The majority of clay pots sold in stores are porous and unglazed, which means they are not waterproof. Terra cotta that is unglazed or unsealed will absorb moisture from the plant and soil, keeping the pot moist, which could harm the exterior ornamentation by causing the paint to blister.
Not only does sealing your flower pot avoid the outside of the embellished design from blistering, but it also aids in keeping the soil moist within.
If you want to use the painted flower pot outside, choose an Outdoor Sealer. This polyurethane-based sealer comes in three finishes Matte, Satin, and Gloss, and is available as a brush-on sealer. It offers the most durability for outdoor projects.
After 48 hours, when the paint has had time to fully cure, cover the painted design with any outdoor sealer to add an additional layer of protection.
[image from theindoornursery.com]