Honestly, the best approach is wet-first cleaning, not dry scraping. Bird droppings can crumble into harmful dust, and moss will release spores if you attack it dry. Put on gloves, lightly wet the bench with a garden sprayer or hose, and let the buildup soften for five to ten minutes.
Then use a plastic scraper, old spatula, or stiff nylon brush to lift the bulk of the droppings and moss. Avoid metal scrapers on painted or wood surfaces because they leave scratches. For bird droppings, spray the area with warm water and a few drops of mild dish soap, scrub gently, and rinse. If the stain remains, make a paste of baking soda and water and work it with a soft brush.
For moss buildup, mix one part white vinegar with one part water and spray it on the mossy areas. Let it sit for 15–20 minutes, then scrub. The vinegar kills moss and loosens its grip without being as harsh as bleach. On metal or painted benches, test the vinegar mix on a hidden spot first. For moss in joints and gaps, use an old toothbrush or soft wire brush.
After cleaning, rinse thoroughly with clean water and let the bench dry completely. If you want extra sanitation for bird droppings, spray the area with diluted hydrogen peroxide or an outdoor disinfectant and let it air dry. To prevent future buildup, keep leaves and standing water off the bench, trim nearby branches for more sunlight, and apply a breathable wood sealer or paste wax to wood and metal surfaces. Avoid pressure washers on older or painted benches, as they can force water into cracks and strip finishes.