Does anyone out there know how to repair a leak in a pre formed pond liner?

I successfully repaired my preformed pond a couple of years ago.
I used a thin copper patch that was about 3 or 4 inches wide and longer
than the crack.
I buttered the DRY crack with RTV silicone sealer.
I then pop riveted the patch over the crack.
You could also use Stainless steel or galvanized sheet metal
screws instead of pop rivets.
You could also use aluminum flashing for the patch.
You could even cut a patch out of the bottom of a section of aluminum
gutter. You "might" even get by with a patch made from a soft drink
can but that's awfully thin.

Good Luck!
Peace!!
Dan D. Louisville KY good ole USA
visit my pond and garden pages!
http://ky-dan.com