I used to use this to seal the window rubbers on old cars when they leaked years ago. I'd been told this stuff didn't exist any more by several car spares specialists I tried recently when I had the same problem on my current car. Mind you, they probably weren't even born when I used to use it.
I was pleased to find it still here on Amazon and was not disappointed.
Unlike ordinary silicon sealers that gum up the outside, or need a very big gap to fill effectively, this stuff is very watery, and creeps inside the joint that's leaking, just like the water does, once inside the crack or leaking joint, it sets and seals it from inside. Magic.
Good for very small cracks or "tired" rubber seals that don't flex well enough to seal any more. Several applications may be required to seal a big leak or crack, but it does work eventually. It helps if the crack is as dry as possible when you apply the cure.
A useful addition to any handyman's armoury.