The best way to clean mold off a park bench that’s been sitting in shade is a two-part job: remove the mold, then remove the moisture. If you only scrub and put the bench back in the same damp, shady spot, it’ll turn green again before long.
Assuming you can move it, shift the bench into an open area. Put on gloves, and if the mold is thick, wear a dust mask and eye protection. Use a stiff-bristle brush to dry-scrub the whole bench first—seat, backrest, underside, and joints—to knock off loose mold and dirt.
For the cleaning solution, I’d use warm water, a few squirts of dish soap, and about one cup of white vinegar per gallon of water. If the mold is dark or stubborn, oxygen bleach is even better; mix it according to the label. Avoid chlorine bleach on wood because it can damage the grain and harm nearby plants. For metal or plastic benches, a diluted bleach solution is usually fine. Just never mix bleach and vinegar.
Scrub the bench thoroughly and let the cleaner sit for 10–15 minutes without letting it dry. Rinse it well with a hose, then wipe off excess water. If possible, leave the bench in direct sunlight until it’s completely dry.
Once dry, apply an outdoor wood sealer or waterproofer if the bench is bare wood. For metal, touch up any scratches or rust spots; for plastic, a regular wash is usually all it needs.
To keep mold from coming back, improve airflow around the bench, trim back nearby plants, keep leaves and debris off it, and try to move it to a spot that gets at least a little sun. If it has to stay in the shade, plan on cleaning it every few months and after long wet spells.