For most chipped urban planter boxes, the best safe planter box paint is a low-VOC or zero-VOC, water-based exterior acrylic paint or water-based acrylic enamel. It is low-odor, cleans up with water, and becomes durable after curing. The right choice still depends on the material.
Metal: sand rust, apply a water-based rust-inhibiting primer, then topcoat with exterior acrylic enamel. Wood: use exterior acrylic latex paint or a water-based exterior wood paint. Concrete: use water-based masonry paint. Plastic or fiberglass: use a bonding primer plus exterior acrylic paint made for plastics.
For touch-up paint for planters with plants still inside, cover the soil with plastic. Water the plants first, paint on a calm day, and never let paint, primer, or rinse water enter the soil. Remove plants if possible. Use a small brush instead of spray paint. Choose low-VOC exterior acrylic paint and let it cure fully, usually 24 to 72 hours, before uncovering the soil. Avoid lead-based paint, oil-based paint in poorly ventilated areas, and products labeled toxic to plants or aquatic life. Once fully cured, a low-VOC water-based acrylic touch-up is usually safe for people, pets, and plants, but it is not edible or harmless if it gets into the soil.