Honestly, it depends on the bench. A lot of older wooden park benches have a slightly rough, weather-worn surface, especially if they are made from slatted timber that has been exposed to sun and rain for years. If you sit down in shorts, those can feel grainy or even lightly textured against bare legs, and sometimes you will catch a small raised splinter or a rough edge near a knot.
On the other hand, many newer benches—especially metal, powder-coated steel, or recycled plastic benches—have a much smoother finish. Powder-coated metal usually feels slick and smooth, almost like a painted metal railing, though it can get hot in direct sun. Recycled plastic benches are often the smoothest option: no wood grain, no splinters, and a consistent surface that is easy on bare skin. Concrete benches can go either way. Some are ground or polished smooth, but plenty have a rough, gritty texture that can scratch the back of your thighs when you stand up.
If you are asking because you want to wear shorts or a skirt and avoid that uncomfortable scrape, my advice is to look before you sit. Run the back of your hand over the surface first. That will tell you more in two seconds than any material label. Also, wooden benches that have been recently sanded and sealed can feel surprisingly smooth, but once the sealant wears off, the grain lifts and they start to feel rough again. So a bench that felt fine in June might feel rougher by late summer.
For bare legs on a bench, I would choose a recycled plastic or powder-coated metal bench if you have the option. They tend to stay smoother over time, do not splinter, and are less likely to leave those little red pressure marks or tiny scratches. If you only have a wooden bench, bring a light blanket, towel, or just sit on the edge with a bit more care. So yes: some park benches are smooth, some are rough, but the material and age of the bench usually tell you the story before your legs do. If you want a truly smooth park bench finish, look for recycled plastic or well-maintained powder-coated metal.