Wooden or metal furniture can be repaired by replacing damaged or badly scratched parts. Most will either bolt or screw together at joints or may be a fitted joint in wooden furniture. Simply take the joints apart, replace the leg and refasten.
With wrought iron furniture, especially outdoor furniture, rust is typically a big problem. Remove as much surface rust as possible using a wire brush or sandpaper, then spray with a rust resisting paint to prevent any further damage. Usually painting the whole surface is the only option to keep the item uniformly painted.