Outdoor planters and urns are filled with soil, watered daily and left out in the wind and rain. It is no wonder that a cast-iron planter will be in need of repair. One of the biggest threats to cast iron is moisture. Unprotected iron will rust quickly. Remove the rust and get your iron urn back into tip-top shape with a few simple steps.
Set a hose nozzle to its highest pressure setting. Hose down the planter to clean the urn thoroughly and remove dirt and debris. Towel off the standing water and let the urn dry overnight.
Scrub any flaking rust with a steel-bristled wire brush. Remove as much rust as you can.
File stubborn rusted areas with a metal file. Use the file on the straight, flat areas of the planter. Sand the rounded corners with 80-grit sandpaper. Wipe away the sanding dust.
Put on goggles, rubber gloves and a mask. Spray phosphoric acid on the urn. The acid will turn any invisible rust into a crust of iron phosphate. Let the acid work on the rust overnight.
Scrub the iron phosphate crust with a steel-bristled wire brush. The crust will fall off of the urn easily. Dust the urn with a damp cloth.
Spray-paint the urn with a metal spray paint. Choose paint that offers rust protection. Apply three to four light coats of paint. Spray in a steady back-and-forth motion, overlapping each pass of the spray slightly.
Line the inside of the planter with a plastic garbage bag to help prevent moisture from ruining the urn. Cut a hole in the bottom of the bag to match the drain hole in the planter.