Are you sure it is rubber? If it is truly rubber and has cracks, then it needs to be replaced or repaired. Any roof that has cracks cannot be magicly healed by a coating. If a surface coating has cracks that do not extend into the structural membrane of the roof, then it should be recoated to keep it intact. That is normal maintenance. If it is rubber, don't let anybody near it with any kind of asphalt, hot or cold. What is on the surface now?