That's a great question! Dealing with graffiti on public bins is frustrating, but you're smart to worry about the finish. Many metal city bins have a powder-coated or enamel finish that can be damaged by harsh chemicals or abrasive scrubbing. Here's a method that usually works well, starting with the gentlest approach.
First, always wear gloves. For fresh spray paint, sometimes a simple mix of warm water and a few drops of dish soap with a soft cloth or sponge can lift it. Rub in a circular motion. If that doesn't work, step up to a dedicated graffiti remover or a citrus-based cleaner. Spray it on, let it sit for the time specified on the label (usually 2-5 minutes), but don't let it dry. Wipe it off gently.
For stubborn marks, isopropyl alcohol (rubbing alcohol) or a non-acetone nail polish remover can be surprisingly effective on some paints. Always test any chemical in a small, inconspicuous area first! Apply a small amount to a microfiber cloth and dab at the graffiti, don't scrub aggressively.
The key is patience. Reapply your cleaner and use several gentle passes rather than one aggressive scrape. Avoid steel wool, metal scrapers, or wire brushes, as they will definitely scratch the finish. For old, sun-baked graffiti, you might need a commercial-grade, biodegradable graffiti remover; follow the instructions carefully.
After cleaning, rinse the area thoroughly with water and dry it with a soft towel to prevent water spots. This process should restore your bin's look while keeping its protective coating intact.