Having spent a lot of money on my new pond, it seems there is little information available on the finishing touches – especially when it comes to reviews of paints and selaers. This is for anyone looking to build a new pond as we’ve been through just about every paint you can buy….And some are certainly better than others!
My original pond used G4, which was blummin expensive, and after only 2 years, the paint had cracked in a couple of places, and the pond had started to leak. I stripped it down, and repainted again using G4 – although this time the paint didn’t last a year. I ended up complaining to my builders merchant, but of course, they didn’t want to know 
I then employed a builder to fibreglass my pond (£1500 and a promise it would NEVER leak). The guy came highly recommended, and although he took his time from day one – it never seemed to hold water properly. I was losing around 1cm every day. The builder blamed the original concrete construction for the problem, although surely if the fibreglass was sound, it should have been watertight? I got a new builder to come and look, and sure enough once patched up, it held water……Until winter, when it cracked again, and started leaking.
I eventually ended up having the whole pond stripped back, re-rendered and started from scratch. I filled up the pond which had been rendered using a waterproofing agent, and to my surprise – it held water for two days!
I needed to find a suitable paint to seal my render away and make the pond fish safe, and I plumped for A1 Pond paint. A load cheaper than G4, although a paint that had to be mixed.
I made a small amount to seal around my waterfall first – and although I managed to paint this in time, the last bit in the bowl had already gone off by the time I’d got to the bottom. They said their small 2.5 litre tin would be enough to cover my pond. I was extremely worried, as I have quite a sizeable build, and sure enough I need another the same size. Their figures seem to convey that 1 litre of paint covers about 6 square metres, but I found this to be more like 1 litre covers 2 – 3 square metres. On reviewing the Ebay and Google feedback it seems most people have to go back for another tin – meaning the cheapness of the product is not as cheap as you first imagine. Infact, it would end up double!
On painting this on, large areas would flake off the render. Apparently my render was the wrong type of render. 
I was told the second coat would solve any flaky issues, but on filling the pond – large chunks dropped off the wall. It was at this point I decided to scrap this paint completely. It had crazed and cracked everywhere. I was even told to PVA over the cracks first, then re-coat (which I did do), and this made no difference. I tried a small area with G4, which sealed perfectly!
At this point I’d had enough. I wrote to Home and Garden (my wife works in Admin) and had a letter published – plus I received a cheque for £250, and two cans of 11 litre black rubberised pond paint for having the “star letter”! I hope i wasnt unfairly treated 
I decided to build a whole new pond from concrete blocks, which was then skimmed using a waterproofing agent AND fibres mixed into the render. Under the render, I asked the builder to place a 2mm thick sheet of polythene (visquine?) to stop any damp from rising into the blockwork. The pond was filled, and left over the winter (even through the -27c temperatures) and the pond didn’t lose a drop 
The pond was painted as soon as the temperatures raised a little, and touch wood so far so good all these months later!
I found this thread whilst reading through the forum, and was reminded of my hassle of finding the right paint. I just wanted to let people know that if your going to build a pond, be prepared to tear all your hair out! I know I did!