This is an effective sealer for concrete ponds g4 is a moisture cured one component polyurethane product which forms a non porous tough durable but flexible seal on concrete this prevents lime leaching into your pond from the concrete and stops water leaking outr you just paint it straight onto the concrete no primer needed g4 can also be used to coat porous flagstones waterfalls statues and other concrete products to make them safe for the pondr for a black finish you need two coats