Putting a brand new pond liner over an old pond liner is not something we recommend but it is possible if you follow these three simple steps.
Step 1: You’re probably fitting a new liner because your old liner is leaking and not holding water anymore, so first you need to find out where and why the pond is leaking. Remove everything from your pond and examine the old pond liner for tears or holes, these are often caused by sharp stones protruding from the soil behind the liner, if you find any stones or objects that could cause damage remove them before fitting the new liner. Examine all of the pond for sharp objects and remove any you find. If the object is near the top of the pond you might be able to peel back the liner from the top to remove it, if the object is towards the bottom of the pond you can cut the liner to get at it.
Step 2: The next step is to find the lowest point in your pond and cut a large hole in the old liner, this will act as drainage for any water that gets between the new liner and the old liner. If you don’t cut a hole for drainage any water that gets between the liners won’t have anywhere to go, eventually it will start lifting the new liner away from the old liner and you will end up with bulges in your pond.
Step 3: Lastly we would recommend installing a new underlay over the old liner before fitting your new liner, this will give extra padding and protect the new liner from any wrinkles that are present in the old liner.
We have an article about choosing pond liners which might interest you.