Have you noticed that after a while a terra cotta planter will disintegrate before your eyes? I have lost many a favored pot to the ravages of water and minerals leaching through the walls of the vessel. To prevent this from happening, coat the inside with elastometric roof coating (found at Ace Hardware)~it creates a barrier between the wet soil and the porous terra cotta. A gallon is quite expensive (about $100), but it will last for a long time if all you’re doing is coating pots. It is worth the investment.