Usually you can use 2 ways to repair a failed silicone sealant joint.
Since polyurethanes will not develop adhesion to cured silicones, the existing silicone sealant would need to be fully removed.
1.Use some mechanical abrasion to remove as much of the existing sealant as possible, when the joint is located within a porous substrate.
2. If the sealant joint is in a non-porous substrate, the sealant should be cut out as close as possible to the bond line. Sanding with a fine grit sandpaper may be needed to remove any silicone that is left within the joint.When applying the new polyurethane sealant joint, it is recommended that the polyurethane extend ¼ inch (6.35 mm) onto the substrate on either side of the joint. This will ensure the material is bonding to fresh substrate.
