A. New liquid urethane rubber does not bond well to fully cured urethane rubber.Some urethane rubbers (such as PMC 121tm 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; 

1. Add newly mixed rubber to cured rubber as soon as possible.
 The longer cured rubber is allowed to cure, chances of a lasting bond diminish. The best chance for a bond between urethane rubber layers is to pour the new liquid urethane over existing urethane before it cures – while it is in it’s “gel” or “tacky” state. 

2. When adding newly mixed rubber to cured rubber, make sure the cured rubber surface is clean. Dust, oils or other contaminants on the surface will prevent rubber from sticking. Wipe the cured surface with acetone and let dry before applying liquid rubber.

Smooth-On offers a flexible adhesive called “Ure-Bond®
 that will repair torn urethane molds. You can also apply to cured urethane rubber as a bonding primer before applying new urethane rubber.