Short answer: yes, a metal park bench can get slippery when it rains, but the level of slipperiness depends on the metal type, finish, and seat design.
Most metal park benches are made from steel, aluminum, or wrought iron. Smooth painted surfaces tend to become slick because rainwater creates a thin, low-friction film. Powder-coated and textured finishes usually perform better in wet conditions, but they are not completely slip-proof. Benches with slatted metal seats allow water to drain or evaporate faster, which can reduce that slick feeling compared to a solid metal sheet.
If you are about to sit on a wet metal bench, do a quick touch test. Wipe a small area with your hand or a cloth and feel the surface. If it feels slippery to your fingers, it will likely be slippery to sit on. If you own or manage a metal bench in a park, choose a textured powder-coated finish, add drainage slots, or apply non-slip strips to high-contact areas. Regular cleaning also matters because dirt, algae, pollen, and mildew can mix with rainwater and make the bench much slicker than water alone.
So yes, use caution, but you do not need to avoid every metal bench in the rain. A quick touch test and a little awareness will usually tell you whether it is safe to sit.