First, I rarely give 5 stars. I have tried several sealants in my time. hard plastic type in local big stores; a small red rubber condom and several others including a nail insert. These yellow flexible caps have NEVER let me down. I even used sealant that was over 1-1/2 years old with this cap, and although the first 3 inches of the jet that came out of the 1-1/2 year old sealant was a bit runny. (unmixed), the rest came out fine, no discolouration, rust etc. They also worked well on tubes of construction glue. air entering through the sealed joint and the curing product).