If you didn't use a pond liner, I'm not sure how else you would get it to seal. You could use epoxy on the wood, but you'd have to be pretty meticulous about filling in all the seams first. 90 bucks for a pondliner sounds like an amazing deal, I guess I should have done more research before I bought mine. I got 45mil liner from Home Depot, I think it is 12'x15' and it ran me about 200 bucks.