Honestly, the best way to protect urban outdoor furniture from rust is to keep moisture from sitting on metal and to catch small chips before they spread. I’d start by checking the material. Powder-coated aluminum, stainless steel, teak, and quality HDPE are much better choices for balconies, rooftops, and small city patios than cheap mild steel. If your furniture is already metal, make sure water can drain away and that the feet are not sitting in puddles.
Clean it regularly with mild soap and water, then dry it thoroughly—especially the joints, screws, welded spots, and feet. City air can leave dirt and pollution on the surface, and that holds moisture. Avoid harsh bleach or abrasive pads on powder-coated finishes. After cleaning, I like to wipe on a non-abrasive automotive wax or a manufacturer-approved protective spray on metal frames; it adds a thin barrier against rain and humidity.
Inspect the furniture every few weeks. A tiny scratch in the coating is where rust usually starts. Touch up chips with matching paint or a rust-inhibiting primer as soon as you see them. If rust has already appeared, sand it back lightly, apply a rust converter, and cover it with a suitable metal paint. Don’t just paint over loose rust.
Covers help, but they need to breathe. A plastic tarp can trap condensation and make things worse. Use a breathable, fitted cover, and elevate furniture slightly so air can move underneath. Keep pieces away from sprinklers, downspouts, and constantly damp walls. In humid or coastal cities, rinse off salt and dry the furniture after storms—even stainless steel benefits from this. Store cushions separately, and don’t let water collect in table recesses, chair legs, or nearby planters.
Finally, make it a small seasonal habit: clean, dry, inspect, and touch up. That’s far easier than dealing with rust later.