Why???????? If you have a leak its better to replace the preform with liner anyways. IF your adding something to the preform then you dont need to seam it to the preform as long as the liner is above the preform and running into it.

greg

I have a preformed as a top pond kind of like a plant bog. It's banana shaped and I have two streams running from the notches that I cut out of the preformed. The streams are made of the liner that needs to be attached to the preformed. I hope that makes it clear as to why!!

Oh gotcha... Well there are a couple of ways, one would be to use seam tape that you use to repair pond liners. Just make sure you clean the sides of the preform. Another way believe it or not is to use gorrila glue and gorilla tape. Wet the liner first and then add a good amount of glue on the wet surface and then attach it to the preform and use the gorrilla tape to hold it in place. Make sure you run a roller over to make sure of good contact. The glue will expand and secure it to the preform. The glue actually preferrs to get wet to work better. I use it for years in all my installs for when i add a biofall and need to attach liner to it. Hope this helps!

Greg