I don't know about keeping the liner white. Won't algae grow on it?

Oh yes, I resealed my former spa pond with white Portland
cement based sealer. Looked like a swimming pool for a
while. A SHORT while.
Then the algae started to grow on it.
Now it's completely black.
I think you'll work yourself to death trying to keep
it clean.

For really clear water a good veggie filter works best.
My pond is four feet deep and it's crystal clear
all the way to the bottom.

Ron
http://radioheaven.homestead.com/Cement_Pond1.html