Don't know why this is happening now but when I use the screenwash to clear the windscreen, the wipers clear it but as the windscreen dries, there's some white "residue" appearing, largely in the bit missed by the wipers but also at some other points in the wipers' range. It has the appearance of some white stuff being smeared by the wipers.
Disappears when you wash the windscreen again and then reappears a few moments later.
Have been using screenwash from the dealers.
What is it? Why's it happening? How do I stop it from happening in future?