You can repair a rusted metal urban planter box without replacing the whole thing in most cases, as long as the frame is still structurally sound. Start by emptying the planter and setting the plants aside. Put on gloves, eye protection, and a dust mask. Scrape away loose soil, rinse the metal, and let it dry. Inspect the base, corners, seams, and drainage holes; rust usually starts where water sits.
Use a wire brush, sandpaper, or a drill-mounted wire wheel to remove loose rust, flaking paint, and dirt. Sand around the edges until you reach firm metal. Vacuum or wipe away dust. If there are small holes, patch them with an epoxy metal repair putty or a fiberglass or metal patch kit; follow the product instructions and sand smooth after curing. For pinholes, a rust converter plus primer may be enough.
Treat any remaining rust with a rust converter. Apply it only to clean, dry rusted areas and let it cure fully. Then apply a rust-inhibiting metal primer. After the primer dries, paint the planter with an exterior-grade metal paint designed for outdoor use. Two thin coats are better than one thick coat. Let the coating cure completely before planting, especially if the soil will touch the painted surface.
Improve drainage so the repair lasts. Clear or drill drainage holes at the lowest points. Raise the planter slightly on rubber feet or bricks so water can escape and air can circulate under the base. If you line the inside, use a durable planter liner or pond liner with drainage holes and weep holes that align with the metal planter’s holes; do not create a sealed bath that traps water. A fabric barrier over drainage holes can keep soil in while letting water out.
Refill with fresh, free-draining potting mix, replant, and water gently. Check the planter every few months for bubbling paint or new rust, and touch up any chips promptly. With good prep, rust treatment, paint, and drainage, a rusted urban planter box can last for years without a full replacement.