I used to work in the caravan industry. I was Quality Manager for Avondale caravans. before I started there, more or less every caravan leaked. We did lots of experiments to determine the best sealants, then changed over to butyl.
As I said, butyl has some interesting properties. If you break a bit off in your hand, it behaves just like plasticine. If you make a small ball, you can roll it around on your desk, but if you leave it there and come back the next day, it's like it melted. You can't get it off. It's like the worst chewing gum , nearly impossible to clean off. What happen is that, over time, the molecules start to adhere to whatever they come in contact with, and they don't like to let go. They creep into every crevice, so it's self-healing as a sealant. It will even seal under water.
As an anecdote, One day I was cruising in my 32ft ketch. I stopped at Ilfracombe, which has drying moorings. While the tide was out, I decided to sort out the speed transducer, which wasn't working. When I opened the access hatch, I noticed that there was a spare tube next to it, but I had never noticed anything else on the underside of the boat, so my curiosity caused me to investigate further. I took the waterproof cap off and looked down the tube, but it was blocked with something. These tubes are about 50mm diameter and are glassed into the hull for various types of transducers. They have a sealed screw-cap to stop water coming in the boat.
I went and got a nice wooden pole (boat-hook) and tapped down through the tube, but nothing happened, so I gave it a big whack, and it cleared. Now I could see through the hole to the sand below the boat. I thought that this spare tube now might be useful, so I climbed down the ladder to have a look at where it was on the underside. To my horror, I'd knocked off a 4" diameter disc of GRP so there was more or less a gap all round the tube where the water could get into the GRP or even into the boat - and the tide was coming in, and my boat would be floating in a couple of hours. Luckily, I had kept a big roll of 2" wide butyl tape from my caravan experiments, so I was able to make a temporary repair, then I went to the chandlers and got a GRP repair kit, which I used when the tide went out gain, and I covered the repair in the butyl tape to keep the water off for a few days while it cured.