A pond liner is a flexible plastic liner laid over the interior of a pond to prevent the water from leaking into the ground and to prevent the pond from becoming muddy. You can add mortar over a pond liner to add strength and you can add rocks and decorative elements to it. Adding the mortar is a simple process, but it is important to keep the mortar wet during the curing process to prevent cracking. Always wear a dust mask, eye protection and work gloves when working with mortar.
Mix the mortar in a ration of three parts sand and one part plastic cement. Add in 1 ½ parts of concrete reinforcement to add stability to the mix. Mix the ingredients in a large container. Add in enough water to create a mud-like paste.
Spread the mortar over the liner with the flat trowel. Spread the cement to a thickness of about 1 inch.
Press large rocks into the mortar to create borders and shelves for plants or fish. Press the rocks around the edges of the mortar to hold them in place. Spread a thin layer of mortar between the rocks for additional bonding and to create a water-tight seal between the rocks.
Water the surface of the mortar every hour. The cement must stay damp throughout the entire curing process or else it will crack.
Seal the mortar with a cement sealer. Apply the sealer with a paintbrush. Allow the sealer to dry for 24 hours.
Fill the pond with water as soon as the concrete sealer dries. This is the best way to maintain the strength of the cement inside the pond.