It can be very difficult to get liquid rubber to stick to cured rubber. Within the first 24 hours of curing time, fresh liquid rubber will stick to cured rubber very well. After 24 hours it becomes questionable. Clean the surface of the cured rubber with denatured alcohol (Note: Denatured alcohol is a good cleaning solvent, but must be handled with extreme caution owing to its flammability and health hazards) to remove any dirt or over spray from other activities in the area. Make sure it is dry and then mix and pour the rubber. Just how well the bond holds up depends on the use of the mold and how much stress is put on the joint during demold.