Bird droppings are acidic and gritty, so the biggest mistake people make is scrubbing them while they’re dry. That can scratch the paint or leave dull marks in the finish. The trick is to soften the mess first and lift it off gently.
Start by soaking a few paper towels or a soft microfiber cloth in warm water. Lay the wet cloth directly over the bird dropping and leave it there for 5 to 10 minutes. If the dropping is thick or has dried hard, add a tiny drop of mild dish soap to the cloth before placing it over the spot. This helps rehydrate the uric acid in the droppings so it releases from the painted surface instead of scratching it.
Once the dropping feels soft, wipe it away gently with the same cloth. Wipe in one direction rather than scrubbing back and forth. If there is still a little residue left behind, dip a clean soft sponge or microfiber cloth into a bucket of warm water mixed with a few drops of pH-neutral soap or mild dish soap. Squeeze it out well so it is damp, not dripping, and wipe the area again using light pressure.
After the spot is clean, rinse the area with a cloth dampened in clean water. Avoid spraying the bench with a strong hose jet or using a pressure washer, because that can force water under the paint or lift the finish. Then dry the bench with a clean, dry towel to prevent water spots.
If the bench looks a little dull after cleaning, you can apply a very thin coat of paste wax or a paint-safe outdoor furniture protectant once the area is completely dry. Just test any product on a hidden part of the bench first to make sure it does not change the sheen.
I would avoid vinegar, bleach, ammonia, magic erasers, stiff brushes, and abrasive cleaners. Even though vinegar is often recommended, it can dull or etch some painted finishes if not rinsed perfectly. For regular care, wipe bird droppings off as soon as you notice them, because the longer they sit, the more likely they are to stain or etch the paint.