I would also suggest using this paint decontamination system. This is an awesome system for decontamination and there is no risk of going through the clear. Oh and no I have no affiliation with this company lol, just trying to help.
Edit: Also, as bbf stated, this has nothing to do with the plastic bumper material. There is a clear coat layer on top of the paint that is extremely durable, but some bugs and bird droppings are very caustic and can do alot of damage fast.