I've been asked to quote for glassing the inside of a concrete structure that they want to turn into a fish pond. I haven't seen the actual structure yet but it was originally some form of water tank (they're quite common out here in Portugal) so I'm not sure it will actually need any fibreglass, maybe just a coat or two of resin to seal it a bit better, unless it is actually cracked in places. I'll have to try and get a look at it over the weekend. It might turn out to be very weak and I need to basically make a complete fibreglass pond within it.

Aparently there is a surface area of 14square meters to glass (sides and bottom), which if my maths are right means it around 5 cubic meters holding 5000 liters of water.

So 2 questions:
1. If it can just be resin coated or skinned with 1 or 2 layers of glass, will polyester bond to concrete or will it have to be epoxy?

2. If it needs a complete pond built in it any ideas on what sort of lay up it would need. In this case I guess I would go with polyester. Never used it before so now might be the time to try it?

Hi!
1) Polyester resin with 1-2 layers of glass should be ok to seal it. Epoxy would be too expensive but you can talk with someone that makes screeds on concrete, sometimes they use epoxy, but poly is ok.
2) 5000 l = 5000kg of water so it is reallyyyyy much as for diy cf and gf. I think it would have to be >20mm of fiber glass and polyester. I don't want even to think about cost of epoxy and cf, that would be thousands of euro.

Bye
Wojtek from Poland

~~~Everytime I hear sound of demoulding part I have eargasm~~~

To be fish safe, wouldnt it would need to be epoxy?

No, why? thousands of swimming pools are made with polyester and fiberglass. Once all the styrene has bled out, i'm sure effects are negligible.

I had NO intention of doing it with carbon!

Bit of further research agrees with you. 1 or 2 layers of 450gsm CSM with polyester is the way to go.

To be fish safe, wouldnt it would need to be epoxy?

Aparently not. There are warnings about polyester having chemicals that are toxic for fish, but so long as it is left to fully cure (2-3 weeks) and then flushed a couple of times it should be fine.

Fair enough, I've never looked into it too far but my wife has as she has a 800l aquarium with smaller ones dotted around the house and always said that there were chemicals that would leach out over time and make the water toxic for them.