Bird droppings are at their worst when:

- The heat & sun are let bake it in to the paint.
- Around Autumn time when the birds are eating fruit & the resultant droppings tend t be more acidic.
- Wiped off with tissue or kitchen roll, a disaster, do not use paper.

I normally put water soaked kitchen paper over dried in droppings, let it soak in & then try hose the dropping off. Never wipe with the kitchen paper though.