First, move the bench to an open, shaded area so your cleaner won’t dry too fast. If the bench is on a deck or near plants, put down a plastic sheet to catch runoff. Using a stiff nylon brush, knock off the loose surface mildew before adding any liquid. Don’t skip this step; dry brushing removes a lot of spores so you don’t just smear them around.
Mix one part white vinegar with three parts warm water and add a small squirt of mild dish soap. Scrub the bench along the grain or joint lines and let the solution sit for about 10–15 minutes. Then rinse thoroughly with a garden hose. If the mildew has left dark staining, switch to an oxygen bleach cleaner mixed according to the label. I avoid chlorine bleach on wood because it can weaken the fibers and harm nearby plants.
After the bench dries completely, lightly sand any fuzzy or stained wood with 120-grit sandpaper. Then apply an exterior wood sealer, outdoor furniture oil, or a breathable finish that contains a mildewcide. For painted metal or plastic benches, use the same vinegar wash, rinse very well, and touch up chips or scratches to keep moisture out.
In a humid climate, the real fix is upkeep. Elevate the bench on pavers or feet so water doesn’t wick up, sweep off pooled water after storms, and clean it at least twice a year. If it is a public bench, report it to your local council or parks department before applying any coating.