Here in the suburbs of a major metropolitan area, and with a lot of agricultural activity further upwind, I find that my car windows develop a film that rain does not remove. Some of the film may be from vegetation (tree sap, etc.), but I can wash the windows and then park in an open parking lot and a driveway with no canapy, and still experience the film.

I wonder whether there are similar issues for solar panels, not solved by rain.

EDIT: Should have read the linked post, first. Rain works well enough for them.