First, identify the type of finish and how deep the scratch is. Powder-coated and painted metal planters can usually be touched up, but if the scratch is deep enough to expose bare metal, you need to treat it quickly to prevent rust.
Clean the area with mild soap and water, then wipe it dry. Remove any loose paint or rust with fine-grit sandpaper, and feather the edges of the scratch lightly so the repair blends in. Do not sand too aggressively, especially on powder-coated surfaces.
If bare metal is visible, apply a thin coat of rust-inhibiting metal primer and let it cure. Next, apply a color-matched touch-up paint or outdoor metal spray paint in light, even coats. For powder-coated planters, a manufacturer-matched touch-up kit is often the best choice. Finish with a protective clear coat designed for outdoor metal.
Let the repair cure fully before placing the planter back into service, and avoid watering or scrubbing the area during curing. For large or highly visible damage, contact the manufacturer or a local powder-coating specialist for a more durable repair.