but one example. A more typical plumbers epoxy is available at most home improvement stores. It usually dries quickly (5-20 minutes) and has a rock-like texture. I’ve found this to be useful when sculpting large areas or filling large holes that don’t require much detail. It can be sanded smooth and then painted.