I’ve dealt with this on powder-coated metal furniture before, and the key is to work from the gentlest method upward. Powder coating is strong, but it is still a cured plastic-like finish, so hard scrubbing and harsh solvents can dull or soften it.
Start by washing the area with warm water and a few drops of mild dish soap. Use a microfiber cloth or a soft sponge to remove surface dirt so you are not rubbing grit into the finish. Rinse and dry completely.
For fresh graffiti, especially spray paint or permanent marker, 70% or 91% isopropyl alcohol is usually the safest first choice. Dampen a soft cloth or cotton pad with the alcohol, hold it on the mark for 30–60 seconds, then wipe with light pressure. Do not scrub hard. Repeat several times, turning the cloth to a clean section each time. This will often lift spray paint without damaging the powder coat.
If the alcohol is not enough, try a small amount of mineral spirits on a soft rag. Test it first in a hidden spot, such as under the bench or on the back edge. Apply lightly, wait about a minute, and wipe. Check the rag for color transfer. If the finish color is coming off, stop. If the surface stays smooth and glossy, it is generally safe for that area.
Some people use WD-40 or a citrus-based adhesive remover as a milder solvent. These can work on fresh paint and marker, but they leave an oily residue. Wash the bench with mild soap and water afterward. Avoid acetone, lacquer thinner, toluene, xylene, paint stripper, and general-purpose graffiti remover sprays unless the label specifically says they are safe for powder-coated surfaces. Those strong solvents can soften the coating quickly.
For stubborn paint, do not reach for steel wool, sandpaper, or a heavy scrub pad. Instead, soak the stain again with your chosen solvent, then use a soft nylon bristle brush or a plastic razor blade. Keep the angle low and use very light pressure. A melamine sponge can be tempting, but it is abrasive, so treat it as a last resort and only after testing a small hidden area.
If you use a pressure washer, keep it on a low setting with a wide fan tip. High pressure can lift powder coating, especially along edges or chips.
After the graffiti is gone, rinse the area with clean water, wash it with mild dish soap to remove solvent residue, rinse again, and dry with a soft cloth. That prevents dull spots and staining.
If the graffiti is old, baked on, or covers a large area, consider calling a local powder-coating shop or furniture refinisher. They can often remove the paint and re-coat the affected section more safely than aggressive home chemicals.