Secondary bond on silicone

I have seen many multi-layered silicone caulk jobs and they usually leak. What is the best solution for secondary bond of silicone on silicone?

 

Properly cleaned (with alcohol or acetone) silicone substrate forms an excellent chemical bond with fresh silicone of a similar type. The bond is as strong as the silicone rubber itself.

Every single silicone failure I have ever seen (and I've seen a lot, mostly on office building windows) has been due to contamination on the bonding surface. A bit of oil from fingertips, some dust, the coating on the glass, the release agent on moulded parts.

In cases where the substrate or the old silicone has been scrubbed clean (two clean white rags soaked in alcohol work well- scrub with one, wipe clean with the second, make sure no contaminants were picked up), I have never seen the resulting bond fail the standard peel tests.