I’d start by assuming it’s dirtier than it looks. City furniture collects exhaust film, dust, pollen, bird droppings, food grease, gum, and sometimes urine or other waste, so wear nitrile gloves, eye protection, and a dust mask if you’re going to brush dry grime. Check with the owner or city first if it isn’t yours.
First, clear the loose stuff. Pick up trash, scrape off gum with a plastic scraper—or harden it with ice—and sweep or vacuum loose grit. Don’t dry-scrape bird droppings or unknown stains; dampen them with water and a little detergent first so you don’t aerosolize anything nasty.
Then match your cleaning method to the material. For metal benches and frames, use warm water with a few drops of mild dish soap and a soft brush or microfiber cloth. Rinse with a clean wet cloth, dry immediately, and check for rust; spot-sand and touch up paint if needed. For painted metal, avoid harsh solvents and test any degreaser in a hidden spot. For wood, use the least water possible—a slightly damp cloth, mild soap, a quick rinse, then dry thoroughly and re-oil sealed wood if appropriate. For concrete or stone, a stiff brush and mild detergent usually work; avoid acid cleaners unless you know the stone can take it. For plastic, mild soap and a soft brush are fine; skip acetone and strong solvents. For fabric or upholstered pieces, vacuum first, use a fabric-appropriate cleaner, rinse with a damp cloth, and let it dry completely in the sun if possible.
For greasy spots, use a small amount of degreaser on a rag rather than flooding the surface. For sticky residue, isopropyl alcohol on a cloth can help on metal or plastic, but test first and keep it away from painted or varnished surfaces. Disinfect high-touch areas after cleaning—not before—because dirt can reduce disinfectant performance. Use a product labeled for the material, follow the dwell time, and wipe away residue.
Rinse with a bucket and cloth rather than a hose if possible. If you must use water, keep runoff out of storm drains and follow local rules. A pressure washer can damage paint, wood fibers, and seals, so keep it low and far away—or skip it.
Finally, dry everything thoroughly and inspect it. Tighten loose bolts, oil hinges, touch up rust, and replace damaged hardware. If it’s public furniture, report structural damage instead of trying to fix it yourself. A regular wipe-down every few weeks will keep it from getting this bad again.