use Pond sheild epoxy by pond armor http://www.pondarmor.com/shop/
watch uarujoey on youtube build a 6' by 6' by 2'. He uses a plywood bottom with pond armor. http://www.youtube.com/watch?v=o8olS_u-y3o&list=PLcWeEm9w62maD1EGxHNXLxomN2YZzUUak this link about 6 minutes explains the product.
watch uarujoey on youtube build a 6' by 6' by 2'. He uses a plywood bottom with pond armor. http://www.youtube.com/watch?v=o8olS_u-y3o&list=PLcWeEm9w62maD1EGxHNXLxomN2YZzUUak this link about 6 minutes explains the product.