Yes, you can and generally should use a sealant on concrete urban planter boxes to prevent water stains. These stains, often white and chalky, are caused by mineral deposits (efflorescence) leaching out with water. A quality concrete sealer creates a protective barrier that minimizes water absorption, thereby reducing the chance of these stains forming.
For best results, choose a penetrating, breathable sealer designed for concrete, such as a silane/siloxane or a concrete densifier. These sealers repel water while allowing moisture vapor to escape from within the concrete, which is crucial for plant health. Avoid film-forming sealers like epoxy or polyurethane for planters, as they can trap moisture and eventually peel.
Before application, ensure the planter is completely clean and dry. Apply the sealer according to the manufacturer's instructions, typically with a sprayer or roller. Reapplication every 1-3 years is recommended to maintain effectiveness. Sealing not only fights stains but also protects the concrete from freeze-thaw damage in colder climates, extending the life of your urban planter.