-
Tyre boot, but that's not really a permanent solution in most cases. Depends on how bad the cut is and what type of tyre boot material was used. I've had Park stick on tyre boots fail after another 300k day and ended up changing to a spare tyre instead.
You can also try glue from the outside. I used to use superglue to fill nicks and stuff in tyres to stop another sharp thing getting in the hole but I'm too lazy for that shit now and I tend to just use more durable tyres in the first place.
What's the best way to reinforce a tyre that's got some glass through it? Got a puncture the other day and have been riding around with an approx 0.5cm gash perpendicular to the casing threads (tyres are the supple kind so not a lot of those) from a glass shard that feels like it's just waiting to get something else through it.. Will an inner tube patch do or is there something more hardy (toothpaste tube?) that I can stick in there permanently?