Remove all cushions from the furniture and move the piece into the middle of the yard or driveway. Using the highest pressure setting on your garden hose nozzle, spray the furniture to remove all dust and debris. Allow the furniture to dry completely.
Brush away visible rust with a steel-bristled brush. Remove as much rust as you can with the brush.
Spray the furniture with phosphoric acid. Phosphoric acid will transform any rust you may have missed or cannot see into iron phosphate. You can purchase phosphoric acid at most home improvement stores. Always wear rubber gloves when working with chemicals. Follow the manufacturer's directions for specific application instructions. Allow the acid to work overnight. Use your steel-bristled brush to remove any phosphate crust from the furniture.
Tape off any areas of the furniture you don't want to paint. Use newspaper and tape to cover vinyl straps or fabric parts of chairs that cannot be removed.
Paint the chair using the can of spray paint. Choose a paint formulated for metal. Krylon and Rustoleum are good choices. As spray paint tends to splatter on nearby surfaces, make sure anything you want to protect is covered with a drop cloth. Apply two or three thin coats of paint, allowing each coat to dry before applying another.