I’ve had the same problem on a north-facing balcony, and the fix is less about one miracle spray and more about breaking the damp, still-air cycle mold loves. In shady urban spots, rain, humidity, dust, and nearby plants all keep surfaces damp longer, so you have to make drying easy.
Start with the furniture itself. If you’re buying new pieces, look for powder-coated aluminum, teak or cedar, HDPE, synthetic rattan, and quick-dry cushions. Untreated wood, porous concrete, and thick cotton cushions are basically mold buffets in shade. Already have furniture? Elevate it on feet or slatted mats, pull it a few inches away from walls, and leave gaps between chairs and tables so air can move. Avoid stacking pieces while wet.
Covers can help or hurt. A plastic tarp that traps moisture is worse than no cover. Use a breathable, waterproof cover that’s sized properly and doesn’t press against the furniture, and only cover dry furniture. During long wet spells, I take cushions inside and lean them upright. If you have a small covered balcony, a cheap battery fan or even a few hours of open airflow after rain makes a big difference.
Cleanliness matters more in the city because pollution and dust hold water. Once a week, wipe frames with mild soap and water, scrub crevices, and dry with a microfiber cloth. For cushions, use removable covers and wash per labels; dry them in sun or a well-ventilated room. Don’t ignore undersides, joints, and between slats.
If mold appears, don’t just paint over it. Treat it outdoors with a diluted white vinegar solution, test a hidden spot first, or use a product labeled for that material. Rinse and dry completely. Never mix bleach and vinegar, and don’t blast delicate fabrics with harsh cleaners. For wood, a fresh sealant helps; for fabric, a water-repellent spray made for outdoor cushions can slow dampness. Avoid sprinkler overspray and trim plants that block airflow.
Finally, give furniture a “sun day” now and then: move it into direct light or a dry spot for a few hours. Mold prevention is mostly moisture management plus routine. If a piece stays wet and airless, mold will keep coming back no matter what you spray on it.