Porous by nature, terracotta pots should be sealed on the inside. This will help retain moisture so plants growing in them will not require watering as often as those in non-sealed pots. If you're constructing a garden fountain using terracotta pots, seal their interiors with three or four coats of good quality, exterior-grade, oil-based polyurethane.
Rinse the inside of the pots to remove any dust or residue. If the pots have been used previously, wash them in soapy water and scrub the insides with a stiff wire brush. Rinse well to remove all soap residue. Set them upside down on a rack and allow to dry for two to four hours.
Paint the inside of the pots with the polyurethane or terracotta pot sealer, using a sponge-type paint brush. Completely cover the interior.
Set the pots, right side up, on a shelf or rack to allow the sealer to dry. The time will vary with the sealer used, but should take approximately two to three hours.
Apply one or two more coats, allowing ample drying time between coats.