Patio furniture does not have to be plastic or metal and made specifically for the great outdoors. Sure, this type of furniture was built to last in the elements but with a few tricks most any type of furniture can live in certain outdoor conditions. A covered patio or porch can accommodate most types of furniture, including rattan, with the help of enough polyurethane and water repelling fabric spray.
Rattan furniture looks great on a patio. The problem is it was made to go inside, perhaps in a sun room. This particular set had been moved outdoors several years ago without taking preventative measures against the elements. Therefor the work was twofold. Not only did it need to be protected, it needed to be repaired. Damage had already been done and mildew had set in.
Mildew does not mean all is lost and the furniture should be tossed. Far from it. What it means is there is some labor intensive work ahead, but the results will be totally worth it. The first thing to do is clean the furniture with a brush. Knock of excess dirt, chipped paint and dust. If things are really bad a mildew cleaner with bleach should be used.
Allow the stain to dry overnight then apply polyurethane. Make sure to choose a urethane that says it is protection from moisture, sunlight and temperature changes. Apply at least two coats with a paintbrush. Coat ALL parts of furniture, even underneath the furniture as well. Allow to dry completely.