Lately thats how my car is most of the time, they are knocking down the old RAF/air traffic control in west drayton so get a lot of dust from there lately, but the rain usually washes it off.

But as said a gentle wash off should get rid of it and give it a dry to stop any water marks. Show car cleaning is done the day before for me, mainly this time as i'm working friday on nights, so have only got all day sat to do a good job