The types of maintenance required for wrought iron patio benches will vary depending on the actual materials used to make the bench. Traditional wrought iron patio benches are made from wrought iron, which is a type of iron that is heated and worked by hand into elaborate shapes and designs. True wrought iron is difficult to come by, and it is quite expensive, so some benches that are now marketed as wrought iron patio benches are actually made from cast iron or even steel. It is important to determine what materials are used for construction of the bench to determine how you must care for it.
True wrought iron often features a grain, much like wood. This occurs because of slag in the iron — slag is a by-product of the iron working process, and the fibers can show through in the final product. If your wrought iron patio benches are true wrought iron, you may see such a grain, and you can care for the bench as you would with other wrought iron products. Wrought iron is likely to rust if exposed to moisture, so the iron is often painted or otherwise coated to protect it from the elements. You may need to sand off the paint as well as any rust that has developed, and repaint the bench to protect it from the elements. The iron may need to be sandblasted to remove the rust without damaging the iron.
If the wrought iron patio benches are actually made from cast iron instead of wrought iron, the maintenance will be similar. The iron will need to be coated with some sort of sealant or protectant, and if that paint or protectant wears out and the iron is exposed to the elements, the paint and any rust that has developed will need to be stripped off and the iron will need another coating of protectant or paint. As with wrought iron, it is important to avoid scratching or otherwise damaging the wrought iron during any restoration processes to avoid further damage or degradation.
Much of the modern wrought iron patio benches available for purchase at stores is actually made of steel. Steel can be galvanized or otherwise treated to make it resistant to rust and corrosion, making it a less expensive and long lasting alternative to true wrought iron. Such a bench will require little maintenance other than keeping the bench generally clean and dry when possible. Steel is quite strong and will resist weather damage, so you may not need to do any maintenance at all.