Now some technical stuff

Kneadit - Epoxy.
You can find it at any BBC hardware.
It is something I always have with me in "my scouts box".
This stuff is magic, you just cut with a knife a slice of the "tube" (which have both the resin and the hardener), you mix them for 60 sec by "kneading" tem, than fill holes and deep scratces just by pushing the thick paste into the cleaned voids. It sets in 5 - 10 minutes, than you can sand the extra ( bee carefull not to leave too much extra) and off you go back on the water