If you’re dealing with squirrels or birds moving into the hollow sections of a park bench, I’d start with physical exclusion, because repellents and scare devices alone rarely solve the problem for long.
First, inspect the bench before doing anything. Look inside open tube ends, gaps under the seat, and weep holes with a flashlight. If you find nesting material, eggs, or babies, wait until the young have left before sealing openings. Sealing animals inside creates odor problems and is inhumane.
Then clean out old nesting material. Use gloves, a long-handled scraper, and a brush. For benches with hollow metal or plastic ends, flush out debris with water if possible, but let everything dry completely before closing things up.
The most effective barrier is 1/4-inch or 1/2-inch galvanized hardware cloth. Cut it slightly larger than the opening, fold the edges if they’re sharp, and attach it over open hollows with stainless steel screws and fender washers or heavy-duty outdoor staples. Hardware cloth blocks both squirrels and birds, but still allows airflow so moisture doesn’t get trapped. Avoid fiberglass window screen or light plastic mesh; squirrels can chew through these fairly quickly.
For small gaps and weep holes, use copper mesh or stainless steel wool. Press it firmly into the opening. It expands to fill the space and does not rust as badly as regular steel wool. I would not rely on expanding foam by itself because squirrels and rodents can chew through it, and it can trap water against metal or wood. If you do use foam to fill a cavity, cover the outside with hardware cloth or a solid cap so the foam is not exposed.
Where a bench has open tube ends, the cleanest fix is a solid cap or plug. Aluminum or PVC end caps work well for round or square tubing. Pressure-treated wood plugs or bent aluminum flashing can cover odd shapes. If you fully seal a metal tube, add a small drain hole at the lowest point and cover that hole with mesh so moisture can still escape.
Next, reduce access and attraction. Trim tree branches that overhang the bench so squirrels cannot drop straight onto it. Keep trash and food waste away from the area, and ask people not to feed wildlife near the bench. If birds are the main problem, consider putting a nest box on a nearby tree or post as an alternative. It won’t stop every bird, but it gives them a more attractive option.
You can try scent-based repellents as a backup, but they need maintenance. Peppermint oil, capsaicin spray, or a commercial animal repellent may discourage squirrels for a while, especially when paired with exclusion. Reapply after rain, and avoid sticky or tar-based products that can get on feathers or fur.
Schedule a maintenance check before nesting season in late winter and again after summer. Look for loose mesh, chewed edges, missing caps, and new nesting attempts. If the bench is exposed to weather, inspect for rust or water damage around any sealed areas. This regular check is usually what makes the fix last.
If you own or manufacture the benches, the long-term design answer is to avoid open hollow cavities in the first place. Specify closed end caps, welded or molded sealed ends, angled surfaces that don’t collect debris, and weep holes covered with factory-installed mesh. That is easier than retrofitting benches one by one.
Overall, the best combination is: clean out, install hardware cloth or metal mesh over openings, cap large tube ends, and check seasonally. That should stop squirrels and birds from nesting inside the hollow parts without harming them or trapping moisture.