I have a pond and would like to seal it for water catchment. What would you suggest.
I would think that some type of swimming pool paint or elastomeric type coating would be your best choice.
It's difficult to give you advice on an exact product to use without actually seeing the pond myself.
I would suggest that you drop by your local supplier of swimming pool or pond maintenance products. Such retailers should easily be found by doing an online search specific to the local area that you live in.