How do I waterproof a fibreglass pond?

[likepizza]

  

 I moved to a new house and the fishpond had cracks in it. I patched the cracks with a fibreglass repair kit. I’m pretty sure I got all the holes and gaps. I put some water in to test it and it seemed to hold well. But overnight the water level dropped about 50mm. Looking at the pond it seems like water has seeped into the fibreglass which is probably 30 years old. Can I use Crommelin pond sealer to fix this?

[Aquaman]

Couldn't be worse then fibreglass resin lol