The most difficult part of fixing a leak in a pool liner is finding the leak. Once the leak point has been found, you can plan the best way to fix it. If the leak is in the wall of the pool, a small patch can be used underwater. This is the easiest method and you should buy a suitable adhesive for PVC that will stick underwater. Cut a patch at least 20mm bigger than the hole and apply a generous layer of glue. Hold the patch over the hole and push it against the liner moving it in a small circular motion to push out any water between the patch the the liner. Keep firm pressure on it for at least 2 minutes. Sometimes gluing a patch underwater takes two or more tries to be successful. If the patch starts to peel straight away, try it again. You need a thick enough layer of glue to spread against the liner under water.
A patch is most successful when done on a dry surface and so if you have access to the hole in the liner when it is dry, the best method is to patch the liner from the back. Not only does this look much neater as the patch is hidden, you have the weight of the water when the pool is fill supporting the patch and holding it in place. When doing a dry patch, allow the glue to touch dry so you can touch it with your finger nail and it doesn’t stick. You cut the patch to size (at least 20mm bigger than the hole) and then mark the liner around the perimeter of the patch with a pencil. Clean the liner surface to be patched with some wet and dry paper to roughen it and remove any pool chemical residue. This will allow the glue to work better. If you have a printed pattern liner, take care not to damage the pattern.
Apply the glue to both surfaces and allow to touch dry. Then carefully apply the patch to the liner by starting at one side and make sure you push out any air between them. Hold firm pressure against the patch for at least two minutes. The patch with get maximum strength after 24 hours, so wait a while before filling the pool back up again. If the patch is on the floor of the pool, it is best to re-fill the pool as quickly as possible once the patch has taken hold to ensure you don’t let the liner shrink whilst the pool is empty. If you have too many holes to patch, like when a liner has been installed on coarse sand and it leaves thousands of pin holes in the floor of the liner, you need to replace it with a new pool liner.