Yes—you can absolutely paint a park bench after purchase. It is one of the easiest ways to customize a basic bench, cover weather stains, or give an older piece a fresh start. The real question is not whether you can paint it, but whether you need a special primer. The answer depends on the material the bench is made from and the condition of the surface.
For a wooden park bench, primer is not always strictly required, but it is strongly recommended outdoors. Bare wood should be sanded smooth and coated with an exterior-grade wood primer. This seals the grain, blocks tannins from bleeding through light paint, and gives the topcoat a much better grip. If the bench already has paint in good condition, you can usually sand it lightly, clean it well, and paint directly over it. Just spot-prime any bare or repaired areas before applying the finish.
Metal benches are where primer really matters. Bare steel or wrought iron should be cleaned, sanded, and treated with a rust-inhibiting primer before painting. Skipping that step allows moisture to get under the paint, which leads to peeling and rust bubbles. Aluminum is slightly different because ordinary primer can struggle to bond. For aluminum, use a self-etching primer or a direct-to-metal paint designed for non-ferrous surfaces.
Plastic or recycled plastic benches are the trickiest to paint. Many are made from HDPE or poly lumber, and paint does not stick well to that slick, non-porous surface. If you want to paint one, clean it thoroughly, scuff it with fine sandpaper, and apply an adhesion promoter or a primer specifically made for plastics. Even with good prep, paint on plastic is usually more fragile than paint on wood or metal.
No matter what the bench is made of, the real secret is not the paint. It is the prep. Wash off dirt, mildew, and grease. Sand or scuff the surface to create some tooth. Wipe away dust with a tack cloth, and only paint when the bench is completely dry. Choose a dry day with mild temperatures, and use an exterior paint formulated for your material. For wood, a quality exterior acrylic latex works well. For metal, use a direct-to-metal enamel or rust-preventive paint. For plastic, choose a paint that bonds to plastics.
So yes, a park bench can be painted after purchase, and you do not always need a special primer for every bench. But you do need the right primer for the material. Skip that step on bare metal or slick plastic, and you will probably be repainting far sooner than expected. Do the prep once, and the new finish can last for years.