In the past few days I have noticed white marks on my windscreen as well as on the side of my car.  I thought it was just dirt but when I tried to remove it, it wouldn't go.

I tried polish; no luck.  In the end, I had to use my finger nails to remove the marks off the windscreen.  However, there are marks remaining on the paintwork which I do not wish to use my finger nails to remove.  It looks like white paint marks splattered at high speed (maybe when I was on the motorway?!).

What do you guys think it may be?  And a solution?