New liquid urethane rubber does not bond well to fully cured urethane rubber. Some urethane rubbers (such as PMC®-121 30/WET) will not bond to themselves under any circumstances because they exude an oil to aid in the release of plaster and concrete.
Other urethane rubbers (such as our Brush-On® Series, Vytaflex® or Reoflex® DRY) will stick to themselves if conditions are right:
Smooth-On offers a flexible adhesive called URE-BOND® II that will repair torn urethane molds. You can also apply URE-BOND® II to cured urethane rubber as a bonding primer before applying new urethane rubber.