We have a small display pond at our shop that had a leak in it almost since we installed it. When it was installed I tried a roll on type of liner which I didn't apply perfectly which caused the leak. I repaired it several times to limp through the season but it eventually had to be replaced. Here are some pictures and descriptions of how we applied the new polyurea spray on liner.:

Because this pond was already half finished all I had to do was remove the exsisting liner material. After this I wanted to raise the water level which I did by adding some block to the top edge of the exsisting cinder block collar. I also added a new Koitoilet II airated bottom drain and also a tangential pond return.